AGRICUL.TURE HANDBOOK NO. 98 NOVEMBER 1956

CHEMISTRY AND TECHNOLOGY

OF CITRUS, CITRUS PRODUC;

AND BYPRODUCTS

UNITED STATES DEPARTMENT OF AGRICULTURE WASHINGTON, D. C.
CHEMISTRY AND TECHNOLOGY

OF CITRUS, CITRUS PRODUC

AND BYPRODUCTS

Prepared by the

AGRICULTURAL RESEARCH SERVICE

AGRICULTURE HANDBOOK NO. 98 NOVEMBER 1956

UNITED STATES DEPARTMENT OF AGRICULTURE • WASHINGTON, D. C.
CONTENTS

Page

Structure and composition of citrus fruits 1

Macroscopic and microscopic structure 2

Nitrogenous constituents 3

Enzymes G

Organic acids 7

Flavonoids 9

Bitter principles 11

Volatile flavoring constituents 13

Pigments 15

Sugars 15

Lipids 17

Pectic substances 18

Vitamins 20

Inorganic constituents 24

Processing of citrus 24

Canning 24

Orange juice 24

Grapefruit juice 28

Blended juices 28

Tangerine juice 29

Lime juice 29

Lemon juice 29

Grapefruit sections 30

Mandarin sections 32

Chilled orange juice 33

Sulfured citrus juices 34

Pasteurized concentrates 35

Beverage bases 36

Bottler's base for carbonated drinks 37

Pasteurized base for noncarbonated drinks 37

Beverage bases preserved with sulfur dioxide _ 37

Frozen concentrates 38

Frozen concentrated orange juice 38

Frozen concentrated lemon juice 47

Frozen concentrate for lemonade 47

Frozen concentrate for limeade 48

Page

Processing of citrus—Continued

Frozen concenti-ates—Continued

Frozen concentrated tangerine juice 48

Frozen concentrated grapefruit juice 48

Frozen purees 48

Powdered citrus juices 49

Citrus byproducts 53

Pectin 53

Pectin manufacture 53

Use of pectic substances 56

Low-methoxyl pectins 56

Pectate pulp 57

Citric acid 57

Citrus-seed oils 60

Flavonoids 61

Essential oils 62

Citrus vinegar 65

Marmalades 66

Brined citrus 68

Grapefruit, lemon, and orange peel 68

Limes 68

Citron 69

Candied citrus 69

Grapefruit, lemon, and orange peel 69

Kumquats 69

Citron peel 70

Waste disposal 70

Dilute liquid waste 70

Methods for disposal 71

Biological treatments 72

Citrus feed 73

Citrus molasses 77

Feed yeast 78

Industrial alcohol 79

Lactic acid 80

Literature cited 81

Index 97

P'or sale by the Superintendent of Documents, U. S. Government Printing OflBce

Washington 25, D. C. - Price 50 cents
CHEMISTRY AND TECHNOLOGY

OF CITRUS. CITRUS PRODUCTS,

AND BYPRODUCTS

Prepared by the

AGRICULTURAL RESEARCH SERVICE'

Production of citrus in the United States

has increased from about 174 million boxes in

1943 to slightly less than 200 million boxes in

1955. Increasing attention is being given to

the processing and utilization of citrus other

than for the fresh-fruit market in order to

effectively market the ever-expanding production of citrus fruit.

In the early 1930's the principal fruit used

for processing consisted of culls from freshfruit packinghouses. This fruit was sound, but

because of offsize or peel blemishes was not considered suitable for the fresh-fruit market.

Processing of these culls into citrus products

added to the returns of the grower. During the

1953-54 season 43 percent of the oranges, 51

percent of the grapefruit, and 53 percent of the

lemons gi'own reached the fresh-fruit market.

The remaining percentage was processed by

canning or freezing, or was made into preserves, citric acid, pectin, oil, or feed. In a few

years processing has progressed from a means

of utilizing culls to becoming the backbone

of the citrus industry. Figure 1 shows a citrusproducts plant.

The purpose of this handbook is to describe

the latest methods for processing citrus products and byproducts. These methods are the

results of researches bj^ the United States De-

^^ Supersedes Circular 577, Citrus Fruit Products.

" Contributors were E. A. Beavens, G. J. Keller, J. G.

Kirchner, R. J. McColloch, J. M. Miller, R. G. Rice,

L. B. Rockland, and J. C. Underwood, chemists, and

E. R. Wolford, bacteriolog-ist, of the Western Utilization Research Branch; 0. W. Bissett, T. J. Kew, W. C.

Scott, L. J. Swift, and M. K. Veldhuis, chemists, of

the Southern Utilization Research Branch; and H. W.

von Loesecke, chemist, of the former Washington

Utilization Research Branch.

partment of Agriculture, State experiment

stations, and private industry.

In carrying out most of the operations described, competent technical aid is necessary,

and the economic situation, both local and national, should be adequately investigated before

considering commercial operations.

STRUCTURE AND COMPOSITION OF

CITRUS FRUITS

The demand for and acceptance of citrus

fruits in the daily diet of human beings is based

largely on their nutritional value, flavor,

aroma, and other esthetic characteristics, such

as color and texture. These quality factors are

dependent directly on the structure and chemical composition of the fruit. Citrus fruits

are a primary source of our daily requirement

for vitamin C, and, in addition, supplementary

nutritional value is obtained from the amino

acids, inorganic salts, carbohydrates, and probably other still unidentified factors present in

the edible portion of the fruit. The color of

the fruit is derived from carotenoid pigments,

chlorophyll, and possibly ñavonoids. The characteristic aroma is obtained from the volatile

essential oils found in the peel. All these chemical constituents plus many others present in

smaller quantities are the result of the combined influences of genetic regulatory mechanisms and the physical, chemical, and biological

environments to which the fruits are subjected

during growth and after harvest. Table 1 summarizes the chemical composition of orange

juice.
AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

FIGURE 1.—Aerial view of a Florida citrus-products plant. (Courtesy of Minute Maid Corp., Plymouth, Fla.)

TABLE l.—Approximate chemical composition

of orange juice

Class of

constituents

Carbohydrates

Organic acids

Amino acids, free

Inorganic ions_

Vitamins

Lipid constituents _ —

Nitrogen bases and glutathione

Flavonoids

Volatile constituents

Carotenoids

Enzymes

Total

Constituents

Number

7

7

17

14

14

18

5

1

33

22

12

150

Total

soluble solids

Percent

76.0

9.6

5.4

3.2

2.5

1.2

.9

.8

.38

.013

UOO

' Approximate.

In fresh citrus fruits the many chemical constituents and enzyme systems, conservatively

estimated at several hundred, are so located in

different cells, tissues, and other portions of the

fruit that they are able to perform their physiological functions without interfering or reacting with one another. However, when the

juice is extracted from the fruit by crushing,

pressing, or reaming, these multitudinous constituents are mixed together where they may

react to bring about undesirable changes in

flavor, color, and aroma. In order to maintain

maximum quality in processed citrus products,

it is essential that we know what chemical

constituents are present in citrus fruits, how

they react with each other, and how their reactions may be controlled.

Macroscopic and Microscopic

Structure

(Fig. 2)

Citrus fruits are covered with a rind, or peel,

to protect the pulp, or edible portion, of the

fruit. The rind is made up of a cuticle on the

outside, thinly covering an epidermal layer

(flavedo) containing numerous oil sacs, or

glands, filled with an aromatic essential oil

having commercial value (p. 62). In addition

to these oil sacs, the flavedo contains coloring

matter, which is not uniformly distributed
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS

FIGURE 2.—Schematic view of the cross section of an orange: a, Oil sacs in flavedo; 6, seed; c, juice sacs; d, center

core; e, albedo; /, segment; g^ segment membrane.

throughout this layer but is concentrated in

minute bodies called chromatophores, green in

young fruit and gradually turning yellow or

orange as the fruit matures (291).^

A white spongy portion of parenchymatous

cells, known as the albedo, lies directly beneath

the flavedo. The cells of this spongy layer are

loosely arranged with large intercellular spaces

and are irregular in shape. This layer contains

approximately 20 percent of pectinous substances, which can be recovered in the form of

citrus pectin (p. 53).

The inner pulp or flesh of the fruit consists

of segments (locules), separated by a membrane of thin epidermal tissue and containing

numerous spindle-shaped juice sacs (vesicles)

and seeds. The juice sacs in each segment are

attached to the segment wall and in contact

with the peel by means of fine threads of varying length. Embedded within the cellular tissue in the central part of each juice vesicle are

oil droplets (90). Yellow chromatophores of

crystalline origin may be observed when these

juice sacs are crushed and examined microscopically (277),

The central axis (core) of the fruit is composed of a white spongy tissue similar to that

found in the albedo. The core and segment

membranes are collectively called the ''rag'' of

the extracted juice.

' Italic numbers in parentheses refer to Literature

Cited, p, 81.

Nitrogenous Constituents

The nitrogen content of whole citrus fruits

varies between 0.1 and 0.2 percent on a wetweight basis (5Jf,). However, the nitrogenous

constituents in citrus-fruit juices compose from

5 to 10 percent of the total solids. Those known

to occur in citrus fruits include proteins, simple

peptides, amino acids, betaines, phosphatides,

and related substances. These classes of constituents are normal metabolites of both plant

and animal tissues, and contribute to the nutritional value of those plant and animal products

used for food and feed. Evidence has indicated

that amino acid-sugar (260) interactions may

be important in the darkening and development

of off-flavors in citrus (iS8) and other food

products (87).

The proteins in citrus fruits are relatively

insoluble, and are found to be associated with

the solid portions of the fruit, such as the seeds,

flavedo, albedo, chromatophores, and pulp.

Amino acids are found in the juice of the edible

portion of the fruit and in the aqueous alcohol

extractable fraction of the peel.

Until recently very little systematic work

had been done on the determination of the

nature, amounts, and reactivities of the nitrogenous constituents in citrus fruits, primarily

because of the unavailability of satisfactory

analytical procedures. During the past few

years a number of new methods have been

evolved for estimating amino acids and other
AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

nitrogenous constituents in biological fluids.

One of the most rapid and convenient of these

new techniques, called filter-paper partition

chromatography, has been particularly useful

for both qualitative and quantitative estimations of amino acids in citrus juices and hydrolyzates of proteins extracted from the solid

portions of citrus fruits.

Several of the free amino acids have been

found in the juice in amounts comparable to

the vitamin C content. Asparagine and glutamine were detected in the orange as early as

1908 (385) and later in lemon juice (J^Oô), In

1913 three nitrogenous constitutents—C.,H^RNO0„ C.H,N,0„ and CsH,,NO,—were isolated

from lime juice {125). However, the formulas

do not correspond with any of the known constituents in citrus juices. Betaine and stachydrin were isolated and identified in orange juice

by Yoshimura in 1918 {18Í. p. 169). These two

constituents and putrescine were also isolated

from, the pummelo in 1927 {ISU) ; putrescine

was later isolated from orange juice {173),

Florida Valencia orange juice was found to

contain arginine, aspartic acid, and choline

{316), in addition to asparagine and stachydrin, whose presence had been known earlier.

A small amount of free histidine has also been

reported {316, i38). Choline and a compound

presumed to be ethanolamine were isolated

from the lipid fraction of Florida orange juice

in 1951 {426).

The development of filter-paper chromatography techniques has facilitated the systematic estimation of nitrogenous constituents

in a wide variety of citrus juices and in the

tissues of California Valencia oranges. Cysteine

and the simple tripeptide glutathione have been

isolated from Valencia and Washington Navel

orange juice {203), and estimated as the free,

reduced compounds in fresh and heated juices

from grapefruit, lemon, lime, and orange

(table 2). Significant losses of aspartic acid,

serine, glutamic acid, and alanine in heated

TABLE 3.—Effect of heat and storage on some

free amino acids in filtered California Valencia orange juice '

Amino acid

Fresli

juico

Loss of amino acids in

Juice ?icatod

I 12 minutes at

200" F.

I

Heated juice

stored for 2

months at

100° F.

7-Aminol)ut>'ric acid

Aspartic acid _ _

Serine

(Jlutamic acid

Alanine

Millif/rams

percent Percent Percent

64 0 0

54 20 20

40 :30 30

26 60 60

26 60 60

' Rockland, L. B., and Underwood, J. C. Unpublished

data.

California Valencia orange juice have also been

observed (table 3). However, no qualitative

changes were observed in free amino acids during the normal ''browning'' of California

Valencia and Washington Navel orange juices

{Í3U). The free amino acids have been characterized in the juices of 6 commercial and 5

hybrid varieties of citrus grovm in California,

including Valencia oran^e^ Washington Navel

orange, tangerine, grapefruit, lemon, lime,

Dweet tangor, and Kara, Kinnow, and Honey

mandarins, and a lemon-orange hybrid {438),

The quality of the free amino acids and of

related constituents in citrus juices appears to

be characteristic of the variety and independent

of the geographical origin of the fruit {438).

No differences were observed in a qualitative

study, in which filter-paper chromatography

techniques were used, between the free amino

acids in fresh single-strength orange juice and

reconstituted, frozen, concentrated orange juice

{351).

TABLE 2.—Cysteine and glutathione content of fresh and heat-treated citrus juices (293) ^

Juice

Cysteine

Fresh

I Milligrams

percent

Grapefruit 0 57

Lemon -32

Lime j -82

Orange (Valencia ) 55

Heated 2

Milligrams

percent

0.27

.28

.41

Loss

Reduced glutathione

Fresh Heated

Percent

53

13

25

Milligrams

percent

5.9

2.8

5.1

Milligrams

percent

5.7

2.1

5.6

' Roman numbers in parentheses here and in other table legends refer to Literature Cited, p. 81.

- In sealed, enamel-lined tin cans for 15 minutes at 212° F.

Loss

Percent

3

25

28
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS

TABLE ^^.-—Distribution of amino acids in late

California Valencia oranges (434)

Amino acid

Albedo

Flavedo

Segment

membrane

Vesicles Juice

Alanine

7-Aminobutyric aci _ _

Arginine__ .__ _

+

+

+

+ -h

+

+

+

+

+

+

+

+

+

-h

+

+

-h

-f

Asparagine

Aspartic acid

Glutamic acid

Glutamine

Leucine

+

-f

+

+

+

+

+

+

+

Lysine _ _ +

+

+

+

+

+

+

+

+

]\iethionine i

+ +

+

-h

+

+

Phenylalanine

Proline

Serine

Threonine. _

+

+

+

+

+

Valine

Unknown 1 +

1 May be ornithine.

The distribution of free amino acids in the

albedo, flavedo, segment membrane, vesicles,

and juice of late California Valencia oranges

is shown in table 4. Only minor differences

were observed in the qualitative amino acid

content of the tissues.

All the known nitrogenous constituents,

other than the vitamins, that are found in various citrus juices are shown in table 5.

A protein fraction was isolated from the

edible portion of the orange as early as 1925,

and some of its general properties were briefly

described UOO). It was soluble in dilute (0.3

percent) alkali ; was insoluble in water, neutral

salts, and weak acids; was not coagulated by

heat in neutral or alkaline solution ; and had an

isoelectric point at pH 4.7. Proteins have also

been isolated from the edible portion of both

California Valencia and Washington Navel

oranges {391). The proteins from these two

varieties appeared to be identical in respect to

their total nitrogen, arginine, histidine, and

lysine content. More recent qualitative studies

have been conducted on the amino acids in the

TABLE 5.—Nitrogenous constituents in citrus juices

Orange

Tangerine

Grapefruit Lemon LimeDweet

tangor

Mandarin

LemonConstituent

I

1 Valencia

Washington

; Navel

Kara Kinno\^^

Hone}^

orange

hybrid

Alanine

7-Aminobutyric acid_

iVrginine

Asparagine

Asparatic acid

Betaine _

{351, 4S8)

{SOU 4S4, 4S8)

{184, 351, 434,

438)

{184, 351, 385,

434, 438)

{184, 203, 351,

438)

{184, P- 160)

{316, 426)

{203, 293)

{351, 434, 438)

{385, 438)

{72, 203, 293)

{184, 203, 438)

{351)

{351, 434)

{434)

{434)

{351, 434, 438)

{173)

{351, 434, 438)

{316; 184, V'

169)

{351, 438)

{351, 391)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

0

{406,

438)

{438)

{438)

{438)

0

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

[438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

{438)

Choline _ _ _

Cysteine {203,

293)

{438)

{438)

{203,

293)

{438)

{438)

{293)

{438)

{438)

{293)

{293)

{438)

{385),

0

{293)

{293)

{4^38)

0

{293)

Glutamic acid

Glutamine

Glutathione

{438)

{438)

{438)

{438)

{438)

{438)

{438)

0

{438)

0

Histidine

Leucine

Lysine

Methionine

Phenylalanine

Proline

Putrescine

Serine

Stachydrin

Threonine

Valine

{438) {438) {438) {438) {438) {438) {438) {438) {438) {438)

"{438) {438) {438)

{125)

{438) {438) {438) {438) {438) {438) {438)

^ Italic numbers in

present.

parentheses refer to literature citations; 0 indicates less than 5 mg. percent of the nitrogen constituent
AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

protein obtained from both the peel and the

edible portion of California Valencia oranges

{A3U). Both the peel and the chromatophore

protein fractions appeared to be qualitatively

identical in terms of the constituent amino

acids.

Enzymes

Enzymes are organic catalysts synthesized

by living cells. They are among the most important constituents of living tissue, since it is

through their action that the synthesis and

alteration of other constituents and the energj^

metabolism essential to life are performed.

Enzymes are considered protein in nature.

Many require complex organic compounds

called coenzymes, which are associated with the

protein component and are equally essential for

the activity of the enzyme. Thus, many of the

water-soluble vitamins are known to be constituents of coenzymes, as, for example, thiamine

(Bi), riboflavin (B.), and nicotinic acid. The

presence of these vitamins in living tissue suggests that related enzyme systems are also

present in the tissue.

Enzymes have a number of properties that

aid in their characterization. They are heat

labile, and heating to boiling (212° F.) will

inactivate most enzymes. Heat treatments of

158° to 176° for 2 to 5 minutes will bring

about the inactivation of many enzymes. Certain enzymes are activated or stimulated by the

presence of such cations as calcium or magnesium and by the presence of reversible oxidation-reduction systems, such as glutathione and

méthylène blue. Many enzymes are inactivated

or irreversibly precipitated by heavy metal

salts, such as lead and mercury. Most enzymes

are reversible catalysts ; that is, they can cause

a reaction to go in either direction under suitable conditions, and therefore are theoretically

capable of forming the same compounds they

degrade. Probably the most characteristic

property of enzymes is their specificity. One

type is group specificity, in which the enzyme

attacks only a certain chemical grouping, such

as a carboxyl (COOH) group or an ester bond,

without showing exclusive preference for the

compound in which the bond occurs. Many

enzymes exhibit absolute specificity, in which

they are capable of effecting a chemical reaction in only one specific compound.

Enzymes are generally classified into broad

groups according to the type of reaction they

catalyze, such as hydrolytic and oxidation-reduction; and they are subdivided within these

groups according to the type of molecule they

attack and the type of bond in the molecule

that is broken, such as ßsi^rases, carbohydrasesy

and decarboxylases.

Pectinesterase (pectin methylesterasej pectase) of citrus fruits has received more study

than any other citrus enzyme (219, 222, 261,

262, 266. 868, JÍ10, ill, il5). This enzyme

catalyzes the hydrolysis of the methyl ester

bond of the pectin molecule, forming pectic acid

and methanol, as indicated in the following

formula, where R represents the polygalacturonide chain of the pectin molecule :

0

/

R_O0-CH3 + H,0 Pectinesterase

^

Pectinesterase exhibits a maximum activity

at about pH 7.0 to 7.5 in 0.15 molar sodium

chloride, and demethylation of pectin in acid

orange juice is accelerated by the divalent

cations (262). Methods of pectinesterase assay

have been devised and modified by numerous

workers (221, 258, 262),These methods involve letting the enzyme act on pectin under

standard conditions of substrate, pH, and cation

concentration. The activity of the enzyme releases the esterified carboxyl groups in the pectin, and these are titrated with a standard base.

The measure of enzyme activity is the amount

of base used in maintaining a constant pH over

a unit period of time. It is expressed in pectinesterase units (PEu) on the basis of milhequivalents of ester bond hydrolyzed per min0

R-C-OH

Pectic

acid

CH3OH

Methanol

ute per unit of enzyme taken (milliliters or

grams of preparation). It may also be expressed in pectin methylesterase units (PMU),

which are calculated on the basis of milligrams

of methoxyl split off per unit of enzyme preparation in 30 minutes. The relation of PEu to

PMU is 1 to 960.

Pectinesterase has received a gi'eat deal of

attention in the processing of citrus products,

because it is believed to be one of the principal

causes of instability known as ''cloud loss'' and

"gelation'' in unheated citrus juices and frozen

concentrates. It is believed that when pectinesterase acts on pectic substances in orangejuice products, the resulting low-ester pectinic

acids and pectic acid form insoluble complexes

because of the acidity and cation content of the
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS

juice. If the original pectin content is relatively

low, the pectinic acids precipitate, carrying

with them other suspended colloidal materials,

which give orange juice its desirable turbid

appearance. Such juice results in a clear, supernatant serum underlaid by an unsightly precipitate. If the original pectin concentration is

somewhat higher, demethylation may result in

the formation of a semirigid gel throughout the

juice or concentrate.

Pectinesterase is found in all citrus fruits.

It occurs in greatest concentration in the juice

sacs and rag, with decreasing amounts in the

flavedo, albedo, and juice {205, 262, 366), Many

workers have studied methods of preventing or

inhibiting cloud loss and gelation in citrus juices

by heat inactivation of pectinesterase {219,

367,368,410, 411,415),

Protopectinase is the enzyme believed responsible for the transformation of protopectin

to soluble pectin in citrus and other fruits during the process of maturation and ripening.

The presence of this enzyme is based on inductive reasoning and the fact that protopectin

is altered in maturing fruit. The enzyme has

never been demonstrated in vitro, and there is

a possibility that it does not differ from pectin

polygalacturo'hase.

Pectic acid depolymerase may occur in some

citrus fruits. There is at least one report of a

pectolytic enzyme in grapefruit, which was believed to be a pectic depolymerase {338).

Acetylesterase and other esterases besides

pectinesterase (pectin methylesterase) have

been reported in citrus by various workers.

Acetylesterase is relatively nonspecific in that

it hydrolyzes methyl, formyl, propyl, and butyl

esters, as well as acetyl esters, and it may be

identical with a similar enzyme reported in

citrus that hydrolyzes methyl butyrate {205).

Acetylesterase is found in oranges, grapefruit,

and lemons; has a pH optimum between 5.5

and 6.5; and is found in all parts of the fruit,

with the amount decreasing from the peel toward the center of the fruit. In contrast to

pectinesterase the enzyme is little activated by

cations.

Phosphatase is a widespread esterase, which

rather nonspecifically hydrolyzes phosphate

esters of many naturally occurring organic

compounds, such as glycerophosphates, dipalmitylglycerophosphoric acid, ribonucleic acids,

and phosphorylated starches, such as amylopectin. This enzyme has been extensively

studied in citrus fruits {14, 15), and occurs in

the peel and also in solution in orange, grapefruit, and lemon juices. It is readily inactivated

by heating. For this reason it has been suggested that the determination of phosphatase

activity in citrus juices might be employed as

an mdex of pasteurization {15),

Proteiîiases are enzymes that hydrolyze protems to smaller peptides or to free amino acids

A protemase has been reported in citrus juices

especially lemon juice {271), However, very

little research has been done on the prote'inases

of citrus, although the fact that free amino

acids are found in citrus juices reflects their

presence. Peptidase, a related enzyme capable

of hydrolyzmg peptides (resulting from'protemase activity) to free amino acids, has also

been reported in citrus fruits {204).

Peroxidase is found in all citrus varieties and

m all parts of the fruit, but it is especially

active m the seedcoats of lemons {91).

Cytochrome oxidase, reported in citrus

{196), is involved in an enzyme system that is

probably responsible for the mai or part of

oxygen uptake by the orange.

Decarboxylases are enzymes common to all

living material, and nearly always are found to

be substrate-specific proteins in conjunction

with the compound cocarboxylase, which is a

complex compound of vitamin Bi (diphosphothiamine), or codecarboxylase, a complex compound of vitamin Eg. Decarboxylases are

known that catalyze the decarboxylation of pyruvic acid and many of the amino acids. Glutamic acid decarboxylase has been reported in

citrus fruits, and probably accounts for the

finding in citrus juices of y-aminobutyric acid,

which would result from the action of this enzyme on glutamic acid. Probably other decarboxylases are also present in citrus fruits.

Phosphoribiosomerase has been reported in

citrus.^ This enzyme catalyzes the equilibrium

between the aldose and ketose form of phosphoribose (a phosphorylated sugar).

Other enzymes are present in citrus fruits,

which are yet to be identified. As enzymes are

responsible for carrying on the chemical transformations essential for life, it is therefore

logical to assume that living material has as

wide an array of enzyme systems as it has constituents that are synthesized, broken down, or

metabohzed for energy.

Organic Acids

Citrus fruits are classed as acid fruits, because their soluble sohds are composed chiefly

of organic acids and sugars. The acidity of

citrus juices is due primarily to their content

of citric and malic acids {151, 311, 392-394)-

Trace amounts of tartaric, benzoic, and succinic

acids {41) y as well as oxalic {287) and formic

acids {142) y have also been reported. Citric

acid accounts for the largest portion of the

* Unpublished observations by B. Axelrod, 1946.
8 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

organic acids in citrus fruits, and may make

up as high as 60 percent of the total soluble

constituents of the edible part of the lemon.

The acidity of citrus juices is ordinarily determined by titrating a known volume of juice

with a standard solution of sodium hydroxide,

phenolphthalein being used as the indicator

(10, p. 390), and the results are expressed as

anhydrous citric acid. With most citrus juices

those substances that react with sodium hydroxide are mainly organic acids, so the titratable acidity of a juice will in general represent

its free organic acid content.

The total organic acid constituents of orange,

grapefruit, and lemon are essentially equal to

the sum of the citric and malic acids, as shown

in table 6.

TABLE 6.—Analysis of the organic acids of

California citrus fruits, expressed as the

average number of grams per 100 gm.

Fruit Citric (•id

Orange (Valencia) {SH2): ,

Immature ; 2. (U

Mature , .!)4

Grapefruit (o,94): , Immature 2.04

:Mature \ 2:^2

Lemon (Eureka) {SfiS):

Immature 4. 78

Mature ().S3

Malic ac id Total

0.17

.15

2.77

1.09

.31

.27

2.94

2.57

.34

.22

5.12

7.03

Although the titratable acidity of citrus juices

represents the total amount of free acids present, it provides no information regarding their

presence in other forms. For example, some of

the organic acids may be in the form of salts,

such as citrates, tartrates, malates, and lactates. Others may be in the form of esters. In

orange juice it was found that the amount of

salts of organic acids (''combined acids") was

remarkably uniform during growth of the

fruit, and any variations in acidity were due

chiefly to changes in the concentration of free

acids, particularly citric acid (392). A definite

relationship was found to exist between the

free acid- ''combined acid'' balance and the pH

of the juices.

In other studies on Valencia oranges it was

shown that the maximum amount of free acids

developed early in the fruit and changed very

little thereafter. However, the concentration

of free acids in the juice decreased considerably

during maturity, increasing fruit size diluting

the juice; whereas there was a corresponding

increase in the pH, owing chiefly to the decrease in concentration of citric acid (397).

Changes in the free acids during maturation

of grapefruit were similar to those of the

orange (39JÍ), However, the free acids in

lemons were found to increase and the pH to

decrease with fruit maturation (393).

The titratable acidity of citrus juices offers

no measure of their "active acidity,'' which depends on the ionic dissociation of acids in solution to yield free hydrogen ions (H+). The

"active acidity," or hydrogen-ion concentration,

is designated by the term pH, which mathematically is equivalent to the logarithm of the reciprocal of the hydrogen concentration in moles

per liter. In common practice the pH may be

determined by two methods—colorimetrically

with suitable indicators and electrometrically

with pH meters. The latter method is particularly useful for measuring the acidity of colored

citrus juices.

Certain solutions possess the ability to resist

a change in pH when a solution of different pH

is added to them. This condition is termed buffer action of a solution. Citrus juices are highly

buffered, as indicated by the fact that they show

relatively little change in pH when hydrogen

(H+) or hydroxyl (OH—) ions are added to

them (18, 396). Mixtures of organic acids and

their salts generally exhibit this behavior of

buffer action. Citric and malic acids, plus their

salts, form the main buffering system of citrus

juices, which have their maximum buffering

capacity on the acid side because of the high

ratio of free acid to "combined acids" (salts).

With such a buffer system citrus juices can be

diluted greatly with water while showing only

minor changes in pH. This is the reason the

pH of citrus juices changes only slightly during

the late stages of maturation and why juices of

varying titratable acidity may have identical

pH values.

The titratable acidity of oranges and grapefruit plays an important part in determining

the legal maturity of these fruits. Since the

color of citrus fruits is not a dependable guide

for ripeness, it is necessary to use some other

measure of maturity. The ratio of soluble solids

(as determined by the Brix hydrometer) to

acidity has been established as a convenient

index of maturity (52, p, 30). California requires a ratio of 8:1 for oranges and 7:1 for

grapefruit before these fruits can be picked and

shipped interstate (53, p. 279; 111). In Florida

fruit maturity is judged by the total soluble

solids, a sliding scale of ratios of soluble solids

to acid, and juice content, and must meet minimum color standards (118, pp, 21-33). In

Texas, the ratio requirements for grapefruit

are very similar to those in Florida and range

from 6.5:1 to 7.2:1 (i32,pp. 131-13^). In Caliiornia, lemons are purchased for processing on

the basis of their citric acid content as deter-
CHEMISTRY AND TECHNOLOGY OF C ,'flUS, CITRUS PRODUCTS, AND BYPRODUCTS

mined by the titratable acidity rather than on

a straight tonnage basis.

New analytical methods have been developed,

including partition chromatography, columnar

chromatography, and ion exchange, which

should prove useful for characterizing in

greater detail the organic acids and their various forms found in citrus juices {21^9, pp. llA131),

Flavonoids

Chemically, flavonoids are organic compounds possessing the carbon framework of a

flavone, or more broadly are Cr-Ca-Ce compounds. The latter definition includes flavonols.

ñavones, benzalcoumaranones, flavononols, flavanones, chalcones, anthocyanidins, dihydrochalcones, and catechins.

Flavonoids occur in the bark, twigs, leaves,

flowers, and fruits of a large number of higher

plants, and are frequently found combined with

carbohydrate molecules as glycosides. Flavonoid compounds may compose as much as 10

percent of the dry weight of citrus fruits. The

flavanone rhamnoglucosides hesperidin and

naringin and their corresponding aglycones

hesperetin and naringenin are the flavonoids

associated with commercial varieties of citrus

fruits (table 7).

Hesperidin was discovered in 1828 {2Jf8), It

may be observed readily with the naked eye as

an amorphous, white crystalline mass adhering

to the segment membranes of an orange after

exposure to freezing temperatures on the tree.

It may also be seen at times as a sludge in cans

of aged single-strength orange juice. Although

hesperidin is insoluble in water and is tasteless,

naringin (in grapefruit) is soluble in hot water

and has an extremely bitter taste. The aglycone

of naringin, naringenin, does not have a bitter

flavor.

The function of flavonoids in citrus and other

higher plants has not been established. It has

been suggested that glucose in combination

with flavonoid glycosides forms a soluble, easily

hydrolyzable compound, and is thus temporarily inactivated until it is brought to that portion

of the plant where the glucose is stored or

utihzed for metabolic processes HUl) It has

also been postulated that flavonoid compounds

are the precursors or intermediary metabolites

m the formation of a class of plant pigments

known as anthocyanidins {129). Suggestions

have been made that these compounds may act

as light filters to regulate photosynthesis, or

they may play an important part as components

oí oxidation-reduction systems {63, 388),

Flavonoids are of technological and economic

interest to the citrus-processing industry

{170). The flavor and general acceptability of

grapefruit juice are strongly affected by the

amounts of bitter principles, such as naringin,

present m the product. Since the peel and pulp

contain much greater amounts of the flavonoid

compounds than the juice, great care must be

exercised during juicing operations to minimize

extraction of flavonoids from these nonedible

portions of the fruit. In addition, the presence

of excessive amounts of hesperidin in orange

juice may at times result in the precipitation of

this flavonoid in processing lines, and necessitate periodic, costly shutdowns of plant operations. It has been suggested {llt,9) that flavonoids and other polyphenolic compounds may

be of importance in the darkening and development of off-flavors in lemon and lime juices

during storage at room temperatures.

Qualitative tests that have been employed for

the detection of flavonoids and related polyphenols include the ferric chloride test {152,

371 ), the cyanidin reaction {Jf6, 371 ), the borocitrate reaction {Í76), and the alkali test {92,

25U, 371 ), Specific methods for the quantitative

estimation of flavonoids in citrus fruits are not

available. Several procedures have been suggested for the determination of flavonoids

separated on filter-paper chromatograms {97,

126-128, 337), However, these procedures have

not been applied to the determination of flavonoids in citrus products.

In spite of their nonspecificity the ferric

chloride and the alkali reactions have been useful for estimation of total flavanones in citrus

products. The distribution of total flavanones

in mature California citrus fruits, calculated

either as hesperidin or naringin, is shown in

table 8. The rind, core, and segment membranes of citrus fruits contain from 75 to 90

percent of the total flavonoids. On a dry-weight

basis these nonedible portions of the fruit may

contain as much as 20 percent of flavonoid

compounds.

The flavonoids in grapefruit and oranges have

been studied more extensively than those in

other citrus varieties. The naringin content of

the rind of California grapefruit has been reported to decrease throughout the growing
TABLE 7.—Flavonoids and flavonoid glycosides in citrus

Common name of

flavonoid

Auranetin

Aurantamarin

Cilronetin

Citronin

Eriodictyol glucoside

Hesperetin.

Hesperidin.

Hesperidin chalcone_

Naringenin

Narin^in (isohesperidin)

Neoh('speridin_

Nobiletin

Ponciridin__

Poncirin

Ponkanetin.

Rhoifolin

Rutin

Tangeretin _

Tricin

Chemical name or formula

3,6,7,8,4'-Pentamethoxyflavone

C,,H7iOi,

5,7-Dihydroxy,4'methoxyiiavanone. _

Citronetin-7-rhamnoglueoside

5,7,3',4'-Tetjahvdroxyflavanone-glucoside.

5,7,4'-Trihydroxy,3'methox3'flavanone

Hesperetin-7-rhamnoglucoside

5,7,4'-Trihydroxyflavanone

Naringenin-7-rhamnoglucoside _

Hesperetin-7-glucorutinoside

5,6,7,8,3',4'-Hexamethoxyflavone.

5,7-Dihydroxy-4'-methoxyflavanone5,6,7,8,4'-Pentamethoxyflavone

5,7,4 '-Trihydroxyflavone-7-rhamnoglucoside.

3,5,7,3',4'-Pentahydroxyflavone-3-

rhamnoglucoside.

3,5,6,7,4'-Pentamethoxyflavone

5,7,4'-Trihydroxy-3',5'-dimethoxyflavone.

Source

Common name of

fruit

Botanical species and

variety

Myitle-leaf orange. _.

Bitter or sour orange.

Ponderosa, Wonder,

or American Wonder lemon.

Lemon

I Lemon

I Sweet orange

Citrus auranlium var. inyrtifolia Ker-Gawl.

C. au raniiuni Linn

('. limon (Linn.) Burm. f

]\Iyrtle-leaf orangeSweet orange

I Lemon

Citron

Sweet orange.

Lemon

Shaddock or pummelo

(Sour orange

J Shaddock or i)ummelo

I Grapefruit - -,

[Trifoliate orange

Trifoliate orange

I\hindarin orange.. _

Trifoliate orange

do . _ _ - -

Chinese honey orange

/Sour orange

(Trifoliate orange

Satsúmelo.

Tangerine

Dancy tangerine.

King mandarin__

C. li)t}on

(\ linwn

C. sinensis (Linn. ) Osbeck (C.

auranlium var. sinensis

Linn.).

(\ auranlium var. myrtifolia--

(\ sinensis

(\ kotokan Hayata

C. ¡i)}ion

(\ mediea Linn

('. sinensis

(\ tanka?i Hayata

C. limon

C. grandis (Linn.) Osbeck

C auranlium

('. grandis

C. paradisi Macf

Poneirus trifoliata (Linn.) Raf _

¡I\ trifoliata .

\ (\ fusea Lour

\(\ retieulata Blanco

\(\ tankan

P. trifoliata

P. trifoliata

(\ poonensis Hort, ex Tanaka_

r. auranlium

P. trifoliata

C. paradisi X C. nobilis Lour.

C. retieulata

(\ tangerina Hort, ex Tanaka.

C. nobilis

Part of fruit

or tree

Literature

citation

Fruit peeL

am

{370, 4^7, 468)

{284, 488)

_do.

Blossom petals.

Fruit

Leaves, fruit

Juice, leaves, twigs, bark.

Fruit

Fruit, flowers

Fruit juice, peelFruit, leaves

Unripe fruit

Fruit

FruitFruit, flowers.

Fruit

Ripe fruit

Leaves

Fruit rind

Fruit rind, oiL

{129. .

{468)

{468)

[88)

{339, 49Ô)

{157)

{191)

{260, 468)

{468)

{129, 178, 248)

{437)

{468)

{8, 226)

{158, 370, 468)

{8,9,129,468,475)

{16,475)

{155)

{226, 282, 427)

{191)

{239, 437)

{437)

{468)

{155, 156)

{149, 198)

{158)

{156)

{242)

(468)

{129, 313)

{129)

>

O

S

o

a

d

>

Ö

w

o

o

CO

00

CÄ

>

o

>

o

O

d

d
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS

TABLE 8.—Distribution of flavanones in citrus fruits (91)

II

Fruit

Grapefruit ^

Lemon 2

Orange 2

Tangerine 2.

Juice

(grams per 100 ml.

Fresh

centrifuged

0.0:^8

.040

.080

.065

Sediment Total

0 00:3

.008

.088'

0.041

. 054

.118

Tissue

(percent, wet hjisis)

Juice

sacs

0 140

.600

Section

meml)i-Mne

1 250

1 ,000

1 500

500

Albedo

2 100

8.000

1.600

n.700

Flavedo

0.600

2.500

1.000

Core

2.600

4.500

n.570

1 Calculated as naringin.

2 Calculated as hesperidin; eriodictyol glucoside may be present and would be included in the calculation as hesperidin.

3 Determination made on whole peel.

4 Fibrovascular and adhering tissue, as tangerines have no core.

season (153), The locality in which the grapefruit is grown has a marked effect on the rate

of change in naringin content during storage

of the fruit at low temperature.

In a study conducted in Texas during the

1948 harvest a marked change in the naringin

content of grapefruit occurred within 1 month

(282). Decreases in naringin content of from

45 to 66 percent were observed in the juices and

various tissues. Juice samples containing

more than 0.07 percent of naringin had an

immature, bitter taste, whereas samples containing less than 0.05 percent had a superior

flavor. No change in naringin content was observed during commercial processing.

The naringin content of the peel and pulp of

California grapefruit was found to be considerably higher than that of Florida grapefruit. The values reported were 0.90 and 0.40

percent in the peel and 0.15 and 0.10 percent

in the pulp of the California and Florida fruit,

respectively (225).

A comprehensive study of the naringin content of the shaddock and 5 Florida grapefruit

varieties has been reported (226), The quantity of naringin in these fruits was found to

remain constant once the fruit had grown to

2 inches in equatorial diameter. Thereafter, as

the fruit grew larger, the naringin content on

a percentage basis became lower. The narmgm

content varied from 6.2 to 1.3 gm. per fruit.

On a percent-by-weight basis Ruby Red grapefruit contained the highest percentage of naringin Almost 90 percent of the narmgm was

found in the albedo, rag, and pulp. The naringin content of the juice decreased or increased at the expense of the pulp and rag the

albedo and flavedo accounting for a uniiorm

percentage of the total. The percentage by volume of naringin in the juice, which could not be

satisfactorily correlated with maturity, varied

between 0.02 and 0.03 percent throughout the

season.

A similar study by the same workers (170)

on the estimation of the total flavanones, calculated as hesperidin, in four varieties of Florida

oranges gave essentially the same results as

their grapefruit studies, except that the total

flavonoids in the nonedible portion accounted

for only 75 to 80 percent of the total and the

hesperidin content of the juice varied between

0.015 to 0.025 percent.

A value of 0.046 percent of hesperidin has

been reported for California single-strength

filtered lemon juice {260).

Bitter Principles

Washington Navel oranges as grown in California are seldom used for the manufacture of

canned juices or concentrates, because when the

extracted juice is heated or allowed to stand,

a bitter or astringent flavor usually develops,

rendering it unpalatable. The intensity of this

bitterness is dependent on the maturity of the

fruit, being most pronounced in juices from

early-season fruit, and it may vary from season

to season. Bitterness may also be found m the

juice of immature Valencia oranges, but it always disappears before this variety reaches

commercial maturity.

The exact chemical nature of the bitter principles and the mechanism of their formation

are still obscure in spite of the large amount of

work carried on by a number of workers {59-

61, 107-110, ISO, 218, 2U0, 21,1, 377). It is believed that unknown precursors are present m

the albedo, center core, and segment membranes of the fruit in a nonbitter, water-soluble

form. When these tissues are ruptured during

the extraction process, nonbitter precursors are

dissolved into the acid juice, where they are

converted by some means yet unknown to the

bitter compounds limonin and/or isolimonm.

As little as 1 part of limonin in 100,000 parts of

water will give a very bitter taste. Limomn,
12 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

not to be confused with the terpene of peel oil,

limonene, and isolimonin have been isolated and

partially characterized, and some of their chemical and physical properties determined.

Limonin has been isolated as a white crystalline substance having a melting point of 555*^

to 557^ F. and with the empirical formula

C^-yHaoOs. Limonin and or isolimonin are soluble in alcohol, acetone, and benzene, but relatively insoluble in petroleum ether and extremely insoluble in water. Limonin forms tasteless

salts with alkaline earth metals, but below pH

6 or 7 the salts revert to the bitter lactone form

of the compound. The water-soluble precursor

might be either a giycoside, which hydrolyzes

in the presence of air and juice acids to release

the bitter compound, or a tasteless, water-soluble free acid, which in the presence of air and

juice acids lactonizes to form the bitter compound {176, 177).

Further work on the chemistry of the bitter

principles in Washington Navel oranges has indicated that limonin and or isolimonin of the

juice may be identical. A related compound,

nomilin, found only in the seeds of oranges and

lemons, has also been reported (107-110). Nomilin has the empirical formula CsH^O,,, and

both limonin and nomilin are believed to contain 2 lactone rings. It has been proposed that

limonin occurs in situ as a water-soluble free

acid which, when extracted into Washington

Navel juice, forms the bitter di-lactone limonin.

In addition to the 2 lactone rings, degradation

studies indicate that limonin contains in its

chemical structure 1 carbonyl group and 3 cyclic ether rings. It is believed that the bitterness of these compounds, particularly limonin,

is associated primarily with one of the lactone

rings. The addition of a base to solutions of

limonin causes the opening of the lactone ring

and the formation of a nonbitter salt.

Another bitter principle has been reported

that is believed to occur naturally in the peel as

well as in the seeds of Navel oranges grown in

Austraha (61). This compound has been

named limonexic acid, and has the empirical

formula CosH.oOxo. Like the other bitter compounds, limonexic acid possesses at least one

lactone'ring, which opens on treatment with

a base to form a nonbitter salt. The finding of

this new bitter principle may explain some of

the conflicting results obtained by workers who

have attempted to debitter Washington Navel

orange juice.

Numerous attempts have been made to prevent the development of or to remove bitterness

from Washington Navel orange juice in order

to make processing feasible from season to season. In one of the first methods the combination

of a special type of burr was used for reaming

the juice to prevent excessive extraction of rag

and pulp, in conjunction with a process of increasing the hydrogen-ion concentration of the

juice to pH 3.8-4.0 by the addition of carbonate

salts to minimize formation of the bitter lactone (177, 179). The difficulty in this method

lies in the fact that it is not easy to prevent

complete bitterness formation without increasing the pH beyond the point where juice

flavor is affected. Another attempt involved

the uä;3 of pectic enzymes. The objective was

to use their action on natural pectic substances

in the juice and thus coagulate the dispersed

colloids that precipitate and, incidentally, to

remove the limonin in suspension (256).

In a third method (256) activated carbons

completely removed the bitter principle or

principles and their precursors. However, the

method presents technical difficulties, the chief

of which are the tendency of carbon ''fines'' to

be suspended in the juice and a partial loss of

vitamin C.

None of these methods are considered successful or feasible for commercial application

at the present time.

Besides seasonal variations in bitterness and

variation of bitterness with maturity, it has

been shown that for a given season or at a

given stage of maturity bitterness varies greatly with the type of rootstock on which fruit is

grown (272). Thus, Washington Navel oranges

grown on rough lemon rootstock never lose the

bitter principles with maturity, whereas Washington Navels grown on grapefruit rootstock

always lose the bitter principles upon reaching

commercial maturity. Washington Navels

grown on other rootstock are variable in the

occurrence and intensity of the bitter principles and the point of maturity when bitterness disappears, as illustrated below.

Time of disappearance

Rootstock of— of bitterness

Grapefruit Upon commercial maturation; 8:1

ratio.

Trifoliate orange___Few weeks after commercial maturation.

Sweet orange Do.

Sour orange Late in the season.

Washington Do.

Navel cutting.

Rough lemon Never.

It is seen that in studying loss of bitterness in Washington Navel oranges with maturation it is necessary to know the type of rootstock on which the fruit is grown. It is also

seen to be theoretically possible to raise Wash

mgton Navel fruit that will not be bitter after

maturation by proper rootstock selection or by

adjusting the processing season to the type of

rootstock from which the fruit is obtained.
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 13

In California most Washin^on Navel

oranges are grown on rootstocks that cannot

be relied on to yield nonbitter juice when the

fruit is commercially mature. It is also impracticable to segregate the fruit for processing

according to the rootstock from which it is

picked. Therefore, it is seldom safe to process

Washington Navel oranges for juice, and in

''bitter" years the entire crop must be sold as

fresh fruit or processed for byproducts.

If it should become more desirable or necessary to process Washington Navel orange juice,

the most satisfactory solution of the problem

of bitterness appears to lie in a gradual changeover to nonbitter rootstocks.

Volatile Flavoring Constituents

The volatile constituents of citrus juice are

defined as those materials that can be removed

from the juice by distillation methods. They

will include a large amount of water, volatile

oils that are steam distilled along with the

water, and other constituents that boil at temperatures lower than water or are removed as

azeotropic mixtures with water.

If the volatile constituents are removed from

the juice by distillation at atmospheric pressures, their composition is considerably altered

in the process. If they are removed by distillation at reduced pressures and correspondingly

lower temperatures, the recovered fraction resembles more closely the aroma of the original

juice; the lower the temperatures of distillation, the more closely the volatiles resemble

the original aroma. Some of the volatile constituents are gaseous in their nomial state, and

elaborate methods must be employed to trap

them.

A citrus juice from which a portion of the

water has been removed by distillation shows a

definite change in ñavor, and it was early recognized that the aroma of orange juice was

associated with the volatile fraction {lí2).

The flavor of a concentrated orange juice that

has been restored to its original consistency

by the addition of pure water tastes insipid and

is lacking in orange character {35If). It is now

generally recognized that the flavor and aroma

of fresh orange, as well as certain off-flavors

of the processed juice, are associated with the

volatile fractions {36, UO, 171). Grapefruit

juice from which a portion of the water has

been removed tastes only sweet and sour {23i).

Examination of other citrus juices for the relationship between volatile materials and flavor

and aroma has not been reported, but it can be

expected that they will show the same relationship as has been reported for orange and

grapefruit juices. No systematic work has

been recorded ni the literature as to what fraction of water must be removed from the juice

to remove all of the volatile flavor.

The isolated volatile fraction consists of constituents that are soluble and those that are

insoluble in water. Whereas some of the aroma

resides in the water-soluble fraction {1U2, 23If,

301), most of the flavor and aroma of citrus

juices are found in the volatile, sparingly soluble oil fractions {233, 23If, 301, 35If), Some

methods of commercial extraction of the juice

from the fruit result in the incorporation of

some peel oil into the juice. However, the juice

itself contains some oil present in the juice

sacs. Experiments with carefully peeled and

washed fruit, followed by analysis of the juice

for oil content, showed appreciable amounts

of oil {36, UO, 301, 35If), For oranges, the

juice-sac oil has been estimated at 0.0006 to

0.006 percent {301, 35If), Globules of oil have

been demonstrated in the juice sacs of various

citrus fruits {90), It has also been claimed

that these globules do not contain oil, only

starch and protein, but no test had been made

for the presence of essential oils {231 ).

Methods for the determination of amounts of

volatile oils in citrus juices have been published

{19, If7, 383), and a review of various types of

apparatus used for this purpose is available

{80), The official method of the United States

Department of Agriculture is given in the

Standards for Grades of Frozen Concentrated

Orange Juice {Ul). The amounts of volatile

oil present in commercially extracted gi^apefruit and orange juices have been reported to

vary from 0.008 to 0.014 percent and 0,016 to

0.075 percent, respectively {301, 35If, 383),

Analyses of the composition of the different

volatile constituents in citrus juices are difficult to attain. Not only are these materials

present in very small amounts, as previously

indicated, but they consist of complex mixtures of closely related substances sensitive to

heat and light.

The gaseous substances in variously extracted orange and grapefruit juices have also

been investigated {34^7). The juices contained

from 22.3 to 41.7 ml. of carbon dioxide, 2.22

to 4.02 ml. of oxygen, and 9.7 to 13.9 ml. of

nitrogen per liter of juice. The ratio of oxygen

to nitrogen in these samples is significantly different from that of air.

For the most part the analysis of the other

volatile constituents of citrus juices is incomplete, qualitative in nature, and sometimes

based on inconclusive evidence. The acetaldehyde content of fresh lemon pulp has been

found to vary between 0 and 1.3 mg. per 100

gm. of pulp {289), Isoamyl alcohol has been

identified in lemon juice by a ferric alum test.
14 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

and isovaleric acid by odor (123). Orange juice

has been more extensively investigated than

other citrus juices, particularly as regards

changes in the composition of the volatile fractions on heating and storage. One of the earliest researches in this connection was that of

Hall and Wilson (i^^). Unfortunately in obtaining the volatile material the whole fruit

was crushed, and the oil layers from successive

distillations were systematically discarded until

the volume of distillate had been reduced to 0,1

percent of the original. The oil that was present in the juice sacs and that contributed

greatly to the flavor was thus discarded with

the peel oil. The following constituents were

reported to be present: Acetone, acetaldehyde,

ethyl alcohol, formic acid, citronellal, esters of

capryllic, acetic, and formic acids, isoamyl alcohol, phenylethyl alcohol, possibly geraniol,

possibly terpineol, and a C^oHigO alcohol similar to linalool.

More recently it has been found that a lowtemperature distillation or a petroleum ether

extraction removed all the odor and flavor

from canned orange juice {171). In this work

a precipitate of carbonyl compounds was obtained from the volatiles with 2,4-dinitrophenylhydrazine, and no change in odor was

detected, although the odor was destroyed by

either potassium permanganate or bromine. No

change in odor was noticed when the mixture

was distilled from an alkaline solution, indicating that organic acids were not concerned

in the odor. Unsaturated hydrocarbons and

alcohols were concluded to be responsible for

the off-odors and off-flavors associated with

canned orange juice.

In further work with canned orange juice

and Hmonene in citrate-buffered model systems,

the infrared spectra of a number of variously

stored samples have been determined {36),

From these data and theoretical considerations

of the reaction of limonene in an acidic aqueous

medium, it was concluded that 1,4-cineol, terpineol, and terpinolene were formed and esters

were hydrolyzed by heat and storage conditions; thus off-flavors were produced.

Hydrogen sulfide has been reported in green

citrus fruits and juices {21, 235), Furfural

has been detected in orange juice stored in the

presence of oxygen {307), and the acetaldehyde and alcohol content of orange juice as

affected by the season has been determined

{Í36).

A comprehensive study has been made on

the volatile constituents in grapefruit juice

{233, 23i). Newly developed Chromatographie

techniques were used, which minimized any

chemical changes during analysis. Table 9

gives the results of this work.

TABLE 9.—Volatile constituents of grapefruit

juice, expressed in milligrams per kilogram of juice

WATER-SOLUBLE CONSTITUENTS

1 Stored 4

Freshly years at

Constituent Fresh Ciinned room

i jUK-e juiee temperature

Acetaldehvde 1 .45 i 0.33

Acetic acid 0 ¡ 1.1)

Acetone 0 0

Acid A (CeHöO^) 0 ' 4.8

Acid B (CeH.Oo) ! 0 i 1.9

AcidC I 0 i (1)

Ethyl alcohol 400.0 - 400.0

Furfural 0 ' (ij

Hydrogen sulfide ( M 0

Methvl alcohol ! .2 ' .2

0.6

23.3

.1

2.9

1.6

0

460.0

8.2

{')

23.0

WATER-INSOLl^BLE CONSTITUENTS

Total oil

Limonene

a-Pinene

a-Caryophyllene

/i-Caryophyllene

C15H24

C15H28

Low-boiling hydrocarbons

N-Methyl methyl anthranilate.

C13H15N

Caryophyllene oxide

Citral._/_

Carvone

CisHoeO

Linalool

CuHo.O

Carveol

a-Terpineol

3-Hexen-l-ol

Geraniol

Linalool monoxide

CloH,o02

Polyox3'genated compounds

Oxides

20.92 ' 26.00 27.59

15 71 1 17 70 11.17

(M ! {') (')

.10 1 .10 .12

1.40 1 40 .87

.11 ' .11 .14

(M ! (M 0

(') ! (M {')

(M 1 {') 0

(1) (') 0

.80 1 .80 .27

2.1 ; 2.1 2.1

2.1 2.1 2.1

19 1

.23 .54

.16 .23 .08

45 ; .42 .64

30 1 .30 .27

.03 ' .88 2.02

.12 0 0

2.05 1 2.05 0

.37 1 2 03 8.95

.20 20 .20

.40 .86 .97

.32 .32 .32

1 Trace.

2 Approximate.

In general, the volatile material consists of

water-soluble compounds, which would not be

expected to contribute to flavor ; terpene hydrocarbons, which have a turpentine-like odor;

and nonhydrocarbons, mostly terpene derivatives, which produce the characteristic odor

and flavor. The objectionable character of

canned and stored grapefruit juice arises from

the decomposition of the terpene hydrocarbon

fraction to a-terpineol and linalool monoxide,

aided perhaps by the formation of methanol,

acetic acid, and furfural from sources within

the juice other than the terpene hydrocarbons.
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS

Pigments

The color of citrus fruits is due to a class

of compounds called pigments, which are located in various portions of the fruit, but generally are found in the cells of the outer peel

(flavedo) and in the juice sacs. These pigments

are not uniformly distributed throughout the

cells, but are concentrated in minute structures

called plastids, or chromatophores. The size

and shape of these plastids appear to be characteristic of the species from which they are

obtained (277). Not all the coloring matter is

in the plastids. For example, the characteristic

color of the peel of mature limes is found in

the cell sap (i^P), and the pigments of orange

and tangerine peel are in the cell walls (i89,

IfOO). In some grapefruit and in the blood

orange the color has been reported to be in the

cell sap.

The pigments of Foster and Marsh pink

grapefruit are lycopene and /3-carotene {280),

the latter predominating in the Marsh pink

grapefruit. This explains why the Foster is

a true pink and the Marsh a salmon pink. Lycopene is also present in the Indian Red pummelo (279), Anthocyanin occurs in the juice

sacs of the blood orange (278).

The color of green fruit is due to the presence

of chlorophylls a and b. As they decrease carotenoids increase and continue to increase after

the chlorophylls have disappeared (291), Although all the pigments in citrus fruits have

not been identiñed, the following have been reported in the peel and juice of oranges: Citroxanthin (215) ; a-, ß-, and ¿-carotene (83, 275,

808) ; cryptoxanthin (83, 291, 1^93) ; zeaxanthin (83, 308, 1^93) ; lutein (83, 308, Í93) ;

violaxanthin (4-93) ; citraurin (493, 494) ; phytofluene (83, 308) ; hydroxy-«-carotene (tentatively identified) (83) ; flavoxanthin (tentatively identified) (308) ; an unidentified acidic

red pigment (423) ; and phytoene (349), The

color of lemons is due principally to two groups

of pigments, the chlorophylls and carotenoids

(290), and limes owe their color to carotenoids

and phlobatannin (149, 290). Tangeretin

(313), ^-carotene, lutein, violaxanthin, and

cryptoxanthin (489) have been reported in

the tangerine, and /^-carotene (491 ) and cryptoxanthin (492) in the mandarin.

Methods for the isolation and analysis of

pigments present in the various parts of citrus

fruits are long and complicated. Usually they

consist of drying ox filtration; extraction with

a suitable solvent; removal of the solvent;

destruction or removal of extraneous material ;

saponification of the residue and separation of

of the pigments by solubility, chromatography,

counter-current extraction, or combinations of

these; and finally, measurement in a colorimetric apparatus. A number of simple and

complex methods have been described in th^e

literature for the isolation and identification of

chlorophyll and carotenoid pigments in citrus

products (83, 330, 429, 490-494),

The amounts of pigments found in citrus

fruits vary with the species, stage of maturity,

seasonal variations, and region in which they

are grown (5). In an analysis of 164 samples

of Florida and California orange juices, the

carotene content varied from 0.32 to 1.65 mg.

per liter of juice (429),

Whole fresh oranges contain around 18 mg.

of carotenoids per kilogram (493), although

amounts several times this figure have been

found; and various quantities have been reported to be present in the tangerine (349, 489,

492),

Carotenoid pigments are important from the

standpoint of nutrition, because some have

vitamin activity.

Sugars

The sweetness of citrus fruits is due to the

presence of glucose, fructose, and sucrose. Total

sugars may vary from less than 1 percent in

certain hmes to nearly 15 percent in some

oranges. Besides the wide characteristic differences among the various kinds of citrus fruits,

there are a host of other factors, such as variety, climate, rootstock, available plant foods,

and maturity, that influence the sugar content.

In addition, location of the fruit on the tree influences sugar content, those receiving the most

sunshine having the highest sugar concentration. In general, juice from the stylar half

of the fruit is sweeter than that from the stem

half, and neighboring segments within a single

fruit have been reported as differing as much

as 2.7 percent in soluble solids.

Other factors remaining constant, probably

the most important variable governing sugar

content is maturity, especially with the sweeter kinds of citrus. In these the acid content

declines slightly during maturation, but the

sugar content increases (144, 146, 148).^ This

fact is used in judging maturity, according to

State and Federal laws and regulations. In

the sweeter fruit juices, such as those from

oranges, tangerines, and grapefruit, the soluble

sohds consist mainly of sugars, but in lemon

juice and especially in lime juice the soluble

solids are chiefly citric acid.

Citrus fruits contain both reducing (fructose

and glucose) and nonreducing (sucrose) sugars.

In oranges at maturity these types of sugars

are present in about equal amounts, but in the

less sweet fruits, such as limes and lemons, reducing sugars predominate. In mature tangerines the reverse is true, the nonreducing sugars
16 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

exceeding the reducing sugars in amount

(362), Table 10 gives the sugar content for

several kinds of citrus juices.

TABLE 10.—Approximate sugar content of

some citrus juices

Fruit varietv Locality

grown

Reducing;

sugars

Nonrcducing

sugars

Grapefruit:

Duncan {L'iJi).

Marsh (744)---

Lemon :

Eureka {^20)

Meyer {407)K..

Villafranca

(407).!

Lime:

Kev (407)1

Tahiti (46>7)i__

Orange :

Hamlin {HS).

Homosassa

{US).

Parson Brown

{imPineapple (i4cÇ)

Valencia {HS)-

Washington

Navel {SifO).

Floi-itla _

._do_._.

Percelit ' Percent

California

Florida _ _

.do,

_do.

.do.,

.do..

ao

._do_..--

California

3.0-5.1

2, 3-4 .8

.78-2.(:

3.22

1.29

- ! . 64

, 1.29

2.()-3.0

I 2.8-3.5

! 3.4-4.8

I

I 2.5 3.9

.! 3.2-5.0

. 4.3-5.8

2 4-4.0

2.6-3.1

.03-. 03

.48

.05

.12

.10

2.0-3.9

1.7-2.7

2.6-4.4

1.5-3.1

2.3-5.2

Total

sugars

Percent

5.0-8.3

5.1-7.8

.81-3.2

3.70

1.34

.76

1.39

4.6-6.7

1.5-6.0

3.9-6.4

5.4-10.3

7.3-10.5

1 Recalculated on a juice l)asis from the original data.

The occurrence of these different sugars is

based on Chromatographie, chemical (47^),

and polarimetric {86) evidence, at least as far

as oranges, grapefruit, and lemons are concerned. There are data U73) to shovv^ that the

edible portion of the lemon contains 1.40 percent of glucose, 1.35 percent of fructose, and

0.41 percent of sucrose; whereas the juice contains 0.52 percent of glucose, 0.92 percent of

fructose, and 0.18 percent of sucrose.

The proportions of sucrose, fructose, and

glucose have been determined in a few samples

of orange juice. These results are summarized

in table 11.

The fact that glucose and fructose are present in about the same amounts suggests that

sucrose has been inverted. However, it is not

TABLE 11.—Sucrose, fructose, and glucose contents of Valencia orange juice, based on

the total sugars present

Origin Year Sucrose Fructose Glucose

California (i?öO)----

Florida {86)

/1946

U947

1944

Percent

49.4

59.5

50.5

Percent

25.3

18.0

25.8

Percent

25.3

22.5

23.7

known whether this relationship holds for

other citrus fruits or, indeed, for all orange^.

When orange juice and presumably juice

from other citrus fruits are pasteurized and

canned, inversion of sucrose occurs in storage.

In one such case {Jf59) it was found that in 9

months there was an 81-percent loss in sucrose,

with a corresponding increase of 69.8 percent

in reducing sugars.

Sugars also occur in the albedo, or white

portion, and in the flavedo, or outer colored

portion, of the peel, as well as in the rag. On

a fresh-weight basis Poore (335) reported 8.68

percent of sugars in California grapefruit peel

and rag combined. In Florida grapefruit peel

he found 6.35 percent of sugars and in the

rag 6.30 percent. These sugars are free, and

these values do not include the higher carbohydrates present, such as cellulose, starch,

hemicellulose, pentosans, and pectin, which

give sugars and sugar derivatives on hydrolysis. Haas and Klotz (137) have shown that

the concentration of sugars is higher at the

stem end of the fruit than at the stylar end—

just the reverse of the sugar-concentration

gradient in the juice. Table 12 gives the sugar

content of California grapefruit, lemon, and

orange rinds.

TABLE 12.—Sugars in the rinds of California

grapefruit (153), lemons (20), and oranges (154)^

Fruit

Reducing

sugars,

expressed

as glucose

Sucrose,

expressed

as invert

sugar

Total

sugars

Marsh grapefruit._.

Lemon (mature)

Valencia orange

Washington Navel

orange

Percent

14.2-26.2

20.0

15.4-27.9

Percent

7.6-16.3

21.6

6.1-13.4

30,4-34.9 i 4.8-6.6

Percent

21.8-42.5

21.6

21.5-41.3

35.2-41.5

1 Data are condensed and are expressed on a dry basis.

2 Nonreducing sugars.

References are given in the literature to the

occurrence in citrus peel of pentoses, which

are 5-carbon sugars, whereas glucose and fructose have 6 carbons. Usually these references

are to substances that can give pentoses on

hydrolysis, or hydrolysis and decarboxylation,

rather than the free, preformed pentoses.

Thus, any pentosans present could be hydrolyzed to pentoses, or a pentose could readily

be derived from pectin by hydrolysis to galacturonic acid with subsequent decarboxylation.

Glycosides, such as naringin in grapefruit

and hesperidin in oranges, are constituents of

juice, peel, and rag and give sugars on hydrolysis.
CJHEJVllSTl ÍOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS |7

Lipids

This term ordinarily means those substances

that are insoluble in water but are soluble in

fat solvents, such as diethyl ether, petroleum

ether, chloroform, and benzene. They are not

volatile in steam, which is one distinction from

essential oils, such as citrus-peel oils. Essential

oils have quite different chemical structures

from lipids, and are usually strongly odoriferous.

There are several classes of lipids, and they

are usually grouped under three general headings :

(1) Simple lipids, including the neutral fats,

which are esters of glycerol (an alcohol) and

fatty acids. Citrus-seed oil is an example.

(2) Compound hpids, which include compounds of fatty acids with an alcohol (sometimes glycerol), but containing other groups in

addition to the alcohol. In this class are phosphatides, such as lecithins and cephalins, both

constituents of orange-juice lipids.

(3) Derived lipids, which may be listed

under several subheadings. Among these are

free fatty acids, alcohols (straight-chain and

sterols), and hydrocarbons, including certain

of the carotenoids. Examples of all of these

subclasses are found in citrus-juice, peel, and

pulp lipids.

Citrus-seed oils contain, like most vegetableseed oils, a small amount (less than 1 percent)

of unsaponifiable matter. In addition, they contain a bitter principle, probably limonin which

can be removed by alkali refining, and the closely related bitter substance nomilin (107)

which somewhat limits the usefulness of the

crude oil in food products.

Physical and chemical constants for a few

citrus-seed oils are given in table 13. It should

be understood that these values are based on

single samples of oil and therefore are not

necessarily typical. These data indicate that

citrus-seed oils are of the semidrying type and

that they are comparable to cottonseed and

many other seed oils in constitution.

In addition to these seed oils, which are

simple lipids, there are deposits of lipids in

other parts of the fruits. These are mixtures

of compound and derived lipids, and they occur

as certain cell inclusions, such as plastids, and

possibly in association with the intercellular

membranes. Thus when juice is extracted,

these compound and derived lipids occur in

suspension and are associated with cellular

debris. For this reason direct extraction of

lipids with solvents is impracticable because

of troublesome emulsions and the collection of

suspended matter at the interface.

A more effective method of lipid extraction

for the purpose of study is to separate the suspended matter by centrifugation or, better yet,

filtration with a filter aid, followed by subsequent extraction of this material with acetone.

TABLE 13.—Physical and chemical values for citrus-seed oils

Florida grapefruit California

orange

hydraulicpress oil

Florida

tangerine

solventextrac'^ed

oil i4e£)

i

A\>st

Indian

lime oil

(103)

Lemon oil

Item

Expeller

oil {W£)

Expeller

oil (SIS)

(usual range)

(104)

Specific gravity (25^ C.)

Refractive index (no, 25° C. )

Acetyl value

0.9170

1.4700

7.7

2.5

194.1

2106'3

0.9197

1.4698

2.4

.95

193.0

.48

2100.9

0.9153

1.4686

0.9165

1.4702

8.2

4.31

193.55

.54

3107.3

66.37

1.1

2.9

"ir4676"

0.916-0.919

11.463-1.466

Acid value

Saponification value

Unsaponifiable matter (percent)

Iodine value

Thinnviinntrpn vnlnp

197.5

.95

3101.7

195.2

.6

111.1

188.0-196.0

.4-8

103.0-109.0

Constituent fatty acids (percent j;-^ !

Arachidic

TTA^rlrnY-v

.9 .5

Lignoceric .1

51.0 36.5

.6

46.6

2.1

39.3

13.1

.3

11.1

26.1

9.6

Linolenic

IVTvrisfir'

Oleic

Palmitic '

Stearic

20.5

20.1

7.6

36.6

20.7

4.7

22.5

19.6

5.2

JAt 40°C.

" Hanus method.

^ Wijs method. , . , n , . ^ ^ -j j^ ^^A 'The fatty acid data for grapefruit are expressed as percent acid glycerides of total glycerides, tor oranges and

limes as percent acids of total mixed acids, and for tangerines as percent methyl esters of acids of total methyl esters. Numerically, the expression for tangerine oil is practically equivalent to the percent acid glycerides oí total

glycerides.
AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

Evaporation of this solvent and extraction of

the residue with petroleum ether serve to eliminate water extracted by the acetone. When

the petroleum ether is evaporated under

vacuum, the lipids are obtained as a viscous

liquid, which in oranges is deep red in color

but which would undoubtedly differ in hue and

intensity when extracted from other citrus

fruits. This method of lipid recovery has been

used as the basis of an analytical method of

lipid determination {Jf21), In amount the yield

from juices usually falls within the range of

0.05 to 0.1 percent, depending somewhat on the

type of juice extractor employed and of course

on the particular fruit sample.

None of the citrus-juice lipids have been

studied in much detail except those from

oranges. Even in this case little is known about

the so-called resin acids, one of the constituents

found in the lipid fraction. Information about

the unsaponifiable fraction is far from complete although some work on the carotenoid

pigments has been reported {88). Table 14

gives the probable composition of two samples

of orange-juice lipids as calculated from analytical data {193, Í26). It must be borne in

mind that these results are not necessarily

typical. Not enough samples of citrus lipids

have been analyzed to show what variations in

composition may be expected.

TABLE 14.—Composition of lijnds from Florida

Valencia orange juice

Constituent

Unsaponifiable matter

Resin acids

Fatty aciils, free

Fatty acids, combined, exclusive of

those in phosphatides

Sterol glycoside

Volatile in steam

Lecithin

Cephalin

Total

1947

Percent

14 81

12 41

21 GO

13 60

1 48

17

15 30

18.00

96 77

1950

Percent

16.53

10.31

34.30

3.42

4.93

'l5"25'"

12.65

97.39

The approximate total fatty acid composition, as methyl esters, of Florida Valencia

orange juice lipids, based on one study {Jf23),

is as follows :

Constituent Percent

Conjugated diene 1.3

Conjugated tetraene .1

Conjugated triene .2

Linoleate 32.5

Linolenate 8.8

Myristate -5

Oléate 18.1

Palmitate 18.8

Palmitoleate 10.4

Stéarate l*'''

Unidentified, molecular weight 344 1.4

Not determined 6.2

Both the rind {276) and pulp {281) of

oranges contain lipids. In the former were

found oleic, linoleic, linolenic, palmitic, and

stearic acids and glycerol, phytosterolin, and

ceryl alcohol, along with small amounts of a

resin and coloring matter. The pulp contained

oleic, linoleic, linolenic, palmitic, and stearic

acids and probably cerotic acid, along with

pentacosane, glycerol, phytosterol, and phytosterohn. Later work {i25) identified this phytosterolin as very likely being y^-sitosteryl-dglucoside. The lipid composition of the pulp

and rind appears to be rather similar, except

that ceryl alcohol is present in the rind but not

in the pulp, and pentacosane is present in the

pulp but not in the rind.

Some work has been done on the behavior of

lipids in the juice during pasteurization {193)

and storage {194^, 195), Pasteurization apparently causes a slight decrease in the amount

of unsaponifiable matter in the lipids. Storage

of canned pasteurized orange juice at room

temperatures, however, causes a progressive

hydrolysis of the phosphatides with the liberation of the phosphorus- and nitrogen-containing

portions of these substances. These fragments

are water soluble and are lost to the lipids.

Storage changes in the fatty acids are minor,

but lipids from canned juice after long storage

show increased peroxide values and other

evidences of rancidity. This discovery led to the

belief that these rancidity changes in the lipid

fraction might be factors in flavor deterioration of canned citrus juices {171, 317), but it

has since been shown that other constituents

are probably more important {36, U2U).

Pectic Substances

These substances are among the most abundant and universally distributed constituents in

the plant kingdom. They are found principally

in the middle lamellae of plant tissues between

adjoining cell walls, where they are believed

to function as a ''cementing'' material, binding

cells together {222).

The complexity of pectic substances, their

fundamental importance in the metabolism of

plants, and their usefulness to man (viz, jelly

manufacture) have stimulated a great volume

of research, resulting in the publication of over

2,000 papers and several hundred patents. It

is not within the scope of this section to give

more than a brief introduction to the pectic

substances and their most important properties. Readers with more than a casual interest

in these constituents should refer to the definitive monograph on this subject {222).

Pectic substances are a family of complex

colloidal carbohydrates consisting of polymerized galacturonic acid, a sugar acid closely
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 19

related to the sugar galactose. The free acid

radicals of galacturonic acid may or may not

be esterified with methanol, and the degree of

esterification is an important variable that

markedly affects the properties of the pectic

substances and is a factor in classifying them

into different groups. The pectic substances include only those polymers of galacturonic acid

that are colloidal by virtue of their large molecular weight. However, within this category

many variations in average molecular weight,

degree of esterification with methanol, and

other characteristics may occur. A uniform,

self-defining nomenclature for all pectic substances has been formulated and adopted by

the American Chemical Society {223), The

official nomenclature is as follows:

Pectic substances: This is a group designation for those complex colloidal carbohydrate

derivatives that occur in, or are prepared from,

plants and that contain a large proportion of

anhydrogalacturonic acid units, which are

thought to exist in a chainlike combination. The

carboxyl groups of polygalacturonic acids may

be partly esterfied by methyl groups and partly neutralized by one or more bases.

Protopectin: This term is applied to the

water-insoluble parent pectic substance that

occurs in plants and that, upon restricted hydrolysis, yields pectinic acids.

Pectinic acids: This term is used for colloidal

polygalacturonic acids containing more than a

negligible proportion of methyl ester groups.

Pectinic acids under suitable conditions are capable of forming gels (jellies) with sugar (65

percent) and acid or, if suitably low in methoxyl content, with certain metallic ions. The

salts of pectinic acids are either normal or

acid pectinates.

Pectin: The general term pectin, or pectins,

designates those water-soluble pectinic acids of

varying methyl ester content and degree of

neutralization that are capable of forming gels

with sugar and acid under suitable conditions.

Pectic acid: This term is applied to pectic

substances that are mostly composed of colloidal polygalacturonic acids and essentially

free from methyl ester groups. The salts of

pectic acids are either normal or acid pectates.

An examination of this nomenclature will

disclose that both pectin and pectic acid are

extreme cases of pectinic acids. In the former

55 to 70 percent of the carboxyl groups contain

methyl ester groups, whereas pectic acid is

practically free of methyl ester groups. The

pectinic acids also include a class of commercially important pectic substances known as

low-ester or low-methoxyl pectins (p. 56). Lowester pectins are pectinic acids in which less

than 50 percent of the carboxyl groups are

esterified, and are characterized by their ability

to form gels on the addition of bivalent metal

ions, such as calcium, to their solutions (208).

However, some bivalent metal ions, such as

copper, also will precipitate pectin.

Pectic substances, being exceedingly complex

organic compounds, exhibit many properties

and reactions, and perhaps the most important

practical property is their abiUty to form gels

(jellies) with sugar and fruit acids (183, 222).

All pectic substances are precipitated as

gelatinous or ñocculent precipitates in 50-percent acetone or 65- to 70-percent alcohol, and

they may also be precipitated or ^'salted-out'^

by high concentrations of divalent or trivalent

metals, properties used in the commercial

production of pectin. The insolubility increases

with decreasing methyl ester content, so that

low-ester pectinic acids and pectic acid may

actually be precipitated as gels by mere traces

of these metal salts. Pectic substances are hydrophilic colloids and as such are good emulsifiers. It is thought that the presence of pectic

substances extracted into citrus and tomato

juices during manufacture helps to suspend

and stabilize other colloidal particles of the

juices forming the desirable turbid ''cloud,''

which bears the characteristic color of the

juice.

One important problem of the citrus industry

involves the action of pectic enzymes (p. 44) in

improperly pasteurized juice and in frozen concentrate that has thawed, whereby the pectic

substances are probably demethylated and

otherwise altered so that they no longer act as

emulsifiers. As a consequence the cloud of the

juice breaks up and settles out, leaving an

unsightly clear serum on top of the juice with

a coagulated mass forming at the bottom. This

phenomenon, known to the trade as cloud loss,

has been a stimulus for much research on the

pectic substances and pectic enzymes in citrus

and tomato products.

The esterified pectic substances are readily

deesterified by dilute bases at room temperature, by hot acid, and by the action of the enzyme pectinesterase. Pectic substances also

undergo a peculiar, poorly understood oxidative

reaction in the presence of peroxides or in the

presence of ascorbic acid (vitamin C) and atmospheric oxygen. This reaction is potentially

important, as it can, under suitable conditions,

lead not only to the degradation of pectins m

food products but to the loss of vitamin C as

well (93, 222).

There are many methods for determining

the amounts of pectic substances in plant tissues and in food products (183, 222, 257, 325).

These methods are usually laborious and time

consuming. They employ alcohol precipitation
20 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

and weighing, demethylation by a base and precipitation by calcium salts, measurement of the

acid groups by titration or by decarboxylation

with hot acid followed by determination of the

carbon dioxide liberated (47^), and measurement of the colored complex formed with the

compound carbazol and sulfuric acid (95).

As with all other physiological constituents

of plants, there must be enzymes capable of

bringing about the formation and transformation of pectic substances necessary for their

metabolism.

Briefly the four known types of pectic enzymes whose activities have been demonstrated

in vitro are as follows :

(1) Polymethylgalacturonase, which is capable of hydrolyzing the glycoside bonds of

fully or highly methylated pectic substances to

yield degraded polymethyl-polygalacturonides

(387),

(2) Pectin-polygalacturonase, which is found

principally in molds and is capable of bringing

about a limited hydrolysis of the glycoside

bonds of pectin and which completely hydrolyzes pectic acid to its monomer, galacturonic

acid.

(3) Pectic acid-depolymerase, found principally in tomatoes and also reported in grapefruit (338), which causes a rapid loss of molecular weight of low-ester pectins and pectic

acid with a relativel}^ small degree of hydrol}^-

sis of glycoside bonds.

(4) Pectinesterase, or pectin-methylesterase,

found both in molds and in higher plants, including citrus, which catalyzes the hydrolysis

of the methyl ester bond, yielding from pectinic

acids, pectic acid and methanol (222, 22i).

The pectic enzymes peculiar to citrus fruits

are discussed more fully in a separate section

(p. 6).

Pectic substances appear to occur in situ,

principally in the form of protopectin, although

significant amounts of low-ester pectinic acids

are also found in the middle lamellae, and varying amounts of soluble pectinic acids and pectin are found in the cell sap and other parts of

plants. Pectic substances appear to be laid

down in the greatest amount during the early

growing stages of the plant or of the fruit, and

are believed to undergo a definite pattern of

changes during fruit ripening. Presumably

unripe fruit is rich in protopectin and contains

little of the other pectic substances. The insolubility of protopectin in the middle lamellae

is believed to account for the firmness of unripe fruit. As fruit ripens it is thought that

protopectin is broken down by the action of

an assumed enzyme protopectinase, which has

yet to be demonstrated in vitro, to soluble pectins and pectinic acids and causes softening

(93, 183, 210, 222). Much work has been carried on to establish the cycle of ripening and

softening of fruit, with somewhat inconclusive

results. However, it does seem clear that usually in fruit ripening a general trend can be

shown to exist of a reduction in molecular

weight of the pectic substances present, even

though it is not always possible to demonstrate

a decrease in total pectic substances or an increase in soluble pectic substances.

Among the richest sources of pectic substances in the plant kingdom are apples and

citrus fruits (183, 222). The edible portions

of the apple are rich in pectic substances, containing from 0.5 to 2.0 percent on a freshweight basis. Both the peel and the pulp of

citrus fruits are rich sources of pectic substances, including protopectins. The flavedo

and albedo of citrus fruits contain the greatest amounts, having from 20 to 40 percent on

a dry-matter basis. The pulp of citrus fruits,

on the average, will contain one-half to onethird of the amount of pectic substances found

in the peel portions.

Vitamins

Vitamins are a group of organic substances

required in minute amounts as a component of

the diet for normal health and well-being

(Í87). Many of the vitamins have been

isolated as pure compounds, which have been

characterized, and some have been synthesized.

These vital chemical compounds consist either

of groups of related individuals or of complexes

of unrelated compounds having diverse physiological activities. Different vitamins have

little in common so far as chemical structure

is concerned. Some are sugar acids, some are

sterols; some contain nitrogen, others do not.

Therefore, vitamins are not, as originally supposed, necessarily amines.

In general, vitamins are not synthesized by

the animal body, and therefore must be obtained from an outside source. They do not

furnish energy, and they are not utilized as

building units for growth of the organism.

Although they are present in relatively small

amounts, vitamins perform specific and vital

functions, and are essential for the transformation of energy and the regulation of metabolism. Certain vitamins function as prosthetic

or action groups in the enzyme systems that

control cell respiration and intracellular respiration. Others have speciahzed functions

not critically related to basal metabolism (26).

Vitamins differ from other well-known essential nutrients in several ways. First, the

amounts required are much smaller than are

the requirements for proteins, carbohydrates,
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 21

fats, and minerals. Second, they may be inactivated by heat, light, chemicals, and heavy

metals. Third, they may exist in inactive forms

as provitamins, which require special enzymatic or chemical treatments before they become

physiologically active. For example, the carotenoid pigment /3-carotene, present in many

foods, must first be converted to vitamin A by

the animal body before it becomes physiologically useful.

Vitamins are generally separated into two

groups, those that are fat soluble and those

that are water soluble. The early discovered

vitamins were designated by letters of the

alphabet, because little was known regarding

their chemical identity. Some received descriptive names, whereas others have been renamed

as their chemical structures have been established.

In the early days of vitamin research the

presence or absence of a particular vitamin was

determined by feeding experiments with such

test animals as the rat, guinea pig, dog, and

chick. These tests are known as bioassays, in

which the presence and amount of vitamins

are expressed in terms of animal response.

For example, the original unit of vitamin C was

the smallest amount that would prevent development of scurvy in a guinea pig. Bioassays

to estimate the vitamin content of foods or

other materials are costly, tedious, and time

consuming. They have now been superseded by

numerous chemical, microbiological, and physiological methods {17Í, U09, UUS).

The demand for and acceptance of citrus

fruits in the daily diet of humans is based

largely on their nutritional value and particularly on their vitamin C content. Although

vitamin C (ascorbic acid) was not isolated

from lemons until 1932 and its structural formula was established a year later, the effectiveness of citrus juices in alleviating scurvy was

recognized in the 1700's. In 1757 Dr. James

Lind, a surgeon in the British Navy, published

a Treatise on Scurvy, in which he described this

dread disease and recommended means for

preventing and curing it, confirming the theory

that scurvy was due to a shortage of fresh

fruits and vegetables in the diet. Later the

British Navy made lemon or lime juice a compulsory ration as a preventive measure against

scurvy during long sea voyages.

The vitamin C content of citrus juices may

vary widely with fruit variety, stage of maturity, climate, soil condition, geograpliic location, and related factors {lUUy US, 1U7, lUS),^

A compilation of the available data on the vitamin content of fresh and heat-processed juices

from orange, grapefruit, tangerine, lemon,

and lime is presented in table 15. In general,

the average retention of vitamin C during the

cannmg process is about 97 percent, with an

average loss of 1 to 2 percent per month during

storage at room temperature {111, 121 2LL

288, 2U, 295, 298-300, 302, 3U-3U3, 363, 36U,

U62, U7U)^ Loss of vitamin C in canned pasteurized fruit juices appears to be closely related to storage temperature ; lowering the storage temperature effectively protects the vitamm (fig. 3). Citrus juices also contain small

but significant amounts of provitamin A, Bvitamins, and other factors for which human

requirements have not been established. The

data in table 15 appear to indicate that biotin,

folie acid, inositol, and niacin are among the

more stable B-vitamins, whereas losses of

thiamine, riboflavin, pantothenic acid, and

^-carotene occur in heat-processed citrus juices.

It has been reported that losses of thiamine in

citrus juices are effectively reduced by lowering the processing and storage temperatures.

Freshly extracted orange or grapefruit juice

will retain from 96 to 99 percent of its original

vitamin C content for at least 1 week when

stored at 40° F. {296). The length of time such

orange or grapefruit juice may be allowed to

stand before use appears to be limited by loss

of palatability and beginning of fermentation,

not by appreciable vitamin C loss.

There seem to be no significant differences in

vitamin potency between freshly squeezed

orange juice and reconstituted frozen concentrates, although published data indicate that

frozen concentrated juices on an average are

sHghtly higher in vitamin C, /^-carotene, and

vitamin B12, and that the freshly squeezed

juice contains on an average slightly more

pyridoxine and inositol {350, 351), A survey of

the vitamin C content of 20 commercial brands

of frozen concentrated orange juice has been

reported (6'). After reconstitution, the vitamin

C values showed wide variations between minimum and maximum values for most of the

brands studied. The averages ranged from 30

to 46 mg. per 100 ml. of reconstituted juice,

with an overall average of 42 mg. The amount

of vitamin C varied from 30 to 50 mg. per 100

ml. within a single brand to an overall difference of 29 to 52 mg., representing the extremes

among the 20 brands studied. In another study

{288) frozen concentrated orange juice was reconstituted and stored from 4 to 8 days at 32°,

41°, and 77° F. Those samples held at 32° for

8 days and at 41° for 4 days suffered no impairment of flavor and minimum loss of vitamin C. However, at 77° palatability was lost

before any significant loss of vitamin C occurred. These results confirm an earlier investigation {296), which demonstrated that

orange juice stored at icebox temperature will

become unpalatable before any significant destruction of vitamin C occurs.
K>

TABLE 15.—Vitamin content of fresh and heat-processed canried^ citrus juices

Orange (irapefruit Tangerine Lemoi)

Vitamin ( )ther designation

Provitamin A.

Bi

B2

B

B

B

6 group-^

Bios L___

Biotin

C

C'holine__

K

P^irtor II.

K

PABA.__

P-P

/3-Carotene_

Thiamine_ _

Riboflavin _

Animal protein factor

Folio acid, folacin, pterylglutamic acid.

Inositol

Ascorbic acid.

Tocopherols

Pantothenic acid

Antihemorrhagic factor.

Para-aminoljenzoic acid .

Niacin _ - -

Units

per 100 ml.

International units.

Micrograms

do

_do.

_do-

-do.

Milligrams,-

Micrograms.

Milligrams..

.. -_do.- --

Fresh

do

MicrogramsMicrograms.

-._.do

190-400

60-145

11-90

25-80

0011-0013

1.2-3.3

98-210

.10-2.0

35-56

10-13

88-121

130-210

0

4

200-300

Processed

2-160

30-100

10-40

16-31

1.5-3.2

104-178

.5-11

34-52

Fresl

I Processed

(2 -21

40-100

20-100

10-27

0-15

10-50

5-30

7-27

F'resh

350-420

70-120

30

23

Processed

60

30

33

Frf

0-2

30-90

60

0-2

40

5-50

.8-1.8

88-150

.4-3.0

36-45

1-2 2

92-112

.3-9

32-45

1.2

135

.5

25-50

1.8 |..

147 i

.5 L

26-31 I 45-60 30-58

60-200 I 290 70-190

Lime

ProcFresh essed

40

(2)

170-300 200-220 i 80-200 ' 200-220 200 100-130 80-100 100

^ Compiled from various published data.

2 Trace.

''^ Includes pyridoxine, pyridoxal, and pyridoxamine.

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CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 23

100

AVERAGE TEMPERATURE

98.6* F.

4 8

TIME OF STORAGE (MONTHS)

12

FIGURE 3-Vitamin C retention in canned orange juice under different storage temperatures (adapted from Ross

(36i)).
24 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

Frozen concentrated orange juice is a rich

source of vitamin C. An average loss of only

3.4 percent of the original vitamin C content

has been observed during the preparation of

frozen concentrates from single-strength juice

(159). Losses during storage of frozen concentrates at 0"^ F. for a year are generally less

than 10 percent (70). Vitamin C is retained

for nearly a week in frozen concentrates held

in the common household refrigerator, whether

they are left in the can or reconstituted into

single-strength juice. At 41° losses of vitamin

C were not serious in 4 days (288). Even

though samples may have deteriorated extensively so far as flavor, gelation, and loss of

cloud are concerned, the vitamin C content may

remain substantially unchanged.

Inorganic Constituents

Inorganic constituents of citrus juices comprise what is known as the ash, the material

left after all organic material has been destroyed. The recovered ash is composed of the

neutral salts in the juice plus carbonates

formed by the decomposition of organic acid

salts, the so-called alkaline ash. Table 16 is a

compilation from various sources of the ash

constituents of citrus juices. Ash will vary with

the pressure used to juice the fruit and with

the amount of pulp left in the juice. In addition to analytical variables, these values depend

on such factors as type of soil on which the

fruit is grown, fertilizer practices, location of

the groves, region of gi'owth, rootstock on

which the trees are gi^afted, variety of fruit,

and the season during which the fruit is harvested.

Potassium accounts for 60 to 70 percent of

the total cation content of the juice. It is probable that most of this element is tied up in

the form of potassium acid citrate (395). The

major portion of the calcium and magnesium is

in a water-insoluble form combined with pectin.

Phosphates, sulfates, chlorides, and nitrates

are some of the anions that combine with a

portion of the cations (395). Bromine, fluorine

(94 ¡xg. per 100 ml. in orange juice), and iodine

are present in citrus juices, probably in the

form of salts (U2, 350, il2). Borates are also

found in citrus juices, orange juice particularly

containing a surprisingly high amount—0.05

to 0.33 mg. of boric acid per liter.

Other elements detected in citrus juices in

trace amounts are aluminum, nickel, barium,

chromium, copper, tin, manganese, vanadium,

silicon, lead, strontium, titanium, zinc, and zirconium. The fact that citrus juices contain

copper (0.3-0.9 p. p.m.) is important, because

it has a role in the destruction of ascorbic acid.

Nutritionally the elements copper, iron, iodine, manganese, and zinc are considered of

significance, since these cations are associated

with basic enzyme systems concerned with

metabolism in the human body. The relatively

low sodium content of citrus juices is of particular dietetic interest, because low sodium

diets are important in treating various chronic

conditions, such as cardiac diseases. Citric acid

is almost completely oxidized in the body. This

is important, because citric acid salts of the

juice are metabolized on ingestion, liberating

the combined cations and thus helping to maintain the alkaline reserve of the body.

PROCESSING OF CITRUS

Canning

ORANGE JUICE

The annual pack in the United States of

canned single-strength orange juice has been

approximately 20 million cases. Certain varieties are preferred for canning. The Pineapple

orange in Florida is considered excellent ; many

seedling trees produce top-quality fruit for

juice. The Valencia orange is unsurpassed in

both Florida and California for color as well

as fine flavor. The flavor and appearance of

the freshly extracted juice are generally good

indications of its suitability for canning. Washington Navel oranges produce a juice that

sometimes becomes bitter (108). Overmature,

undermature, or stale oranges (fruit that has

stood too long after picking) do not make

good juice (^59).

TABLE 16.—Total ash and ash constituents of some citrus juices

Citrus juice Ash

CalciumChlorine Iron

Magnesium

Phosphorus

Potassium Sodium Sulfur

Marsh grapefruit, Florida (4^)

Seedy grapefruit, Florida (362) _.

All varieties of lemon, California {W}-

Valencia orange, California {41'^)

Valencia orange, Florida (362)

Tangerine, Florida (36^)

Gm.. per

100 gm..

0.4

.4

.3

.4

.4

.4

Mg. per

100 gm.

of ash

8

15

26

14

9

12

Mg. per

100 gm..

of ash

1

2

3

6

3

3

Mg. per

100 gm.

of ash

0.1

.08

.35

.25

.1

.28

Mg. per

100 g m-.

of ash

5

9

10

13

5

7

Mg. per

100 gm.

of ash

10

29

13

20

32

15

Mg. per

100 g m..

of ash

90

172

164

170

172

177

Mg. per

100 gm.

of ash

4

7

21

6

7

6

Mg. per

100 gm.

of ash

2

4

7

8

4

6
FEED MILL

DCAERATION AND

DEOILING PASTEURIZER

SUGAR

(FOR SWEETENED JUICE)

SAMPLE TO

LABORATORY

^ 1 A « Ä

' »^

)

CAN FILLER CAN-CLOSING MACHINE CAN COOLER

FIGURE 4.—Flowsheet showing steps in the canning of single-strength orange juice.

WAREHOUSING

LABELING ;^,^Q RACKING

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26 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

Fruit is delivered to the canning plants by

trucks holding from 35,000 to 40,000 pounds.

At the plant it is unloaded for inspection and

grading, and unsuitable fruit removed before

the oranges are run into storage bins prior

to canning (fig. 4).

An automatic sampler removes some of the

oranges at this point for determination of soluble solids and acid. By knowing the ratio of

soluble solids to acid for oranges in each bin,

fruit from various bins can be blended to produce juice of more uniform flavor. The bins

also provide a supply of fruit for continuous

operation.

As the fruit comes from the bins, it is carefully scrubbed with a detergent on a rotary

brush washer and rinsed with potable water.

It is again inspected and damaged oranges are

removed.

Plants processing orange juice are equipped

at several points with facilities for the inspection and discarding of damaged fruit. A convenient arrangement is to pass the oranges

over a roller conveyor so that all sides of the

fruit are exposed. These roller conveyors or

grading tables are well lighted, installed at a

convenient height, and of such width that all

fruit can be reached by the inspectors. For

adequate inspection fruit should not cross the

grading tables at a rate in excess of 30 feet per

minute (4-3). Causes for rejection include

ruptured peel and evidence of mold or rot.

Examination of the juice in the control laboratory is helpful in revealing the effectiveness

of the grading.

Prior to juice extraction, the oranges must

be separated according to size, and this is accomplished by machines that may be purchased

from most canning-supply manufacturers.

Several effective juice extractors (figs. 5 and

FIGURE 5.—Assembly of juice extractors in a citrus-processing plant. (Courtesy of Florida Citrus Canners Coop.,

Lake Wales, Fla.)
CHEMISTRY AND TECHNOLOGY OP CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 27

FIGURE 6.—Another type of juice extractor. The guards

have been removed to shovv^ the heads for extracting the juice from the fruit.

6) are employed by the industry and operate

in various ways. Some halve the fruit and remove the juice by reaming or by pressing; one

machine inserts a tube through the peel, and

by squeezing the whole fruit it forces the juice

out through the tube. Although extracting machines may differ in design and are rather

complicated, they all possess certain features

in common. They are fast, rugged, easily

cleaned, and can be adjusted to reduce the incorporation of peel extractives into the juice.

The canner usually leases extracting machines

from the manufacturer on a royalty basis.

Extracted juice is free from peel, but may

contain seeds and coarse pulp. These are removed by finishers, which vary in design. One,

of a screw-type design, consists essentially of

a conical helical screw enclosed in a cylindrical

screen with 0.020- to 0.045-inch perforations.

The finished juice flows into blending tanks,

where it is tested for soluble solids and acid.

The juice may be packed unsweetened, or sugar

may be added at this point to make sweetened

orange juice for canning. The requirements for

soluble solids, acid, and color are given in detail in the United States Standards for Grades

of Canned Orange Juice {H6).

After blending, the juice is deaerated (fig.

7). As practiced today deaeration is often accomplished in the same operation as deoiling.

Air incorporated in the juice during extraction

and finishing can be effectively removed by an

apparatus developed for the purpose {303, 3kU,

SU7, h59). There are several benefits from

deaerating orange juice, such as improvement

in uniformity of can fill by elimination of foaming in the filler bowl ; improvement in efficiency

of the heat exchanger with less danger of

scorching the juice; and when enamel-lined

FIGURE 7.—Pulley-type deaerator (U. S. Public Service Patent No. 2,060,242) for citrus juices. (Courtesy of American Machinery Corp., Orlando, Fla.)

cans are used, less separation of enamel from

the cans {358).

Excessive amounts of peel oil in the juice

may be objectionable. However, some peel oil

is considered necessary for maximum flavor

(S5, 358), and according to the United States

Standards for Grades of Canned Orange Juice

it may contain up to 0.030 percent of recoverable oil U-46)- To control the amount of peel

oil in the juice, deoiling sometimes becomes

necessary, and this may be accomplished by

vacuum distiflation {3U6). After condensation,

the oil can be separated and the aqueous distillate returned to the juice. There are several

deoiling units on the market, which may be

purchased by the processor.

The deaerated or deoiled juice is next pasteurized. Pasteurization of single-strength

orange juice, as now practiced, is not primarily

directed toward the destruction of spoilage organisms. It was realized years ago that heating

at temperatures as low as 150° F. is sufficient

to destroy most spoilage organisms. The orange
28 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

juice thus preserved, however, separates into a

clear liquid v^ith a sediment at the bottom.

Evidently after the spoilage organisms are destroyed, the juice is subject to another type of

deterioration, and it was found that heating

to higher temperatures (up to 200*^) conferred

additional stability. It was later realized that

these chemical changes were brought about

by the presence of naturally occurring enzymes,

and the recommendations for heating to 190°-

200° reflect a better understanding of the problem of stabilization. Today it is recognized that

the particular enzyme pectinesterase (p. 6)

may be associated with deterioration, and processing is directed to the conditions necessary

for destruction of its activity. A simple test

has been devi^a^î (228) to check the efficacy of

the pasteurization process by testing for the

activity of pectinesterase.

In present commercial practice the juice is

rapidly heated to about 197° F., the exact temperature being dependent somewhat on the particular equipment in use and the rate of juice

flow. Juice is generally in the pasteurizer for

about 40 seconds.

Flash pasteurization is carried out with

either tubular or plate-type heat exchangers.

A study (165) of pasteurization of citrus juices

indicated that turbulent flow is necessary to

heat the juice rapidly without local overheating. Inevitable variations in the flow of juice,

which could result in scorching or underheating, are reduced by the design of modern heat

exchangers and automatic controls.

After the cans are filled with hot (180°-200°

F.) pasteurized juice, they are inverted to allow the hot juice to sterilize the inside of the

fids, and then cooled. The juice is hot during

pasteurization and while the cans are filled,

closed, and held inverted—a total of 2 to 3

minutes in commercial operations. Cooling to

90°-100° is accomplished rapidly by spinning

the cans on a conveyor under sprays of cool

water. It is desirable to have the cans emerge

from the cooler with some residual heat (not

more than 100°) to facilitate drying and prevent subsequent rusting.

Sometimes a jet of steam may be played beneath the can lid before sealing. This cleans

the underside of the lid and aids in obtaining

a vacuum in the can (331).

Time and temperature of storing canned

juice are of prime importance in maintaining

quality of the product. Modification of orangejuice flavor during actual canning is slight, and

off-flavors that develop are chiefly the effects of

subsequent storage temperatures. Rate of deterioration increases with increased storage

temperatures, and juice should be kept as cool

as economically possible.

Storage studies of canned single-strengÜi

orange juice at various temperatures and periods of time (lU, 136, 2U. 299, 302, 36U) have

shown that in 12 months only slight changes m

flavor occur at or below 60° F.

Riester, Braun, and Pearce (358) state, The

importance of adequately controlled low-temperature storage cannot be overemphasized. It

is difficult to conceive of an improvement m

production technic, even though revolutionary,

the effects of which would not be largely nulhfied by high-temperature storage.'^ Many warehouses for canned citrus juices have been insulated and ventilated to maintain storage temperature as low as practical.

GRAPEFRUIT JUICE

The methods described for canning singlestrength orange juice are also applicable to the

canning of grapefruit juice. The main difference is that the juice extractors must be adjusted to handle the larger fruit. Sometimes

canning plants are equipped with juice extractors designed to handle the whole range of

fruit sizes from small oranges to large grapefruit, and therefore can switch back and forth

from one juice to the other.

As previously mentioned grapefruit contains

a bitter glucoside, naringin, most of which is

found in the albedo, rag, and pulp (226) ; pectinesterase activity is also associated with the

solid material (262), Consequently, in extracting the juice and separating it from the solid

portions efforts are made to minimize the extraction of naringin and the incorporation of

too much pulp (358),

There is no general agreement on the value

of deaeration for grapefruit juice.

Pasteurization is concerned with the inactivación of pectinesterase (326), just as in the

processing of orange juice. The same rapid test

(228) for residual enzyme activity as an index

of pasteurization will be helpful.

Canned grapefruit juice is an excellent

source of vitamin C, and its retention during

processing (298) and in storage is similar to

that observed with canned orange juice (2Jfíy

299, 302, 36A).

BLENDED JUICES

Blended grapefruit and orange juice is produced by the procedure described for orange

and grapefruit juices, except for the blending.

The general practice is to use approximately

equal quantities of the two juices. However,

when the oranges yield light-colored juice, the

proportion of orange juice is increased. Late in

the season when more deeply colored juice is

available, the proportion of orange juice is
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 29

decreased. It is desirable to keep the appearance and flavor as uniform as possible over the

entire season.

Canned blended grapefruit and orange juice

of United States grade A may contain 0.030

percent by volume of recoverable oil {HU).

Canned blended juices are excellent sources

of vitamin C, as about 98 percent of the vitamin is retained during canning; retention in

storage is equal to that of either juice canned

separately (372).

Tangerine-orange blends have a good flavor,

but cannot compete on the market with grapefruit-orange blends because of economic

reasons.

TANGERINE JUICE

Despite earlier difficulties (11) it has been

found (79) that an acceptable canned singlestrength tangerine juice can be produced. An

important factor in preparation appears to be

avoiding excessive extractives from the pulp

and peel. Tangerine juice should contain not

more than 0.020 percent of recoverable oil and

not over 7 percent of free and suspended pulp

(UO).

Since the fruit is fragile, it is usually delivered to the plant in boxes rather than in

bulk by trucks. The fruit is conveyed directly

into wash tanks, bypassing the storage bins.

Processing is similar to that described for producing canned single-strength orange juice.

Extractors must be specially adjusted to handle the smaller fruit.

LIME JUICE

There are only two acid fruits of the lime

group Citrus aurantifolia (Christm.) Swingle

that have attained commercial importance.

These are the Key (Mexican or West Indian)

lime, a small round fruit averaging about 1 inch

in diameter, and the Persian (Tahiti) lime,

which is considerably larger, being about the

shape and size of the commercial lemon. These

fruits are very acid and contain from 5 to 8

percent of citric acid.

Only a limited amount of canned lime juice

has been marketed largely because of changes

that take place in appearance and flavor at

room temperature. The procedures of preservation have been based on those commonly used

in processing canned orange juice. Use of the

Persian variety has permitted extraction of

juice with standard juicing equipment. The

juice, after passing through a finisher, is pasteurized at about 195° F., filled into cans, and

cooled. Refrigeration at 35° greatly extends

its storage life; samples have been stored as

long as 15 months at this temperature (34^)

without appreciable deterioration.

Since the Key, or Mexican, lime is smaller

than the Persian, the use of conventional juice

extractors is impracticable; therefore, juice is

generally extracted by crushing the whole fruit

in a screw press. The juice obtained may have

a high oil content, but this can be reduced by

vacuum deoiling. Sometimes lime juice is

clarified by mixing with a filtering aid and

filtering in a plate-and-frame press. This clear

juice is used in compounding formulas for bottled drinks and fountain use.

Lime oil, both cold-pressed and distilled, is

usually a byproduct of juice manufacture. The

peel is used for recovery of cold-pressed and

distilled oil, and the filter cake from clearjuice production may be used for distilled oil.

In the West Indies lime juice is prepared by

crushing the whole fruit as in the preparation

of oil, except that stone rollers are used. The

expressed juice is screened through a 3/16-inch

mesh screen and then through a 1/8-inch mesh.

It is pumped into wooden storage tanks, where

it is allowed to remain for 16 to 30 days. During this period a sludge settles to the bottom

and light pulp rises to the surface. Fermentation is retarded by the high acidity and low

sugar content of the juice; the oil present also

acts as an inhibitor of fermentation.

After the pulp settles, the intermediate clear

layer of juice is drawn off, filtered through

cloth, and stored in paraffin-lined barrels holding from 50 to 100 imperial gallons. Sulfur

dioxide (350 p. p. m.) or 0.1 percent of sodium

benzoate is added, depending on the import

regulations of the country to which the juice

is to be sent.

In commerce the following tj^es of West

Indian lime juice are recognized (also see

p. 37) :

(1) ^^Settled'^ or '^racked'' juice. This is the

clear juice obtained as previously described.

It should be clear and sparkling and with a

typical lime flavor. Often it has a peculiar

musty lime flavor quite different from lime

juice prepared in the United States. This

peculiar flavor is very acceptable to the British

palate ; in America it is not so widely accepted.

The acidity of this type of juice should run

from about 9.4 to 12 percent, calculated as

anhydrous citric acid.

(2) 'Top juice with pulp/^ This contains

light fine pulp and oil floating as a thin film on

the surface. Most of this juice is used in the

West Indies for making distilled lime oil.

LEMON JUICE

The commercial production of lemon products in the United States is confined almost
30 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

exclusively to California. The Eureka lemon

is the principal variety grown, comprising approximately 88 percent of the total production.

Lisbons make up about 8 percent of the total,

and miscellaneous varieties comprise the remamder. The lemon industry in California is

unique in that the fruit is picked for size only

and held under controlled storage conditions to

undergo coloring from green to yellow instead

of being left on the trees to color. Lemons

sorted at the packinghouses may be used as the

raw material for processing, although under

unfavorable market conditions large tonnages

of gi'ove-run fruit may be diverted from the

fresh market to processing.

In preparing natural single-strength lemon

juice, the initial operations of fruit handling,

inspection, washing, sizing, and extraction of

the juice are the same as those described for

other citrus juices. After the juice is extracted

from the fruit and screened to remove rag and

seeds, it is usually held in storage tanks for a

short time for the purpose of blending. If the

juice is to be frozen, brine-jacketed tanks

are used to chill the juice. When required, the

juice can be deaerated in these tanks by applying a vacuum of 20 to 25 inches of mercury

for about 30 minutes.

At least six lemon-juice products are now

being manufactured in California, including

pasteurized or frozen single-strength juice,

concentrate, and lemonade concentrate.

In the preparation of canned frozen singlestrength juice, the chilled juice is drawn from

the coldwall holding tanks and further cooled

to 30° F. by passage through a heat exchanger.

It is then filled into cans, sealed, frozen, cased,

and stored atonto-10°.

In the preparation of canned pasteurized

juice, the juice is generally centrifuged to remove a portion of the solid suspended material

and then pumped through deoilers, where a

large portion of the volatile oil is flashed off

by heating the juice to approximately 175° F.

for a short time. The presence of this oil would

create off-flavors in the canned product. This

treatment also stabilizes the cloud and pasteurizes the juice. The hot (175°-180°) juice

is filled directly into cans, sealed, cooled as

rapidly as possible with cold water, and cased.

No further processing is necessary. The filled

cases are usually stored at around 40° to prevent changes in the product, which occur too

rapidly at higher storage temperatures. The

principal deterioration of lemon juice is a

browning of the juice, resulting in an off-flavor

and poor appearance. Cool storage retards this

change.

Large amounts of single-strength bottled

lemon juice are produced. This product is not

made from the natural single-strength juice

previously described, but by reconstituting a

concentrated lemon juice. Either pasteurized

concentrates (p. 35) or frozen concentrates are

generally used. Several reasons have been

given for using reconstituted lemon juice instead of the natural juice in making this product. The most important is uniformity of quality. For example, the acid content of the reconstituted juice is controlled by blending to insure uniformity from bottle to bottle. A longer

shelf life is reported for this product. Decentralized bottling plants convenient to major

markets may be established, in which case it

could be more economical to ship the concentrate to be reconstituted at the bottling plant

than to ship the natural-strength juice. If the

concentrate has not been heated, it is pasteurized at approximately 176° F. for 1 minute to

stabilize the cloud.

Water that is purified through carbon filters

and deaerated in a vacuum tank is added to the

concentrate to reconstitute it to natural

strength. The reconstituted juice is then homogenized at about 3,500 pounds per square-inch

pressure, and sufficient sodium benzoate or sulfur dioxide added as a preservative. The juice

is filled into bottles (usually green in color)

and capped. No artificial coloring or flavor is

added. The presence of sulfur dioxide or sodium benzoate must be declared on the label in

accordance with regulations of the Federal

Food, Drug, and Cosmetic Act.

GRAPEFRUIT SECTIONS

Seedy grapefruit, such as the Duncan, is preferred for section canning. Upon arrival at the

processing plant the grapefruit is inspected,

and the sound fruit is separated according to

size into bins. Uniformity of size of the segments in the canned product is improved by

using fruit of one size at a time. Sizes considered ideal are fruits that measure from 3iyi6

to 413/^g inches in diameter. Small and very

large sizes are commonly used for juice production. Since part of the subsequent operations is handled on a piecework basis, small

fruit is too expensive to use, and excessively

large fruit yields too few segments per can.

Two methods are used for peeHng the fruit

{Ip56, pp. 61-71 ). The "cold peel" method is one

in which the peel is sliced off in such a manner

that both the albedo and outer membrane of

the segments are removed. This method results

in a lower yield, and segments sometimes show

excessive trimming. However, it is employed

when the fruit is tender and will not stand lye

peeling.

The more generally used method of peeling
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 31

FIGURE 8.—Peeling grapefrait in preparation for sectionizing. (Courtesy of American Machinery Corp., Orlando,

Fla.)

is to "scald" the fruit either by steam or in

water (196° -210° F.) for about 5 minutes for

the purpose of phimping the peel for easier

peeling. It is then cooled slightly with water

sprays as it emerges from the scalding tank.

Peelers deftly remove the peel (fig. 8) with the

aid of a short knife by slitting or scoring the

stem end of each fruit and pulling off the peel.

Some of the adhering albedo is stripped off

with the short blade of the knife and the

thumb. A machine to peel the fruit is in an

experimental stage.

Baskets of peeled grapefruit are passed

through a lye peeler. Banks of sprays successively spray the grapefruit with hot (200°-212°

F.) alkali of about 1/2- to 21/2-percant concentration for about 12 seconds. A variable

speed conveyor carries the baskets under about

10 feet of these alkali sprays, followed by 15

feet without treatment to allow the alkali to

dissolve adhering albedo, and then under 40

feet of water sprays to rinse the fruit. The

"canner's alkali" is a mixture of sodium hydroxide and sodium carbonate, which is believed to be easier to remove from the fruit by

the water rinse than sodium hydroxide alone.

The alkali solution from the sprays iircollected

in a tank under the conveyor and circulated by

pumps back through the sprays. During a

period of operation the dissolved fruit solids

build up in the lye solution, so that it is desirable to start with a fresh solution each day or

even more often. Because of such variables as

changing composition of the lye solution and

differences in fruit, lye peeling must be carefully controlled to obtain the desired results.

This control is based largely on personal experience by observing the fruit as it emerges

from the lye bath.

The peeled fruit is conveyed in baskets to

sectionizing tables. Here a limited amount of

fruit is allowed to accumulate for three purposes: First, the peeled fruit when warm is

fragile and apt to break up during sectionizing ;

second, a supply ahead enables the sectionizers

to work without interruption ; and third, delay

permits dissipation of final remnants of yellow

color remaining after lye treatment.

Successful sectionizing depends on the skill

and experience of the operator. The grapefruit

is either held in the hand or placed on a revolving

spindle inserted through the center of the fruit

(fig. 9). A stainless-steel or aluminum spatula

with a wide base is inserted betvv'een the fruit

cells and membrane of each segment and the

segment lifted out. Seeds are wiped from the
32 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

FIGURE 9.—Sectionizing grapefruit. (Courtesy of Florida Citrus Canners Coop., Lake Wales, Fla.)

segment with the edge of the spatula after the

section has been transferred to the other hand,

or removed by brushing or tapping two segments against each other. The sections are

packed by hand into unHned cans containing

the requisite amount of sirup made by dissolving sugar either in water or in juice. Density

of the sirup is adjusted to give, after storage,

a "cutout" (drained) sirup to conform to

standard designations: Heavy sirup (18° Brix

or more), light sirup (16°-18° Brix), or shghtly sweet (12°-16° Brix). It is necessary for

operators to wear rubber gloves to prevent skin

irritation. Machines have been developed for

sectionizing grapefruit, but are not as yet in

extensive use.

The fruit is arranged in the cans by placing

sections around the periphery in such a manner

that a hole is left in the center. Whole segments and broken segments are customarily

packed in separate cans and appropriately labeled. Care must be taken not to overfill the

cans, as the sections may be broken when the

cans are lidded, and they may appear bulged,

which usually suggests spoilage to the lay person. If insufficient sections are added, a loose

pack results, oxygen in the head space increases

corrosion of the cans, and the packer may be

penalized for slack fills (4.45).

Filled cans are either vacuum closed and

processed, or exhausted by heat, closed, and

processed. Vacuum closure is accomplished by

injecting steam to displace the air beneath the

lid as it is placed on the can. After No. 2 cans

are sealed, they are heated to about 185° F. for

35 minutes in a cooker-cooler (fig. 10). If the

cans are heat exhausted, the filled but open

cans are conveyed through a tank, called an

exhaust box, of hot (180°) water. The temperature of the sirup in the center of the cans

emerging from the exhaust box should be 146°.

No. 2 cans are exhausted for 27 to 35 minutes.

They are sealed and heated for 16 to 30 minutes

at 176° to 185°. They are then cooled to about

100° by being conveyed through a cooling tank.

Cans are handled and cased as gently as possible to avoid breaking the sections. It is desirable to let the cans stay undisturbed for 10

days to 2 weeks, during which time the sections

become decidely more firm.

MANDARIN SECTIONS

This product has not been commercially successful in the United States, but before World

War II significant quantities of such a pack

were imported from Japan. The canning method

used there has been described by Boswell (39).

The washed fruit is passed through a steam or

hot-water bath for about 1 minute to facilitate

peeling, and then cooled in cold water. The

fruit is peeled by hand, and then is placed in

shallow trays until the surfaces have dried. The

sections are separated by hand, and immersed

in cold (68°-77° F.) 21/2-percent hydrochloric

acid for about 2 hours to remove the stringy

fibers adhering to the peeled fruit. The acid is

subsequently removed in running water. The

washed segments are immersed for 25 minutes

in a 1-percent solution of sodium hydroxide at

113° and then again washed. The cans are

filled with these sections, and are subsequently

inverted to drain off excess water. Each can

contains 325 gm. (11.46 ounces) of fruit, to

which is added 125 gm. (about 41/2 fluid

ounces) of hot 17° Brix sugar sirup. The cans

are vacuum closed and processed at 176° for

14 minutes.

The separated sections are extremely fragile,

and much agitation or vigorous stirring will

cause breakage. Washing and handling must

be done with large volumes of gentle currents

of water. A whirlpool floatation type of separator is best for washing and removing small

fragmer/-s of membrane and peel. Much flavor

is removed with all this manipulation, but the

pieces are attractive in appearance.
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 33

FIGURE 10.—Part of a cooker-cooler used in the processing of grapefruit sections. (Courtesy of American Machinery Corp., Orlando, Fla.)

Chilled Orange Juice

Although most citrus juices consumed in this

country are preserved in one form or another

by pasteurization, concentration, or freezing,

considerable quantities of orange juice are distributed in California and in the Middle Atlantic and Northeastern States in the form of

chilled single-strength juice. The juice may be

packaged in milk bottles or waxed cartons, and

shipped up to distances of 1,200 miles in refrigerated trucks.

Carload lots of unpasteurized orange juice,

packed in 1-gallon hermetically sealed tin cans

refrigerated with ice and salt, have been successfully shipped from southern California to

New York City for distribution by dairies.

In Florida bulk shipments of orange juice

are made in stainless-steel refrigerated tank

trucks lined with plastic or glass {U86). Sometimes the juice may be shipped in 1-quart

waxed cartons. Most of the orange juice is

purchased by hotels, hospitals, restaurants, and

dairies. In Florida reconstituted frozen concentrate may be added to standardize the

product or used to supply the market when

suitable fresh fruit is not available for juicing.

Sometimes the juice is given a heat treatment

to improve cloud stability and otherwise extend

storage life. In large cities this type of juice

may be produced in small processing plants and

distributed locally through juice bars, grocery

stores, and dairies.

Chilled orange juice must compete with

single-strength canned citrus juices, frozen
34 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

concentrates, and fresh fruit itself. The main

advantage of the chilled juice is one of convenience, in that it enables restaurants, hotels, hospitals, and other large users to prepare this

food easily for their customers. Many families

are interested in the convenience of having

high-quality orange juice delivered to their

doorsteps.

Strict sanitary control of all procedures, from

selection of fruit through processing and shipping, is necessary to insure superior quality of

the product. The fruit should be thoroughly

washed with a good detergent, rinsed, and

dried. Cutting and reaming the fruit should be

carefully controlled to maintain oil content of

the juice at a low level. Heavy solid particles

and seeds are screened out. If the juice is to

be stored for more than 3 to 4 days, it should

be deaerated to remove dissolved oxygen, which

hastens flavor deterioration. The extracted

and screened juice should be chilled as quickly

as possible to 30^-32'^ F., preferably through a

heat exchanger, filled into bottles or cartons

with a minimum of head space, and tightly

sealed. The juice should be kept at around 30*^

until consumed in order to preserve maximum

quality. This temperature is lower than that

generally maintained in most commercial and

household refrigerators. Orange juice, because

of its sugar content, will not freeze at this

temperature.

Orange juice prepared by this method will

retain most of its fiavor and practically all

its vitamin C content for storage periods up to

10-14 days when kept at the recommended refrigeration temperatures. During this period

the microbial population of the juice usually

tends to decrease.

Sulfured Citrus Juices

Sulfur dioxide is widely used for the preservation of citrus juices in the Mediterranean

countries and in England. Its use in the United

States is limited, because foods containing sulfur dioxide are not permitted under the purefood regulations of certain States. Anyone

planning to use this preservative in citrus

juices for sale should consult applicable regulations for complete details.

The use of sulfur dioxide as a food preservative has been subject to regulation since the

15th century. The preservation of citrus juices

with sulfur dioxide began to assume importance

early in the 20th century, when Great Britain

imported lime juice for use as a part of the

official ration in its Navy, with a view to the

prevention of scurvy. After the introduction

of lime juice preserved with sulfur dioxide,

lemon and orange juices and then grapefruit

juice so preserved were imported for use in

the preparation of beverages (also see p. 37).

Sulfur dioxide is also used to preserve sliced

and cooked citrus fruits for use in marnialade

manufacture. In recent years it has been used

extensively for the preservation of concentrated orange juice in bulk. In the citrus areas

bordering the Mediterranean there are few

processing plants with modern equipment for

canning, concentrating, or freezing citrus

juices; therefore, much of the processed juice

is preserved with sulfur dioxide.

Sulfur dioxide is more effective against mold

spores and bacteria than against yeast. It inhibits darkening and helps ^n reducing losses

of vitamin C. However, it does not affect pectic

enzymes, which are responsible for breakdown

of tissue or which cause loss of cloud in citrus

juices. Also, sulfur dioxide cannot be considered to have a permanent preservative effect

in foods stored under conditions permitting reinfection. Being a gas, it tends to escape during storage in nonhermetically sealed containers, and it is quite likely that some of the sulfite

is lost by reduction to sulfate. It also reacts

with some of the constituents of fruit, notably

aldehydes, ketones, and sugars, to form compounds lacking in preserving power. Since high

temperatures accelerate both these actions, it is

advisable to store sulfur dioxide-preserved

foods in cool places.

Downer (98) states that the preservative

effect of sulfur dioxide in acid juices is due to

the undissociated sulfurons acid molecule. The

effectiveness of sulfur dioxide as a preservative

is a function of juice acidity, as a large concentration of hydrogen ions is necessary to repress ionization and permit the existence of

appreciable numbers of sulfurons acid molecules. Although some investigators report that

sulfur dioxide exerts a very slight destructive

effect on vitamin C in citrus juices. Hamburger

and Joslyn (US) found no evidence of such

action. Apparently sulfur dioxide competes

with the vitamin for any free oxygen present

and therefore inhibits rapid early oxidation

of the vitamin. The final minimum value of

vitamin C is independent of the presence or

concentration of sulfur dioxide.

The amount of sulfur dioxide required depends on the acidity (pH) of the juice and the

amount of suspended pulp. Clarified lime juice

can be preserved safely with as little as 350

p. p. m., but pulpy orange juice requires as

much as 1,500 p. p. m. (0.15 percent). Thorough

mixing of preservative with juice is essential,

as an unprotected pocket may begin to ferment,

after which no amount of preservative will

prevent the loss of the entire contents of the

container.
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 35

Aging or subjecting citrus products to elevated temperatures accelerates the inversion of

sugars, with an increase in aldehyde (glucose)

content. Therefore, products so treated require

larger amounts of sulfur dioxide for adequate

preservation. Cooked pulps for marmalade are

usually treated with 2,000 p. p. m. or more of

sulfur dioxide. When sulfur dioxide is used

to preserve concentrated orange juice, 2,000 to

2,500 p. p. m. must be added to give a residualfree sulfur dioxide sufficiently high to prevent

infection for any appreciable storage period.

Although combined sulfur dioxide does not exert preservative effect, it does affect flavor. Excessive amounts will also cause bleaching. It

is desirable, therefore, to avoid such high dosages wherever possible. Ingram (200) recommends that orange concentrate be heat processed and hermetically sealed until ready for

use. It may then be treated with sufficient sulfur dioxide to counteract any contamination

picked up during repackaging or subsequent

processing. Where infection may occur at

several stages, the best plan would seem to be

to handle at low temperature to minimize the

activities of micro-organisms and to add sulfur

dioxide as late in the process as possible (200).

For large-scale operations the most economical way to apply sulfur dioxide is as the

pure gas. This is accomplished by placing the

cylinder containing sulfur dioxide on a scale

so that loss of weight can be measured. A copper tube is connected to the valve of the

cylinder and extended into the juice to be sulfured. Care should be taken to see that no

iron comes in contact with the juice, as it will

cause discoloration in the sulfured product. The

operation should be carried out in a well-ventilated room, because there is always escape of

some gas, which is extremely irritating to mucous membranes. Gas should be bubbled slowly

through the juice to assure complete absorption.

Another way of adding sulfur dioxide is by

means of water solutions of a sulfite salt. The

following method will be found satisfactory.

Place a strip of adhesive tape at the 5-gallon

level of a 5-gallon bottle of water. Pour out 1

gallon into a bucket. To the bottle add one of

the following : (1) 10 pounds of sodium metabisulñte (theoretically 64 percent of sulfur dioxide), (2) 101/2 pounds of sodium bisulfite (theoretically 61.5 percent of sulfur dioxide), or (3)

12 pounds and 5 ounces of potassium metabisulfite (theoretically 57.6 percent of sulfur dioxide). Shake until dissolved and pour back

the water from the bucket until the 5-gallon

mark is reached. The mixture thus obtained is

a stock solution and should be maintained tightly stoppered. Each fluid ounce of this solution

will contain 0.01 ounce of sulfur dioxide.

As an example of its use, suppose it is desired to add 800 p. p. m. of sulfur dioxide to

a barrel of juice. Put 1 quart of the solution

and 400 pounds of juice into the barrel. The

next day, after foam has settled, nearly fill the

barrel with juice and determine the net weight.

This will be found to be between 444 and 475

pounds. Subtract 400 pounds from the net

weight, and for every 12.5 pounds over 400

pounds add 1 fluid ounce of stock solution. For

example, if the net weight is 457 pounds, subtract 400 pounds. Then the difference, or 57,

is divided by 12.5, which gives 4,56. Therefore,

add 4.6 ounces more of the stock solution. This

will give a final concentration of 800 p. p. m. of

sulfur dioxide.

Pasteurized Concentrates

In general, the process used commercially for

the manufacture of pasteurized concentrates is

essentially the same as that used for frozen

concentrates (p. 38), except that the former

are heated sufiiciently to completely inactivate

pectic enzymes and to destroy spoilage organisms. This is necessary, since these concentrates are generally intended for storage at

temperatures above freezing (usually at about

40° F.).

Procedures for washing, sorting, sizing, and

inspecting the fruit and extracting and finishing the juice are the same as those used in the

preparation of other citrus products. If the

concentrate is to be used for the manufacture

of compounded products, the fruit may have a

wider range of soluble solids-acid ratios than

that intended for single-strength or frozen

concentrates, since these juice variables will

be adjusted in the final product. If the concentrate is to be used as a pure-juice beverage,

the usual precautions must be observed in controlling these fruit qualities (253; A35, pp, 677-

695).

The juice is evaporated to the desired concentration in a vacuum evaporator. Older types

of evaporators operate at 120° F. in the optimum temperature range for enzymes present.

To prevent enzymatic deterioration during

evaporation, the juice is heated to 190°-200°

just before it enters the evaporators. Newer

falling-film, low-temperature evaporators operate at about 70°, and it is not essential that a

preliminary heating be conducted. In either

case the concentrate is heated and filled hot

into cans to prevent spoilage.

As discussed under pasteurization (p. 27),

single-strength juice must be heated to approximately 200° F. to prevent cloud loss. If
36 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

the heat treatment is performed on the concentrate, there is reported evidence (219), generally known to the industry, that less heating time

IS required to inactivate pectic enzymes than

IS necessary for single-strength juice.

Heat treatment is directed not only toward

inactivation of pectic enzymes to prevent cloud

loss but also toward destruction of spoilage

micro-organisms. A common practice is to

flash-heat the single-strength juice as it is fed

to the evaporator, and if the concentrate is to

be preserved in cans, it is reheated to approximately 170° F., filled hot into cans, hermetically

sealed, and cooled. Another practice is to prepare an unheated concentrate, fill into oak barrels or steel drums lined with polyethylene

bags, and hold in frozen storage. When needed,

the product is thawed, flash pasteurized, and

packaged as required.

Concentration of citrus juices is carried out

in either of two general types of vacuum evaporators U35, pp. 677-695), One is a low-temperature, falling-film type, which operates in

a temperature range of 60° to 80° F. (p. 40).

The other is the conventional type, which operates in a temperature range of 100° to 120°.

This type is now nearly obsolete, because the

product obtained cannot compete in quality

with that prepared in the falling-film type of

evaporator.

The two types of concentrates are generally

used for different purposes. The product produced at the higher temperatures is used exclusively in beverage bases, condiments, and

candies. The low-temperature product, being

superior in flavor, is marketed as a concentrated pure-fruit juice in addition to these

other uses. The low-temperature process is

gradually becoming the principal method for

manufacturing citrus concentrates intended for

room or cool storage.

A wide range of concentrated citrus juices

is produced, the degree of concentration depending on the ultimate use. Concentrated

orange juice packaged in No. 10 cans, gallon

cans, or paraffined barrels for bulk handling

is commonly of 60° to 70° Brix, although higher concentrations are available for special use.

The degree of concentration of grapefruit juice

is generally lower than that for orange juice

(253). Highly concentrated grapefruit juice

tends to brown during storage, the degree of

browning being in direct proportion to the

degree of concentration.

Concentrated citrus juices should be stored

at temperatures of 35° to 40° F. to prevent

development of off-flavors and browning. Studies on the storage deterioration of pasteurized

concentrated orange juice indicate no significant effect of degree of concentration (13°-71°

Brix) at either 40° or 60° F. on development of

off-flavors. At 80° and above a marked change

in flavor occurs with increase in concentration

(81), No significant browning is noted in concentrates held 1 year at 40°. At 60° and 80°

darkening increases as concentration increases

(8U8U)^

Other types of spoilage may occur in canned

concentrated orange juice. Cans of concentrate

stored at room temperature develop swells

either by chemical decomposition or by yeast

fermentation (68), Gas formation is caused

by fermentation on storage at 80° and 95° F.,

but not at 40° or 120° (82, 8If).

There are United States standards for grades

of concentrated orange juice for manufacturing

(H3) and tentative grades for canned concentrated grapefruit juice (Jf39),

In the preparation of pasteurized concentrated lemon juice, single-strength juice is pasteurized at approximately 175° F. to stabilize

the cloud and destroy micro-organisms before

it goes to the evaporator. Concentration is

carried out in low-temperature, falling-film

evaporators and conventional vacuum pans.

Low temperatures are especially desirable for

lemon juice to prevent browning during evaporation. Numerous variations are practiced in

concentrating lemon juice. The heated juice

may be fed directly to the evaporator and flash

cooled, or it may be cooled in a separate heat

exchanger and fed to the evaporator as needed.

After the juice is concentrated, it is filled into

barrels and stored at 40°. This product is used

almost exclusively in the bottling trade for soft

drinks.

In the preparation of a concentrate for

lemonade, single-strength lemon juice is mixed

with concentrated juice, pumped through a deoiler, and then rapidly cooled to 50° F. It is

then pumped to a tank, where dry sugar is added to bring the concentration to approximately

50° Brix. This siruplike concentrate is heated

to approximately 160° F., filled into cans, sealed,

and cooled. The concentrate is generally stored

at 40°.

Beverage Bases

Beverage bases have been manufactured for

many years by citrus-fruit processors. The

popularity of these products has been based on

the wide acceptance of carbonated fruit-base

drinks (soda pop). Orange has proved the most

popular and is the most widely used for all

fruit concentrates for beverage purposes. These

fruit concentrates have a wide variety of uses

in the preparation of beverages and foods.
CHEMISTEY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 37

BOTTLERS BASE FOR CARBONATED DRINKS

Juice for bottler's base, which should be low

in peel extractives, is pasteurized at 200^ F.

for about 4 seconds to inactivate pectic enzymes and stabilize the juice cloud, which is essential to achieving a cloudy, fruitlike appearance in the final carbonated drink. The pasteurized juice is concentrated under vacuum at

about 110° F. to 65° Brix. The concentrate thus

obtained is mixed in a refrigerated tank with

the desired amount of sugar, citric acid, citrus

oil, brominated olive oil to prevent separation,

certified food coloring, and benzoate of soda;

homogenized; again pasteurized at 170° F.

(125'^ if benzoated) ; filled hot into gallonsize citrus enamel cans ; and hermetically

sealed. These products should be stored at 55°

to 60° for preservation of their flavor and to

prevent browning. One gallon of such a base

makes from 20 to 100 gallons of citrus beverage

when mixed with carbonated water (192, J^SS),

Carbonated citrus beverages contain approximately 6 percent of actual fruit juice. Their

composition is given in table 17.

TABLE 17.—Average composition of citnisflavored carbonated beverages (433)

Gas

Flavor Samples Sugar volumei

Citric

acid

pH

Gm. per

A'umber ° Brix Units liter

Lemon 348 11.18 3.2 1.20 3.07

Lemon and lime_ 20 11.04 3.2 1.75 3.01

Lime 54 11.10 3.7 2.28 2.90

Lime, lithia 54 9.17 4.0 1.40 3.02

^ Unit volumes of gas dissolved in each unit volume of the

beverage.

PASTEURIZED BASE FOR NONCARBONATED

DRINKS

This product, often called a beverage concentrate, is a mixture of natural juice with

sugar sirup, acidified with citric or tartaric

acid, flavored with citrus oils, and colored with

certified food colors. Some of the better grades

are acidified with lemon or lime juice. The mixture is deaerated, flash pasteurized to stabilize

the juice cloud and prevent fermentation, and

filled into citrus enamel cans, which are sealed

and cooled immediately. To prepare the finished

beverage, generally 1 part of the concentrate

is mixed with 5 parts of water.

BEVERAGE BASES PRESERVED WITH SULFUR

DIOXIDE

There has been an export market, particularly to England, for a limited amount of bases

for use m the preparation of beverages, such

as orangeade, lemonade, limeade, squashes, and

variously flavored barley water. The beverage

base is diluted with water, sweetened, acidified,

and flavored to suit the trade to which the

bottler is catering.

The trade calls for three types of products—

^^clear juice,'' ^'green juice," and "whole fruit

with top oil" (see also p. 29). Clear juice is

made by pressing the cleaned fruit through a

screw press, screening, and separating the undissolved oil. The resulting liquid is treated

with 1,200 p. p. m. of sulfur dioxide and allowed to stand until the clear juice can be

siphoned or decanted off. On analysis the

product will show about 800 to 900 p. p. m.

of sulfur dioxide as a result of handling, and

this amount will be further reduced by the

time it reaches the bottler.

Green juice, or pulp juice, usually consists of

the material obtained by passing peeled fruit

through a screw press and then through a

screen with 1/8-inch holes. It is a turbid product, and may be packed with added juice sacs

if these are desired.

The whole fruit with top oil is similar to the

green juice, except that the fruit is not peeled,

or is only partly peeled, before it is passed

through the screw press. It contains the full

amount of juice sacs and some peel and oil.

These bases are usually packed in paraffined

white oak barrels or in steel drums lined with

polyethylene bags, and preserved with approximately 1,200 p. p. m. of sulfur dioxide.

Citrus beverages prepared from bases as

described are poor sources of vitamin C, or

ascorbic acid (table 18), as compared to canned

single-strength juice or reconstituted frozen

concentrate. They lose their vitamin C content

TABLE 18.—Analysis of citrus-:iuice beverages ^

BrixPro- Contains Soluble Citric acid Ascorbic

ducer juice of— solids acid ratio acid

Mg. per

° Brix Percent 100 ml

A Orange 13.4 0.31 43.2 1.42

B /Orange 12.8 .77 16.6 27.8

1 Tangerine.

/Orange

15.2 .72 21.2 14.8

C 13.1 .96 13.7 38.2

\Grapefruit_ 11.3 1.03 11.0 16.5

D /Orange 13.2 .53 24.9 12.1

\Grapefruit. 11.7 .48 24.4 3.56

E Orange 10.8 .35 30.8 1.95

F .15

.24 ""58'3'

0

G Orange 14.0 .89

H /Lemon ^ 10.4 .32 32.5 .36

) Orange ^ 15.7 .10 157.0 0

'Condensed from Roberts {361),

'^ Carbonated.
38 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

FIGURE 11.—Part of the pilot plant at the Citrus Products Station (U. S. Dept. Agr.) at Winter Haven, Fla.

rather rapidly, losses varying from 7 percent

to nearly 70 percent in a week's time depending

on the product (361). Investigations in this

field have been reported elsewhere (62, 265).

Frozen Concentrates

FROZEN CONCENTRATED ORANGE JUICE

Frozen concentrated orange juice was the

first citrus product of its type to be produced

in large commercial quantities, and continues

to be the leader among frozen juice concentrates. Its development has been so spectacular

that it has been the subject of several feature

stories in popular magazines and of many reports in trade and technical journals. Production began in the 1945-46 season, and increased

steadily until over half the Florida orange crop

is used for frozen concentrate.

The development of frozen concentrated

orange juice came about as a result of a definite need. Pasteurized concentrated orange

juice (p. 35) had been used for some time in

beverage bases and candies, but only limited

quantities were reconstituted into singlestrength juice and used as such. This product

suffered in flavor during manufacture and

would deteriorate rapidly in storage (81).

Frozen single-strength orange juice has been

marketed on a limited scale, and showed good

retention of quality in storage, much better

than the canned product held in common storage. However, it did not strike popular fancy,
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 39

possibly because it freezes solid and requires

time to defrost.

Early in 1944 investigations were begun at

the Citrus Products Station of the United

States Department of Agriculture, Winter

Haven, Fla. (fig. 11), in cooperation with the

Florida Citrus Commission, on methods of improving the quality of concentrated orange

juice. It was found that the usual temperature

(120° F.) at which vacuum evaporators were

operated resulted in ñavor damage during concentration, and that this damage was largely

eliminated if the temperature during evaporation was lowered to 60°-80°. Even though no

oíf-ñavors were formed, the product still lacked

the flavor of fresh juice when reconstituted.

Volatile fractions, especially peel oils, contributing to the aroma and flavor were largely lost

during concentration.

It was found that the addition of a small

portion of fresh juice ^ would restore sufficient

natural flavor so that the reconstituted juice

resembled fresh juice rather closely. Attempts

to recover volatile constituents lost during

evaporation and return them to the concentrate

instead of adding fresh juice to enhance the

flavor have not been successful (301). The use

of carefully selected cold-pressed orange oil

has been shown to restore the volatile flavor

and aroma lost during concentration of California orange juice (35i).

A patent (263) covering the use of low temperature during evaporation and the addition

of fresh juice to the concentrate for the enhancement of flavor was granted and assigned

to the Secretary of Agriculture of the United

States. In the process developed evaporation

was continued until a concentrate of 55° to 65°

Brix was obtained, then fresh juice was added

° First suggested by L. G. MacDowell, director

research of the Florida Citrus Commission.

of

BLENDING

TANKS

HEAT

EXCHANGER

LOWTEMPERATURE

FALLINC-FILM

EVAPORATOR

SS^'-BRIX

CONCENTRATE

EXTRACTOR

CULL S TO

FEED MILL

FEED MILL

SAMPLE TO

»-LABORATORY

SAMPLE TO YI //

LABORATORY \/ /

WAREHOUSE

-lOV.

42*-BRlX

CONCENTRATE

CAN-CLOSING

MACHINE

FIGURE 12.-Flowsheet showing steps in the production of frozen concentrated orange juice.
40 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

until the concentration was reduced to 42°

(± ¥2"^). The fresh juice added, called ^'cutback'' m the industry, amounted to 7-10 percent of the total juice used.

At 0"^ F. the 42°-Brix concentrate is not

frozen solid, but contains considerable ice and

is rather stiff. The heat in the water required

for reconstitution is sufficient to melt the ice,

and the melting ice absorbs enough heat to assure a cool drink. Reconstitution is accomplished by adding 3 cans of water to 1 can of

concentrate, which makes a product of 11.7°

to 12° Brix.

Many of the steps in the production of frozen

concentrated orange juice (fig. 12) are the

same as those in the production of canned

single-strength citrus juices (p. 24). Fruit

is usually received at the plant in bulk in large

trucks holding up to 36,000 pounds. The trucks

are tilted or backed into pits so the fruit flows

out by gravity onto conveyors, where broken,

bruised, or otherwise unsatisfactory fruit is

removed. The fruit is elevated into bins where

it is held until needed, because a limited

amount of storage is necessary for the 24-hourper-day operation. It is processed as soon after

harvest as possible, usually within 24 hours.

Fruit from different bins, which has been previously analyzed for soluble solids and acid, is

mixed in order to give a more uniform product.

The fruit is first immersed in water or sprayed

to wet it and to remove loose particles. Then

a detergent is applied while the fruit passes

over continuously rotating nylon or bristle

brushes. A water rinse follows, and then a

germicide is applied. Water with 12 to 50 p. p. m.

of chlorine may be used, or a 250-p. p. m. quaternary ammonium compound solution may be

applied as a fine mist, to be removed by rinsing

just before the fruit goes to the extractors (27,

US),

Extractors are the same as those used in the

preparation of juice for canning. The juice

passes from the extractors to the finishers,

where pulp, seeds, and rag are removed. Two

cylindrical, screw-type finishers are commonly

used in series. The first finisher has a fine

screen with about 0.020-inch perforations. The

juice from this machine is comparatively free

of coarse pulp and passes to the evaporator.

The pulp from the first finisher, along with

some juice, passes to the second finisher, which

is fitted with a screen having i/ig-inch to %-

inch perforations. This juice is comparatively

high in coarse pulp particles and is used as cutback. The cell fragments make the reconstituted concentrate resemble fresh juice in appearance as well as in flavor.

In some plants juice passes directly from the

finisher to the evaporator, whereas in other

plants it goes to holding tanks of about 1,000-

gallon capacity. If holding tanks are used, they

are of the coldwall type, where the juice is

cooled to about 40° F. Use of holding tanks

permits analyses to be made for solids and acid

as a check on the efficiency of mixing fruit

from the bins to produce juice of desired soluble solids-acid ratio.

Evaporators are of various types, but all

have several features in common and accomplish approximately the same results (381),

All make use of falling-film heat exchangers,

in which the juice runs in a thin film down the

inside of a tube while the tube is gently heated

from the outside. Since the film of juice is thin,

local overheating is prevented. Another advantage is that the heat-exchange rate is high

and the temperature of the heating medium

can be kept low, so that the chance of overheating is further decreased.

The temperature of the juice during evaporation depends on the design of the unit, but

is kept in the range of 60° to 80° F. In order

to accomplish evaporation at these temperatures, evaporators of advanced engineering design have been developed (fig. 13). All reuse

energy in one way or another. Some use the

hot side of ammonia compressors to provide

heat to evaporate water from the juice and the

cold side to condense the vapors (2, 7U, 16U,

220) ; others use large steam boosters or thermocompressors to raise the temperature of the

vapors coming from the juice in one stage so

that they can be used to furnish the heat required for evaporation in the next stage (12y

13). Stainless steel is the standard material of

construction for all parts coming in contact

with the juice or concentrate.

The juice is concentrated to 55° to 63° Brix,

depending on the policy of the manufacturer

or the specifications of the purchaser. Evaporators operate in a continuous manner, with

fresh juice being added at one point in the

system and concentrate being constantly removed at another. The evaporators operate in

2 or more stages, 4 and 6 being quite common,

concentration being carried to a predetermined

point in each stage, and the partially concentrated juice then passing to the next stage,

where more water is removed, and so on until

the desired concentration is reached. These

stages generally operate with a common vacuum source and at about the same temperature.

Some units operate as two or more effects, with

the vapors from one effect providing the heat

of vaporization for the next, and thus save fuel

(220), The volume of juice is held at a minimum in each stage, so that the total time required for evaporation is 20 minutes or less.

The concentrate is collected in a coldwall
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 4|

FIGURE 13.—Battery of evaporators for concentrating orange juice in the preparation of frozen concentrate. (Courtesy of Florida Citms Canners Coop., Lake Wales, Fla.)

tank, where the temperature is held at 35° F.

or below, and here its concentration is determined by a refractometer and expressed as

degrees Brix, corrections being made for temperature and citric acid content (JflS). Cutback juice is then added until a concentration

of 42° Brix (± 1/2°) is attained. If the oil

content is below the desired level (0.025 ml.

per 100-gm. concentrate), cold-pressed orange

oil may be added, care being taken to agitate

the concentrate sufficiently to insure even distribution. When the adjustments have been

made for concentration, peel-oil content, and

acidity, the concentrate is further cooled in the

coldwall tank or a continuous cooler (fig. 14)

to about 20° to 25° F.

Because 42°-Brix concentrate is rather thick

and begins to freeze at about 18° F., pistontype fillers with positive action are used to

handle the viscous product. Steam is injected

beneath the lid of the can before sealing to

sweep out the air in the head space and sterilize the lid. When the steam condenses, a vacuum is developed in the can.

Final freezing may be done while the cans

are conveyed on a perforated belt in an air

blast at —40° F. (fig. 15), or the cans may be

frozen by immersion in a refrigerated denatured alcohol-water mixture; agitation and

liquid contact promote rapid freezing. After

the product has been frozen, it is stored in a

refrigerated warehouse at 0° or below. Similar

temperatures are maintained during transportation and distribution.

To produce a uniform product it has been

found desirable at times to save the concentrate
42 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

FIGURE 14.—Continuous slush freezer for chilling concentrated citrus juices before canning and freezing.

from one part of the season for use when the

solids-acid ratio of available fruit is either too

high or too low to prepare a concentrate of the

desired quality. A concentrate stored for such

purpose has come to be known in the industry

as "addback." Sometimes addback may come

from a different producing area. A concentrate

of 55° to 63° Brix is filled into bulk containers

(5 to 55 gallons) lined with polyethylene bags

and placed in frozen storage. When needed,

the concentrate is brought out, thawed, and

mixed with fresh concentrate along with the

cutback juice.

Concentration by Freezing

Concentration by freezing has attracted the

attention of engineers for some 35 years, and

as long ago as 1914 Gore concentrated cider by

this method (132). The process is interesting

because no heat is applied, which might affect

the delicate flavor, and theoretically less energy

transfer takes place in freezing than in evaporating water. Considerable effort has been

made to develop this method on pilot-plant and

full-plant scale, but commercial operations have

been limited. Sufficient material has been processed and tested in market surveys to indicate

that an acceptable product can be produced in

this way.

However, several difficulties have been encountered in commercial application of the

method, and some of the expected advantages

over vacuum evaporation have not been realized. Much of the suspended matter and soluble solids remain in the ice and leave the concentrate light in color and low in characteristic

orange flavor. Large crystals facilitate separation of the ice, but such crystals form slowly

and much of the product needs to be held in

the processing operation. The degree of concentration that can be attained in a single

freezing and centrifuging is limited, but it can

be further increased by freezing the concentrate and centrifuging again. Usually two

freezings are necessary to obtain a concentrate

of 45° Brix. The ice from the second freezing

needs to be reworked in order to avoid excessive loss of soluble solids. Flavor and texture

can be restored with pulpy cutback, but the

amount that can be used is severely limited by

the impracticability of overconcentrating, as in

the case of vacuum concentration.

Most of the work has involved freezing in

containers immersed in refrigerated brine

{A08), but the application of high vacuum to

induce rapid evaporation to cool and freeze

has also been tried. A "step-freeze" process

{2^6) has been developed, in which freezing

occurs rapidly, and the product is conducted

automatically from one stage to the next.

Quality Control

Control of quality during the manufacture of

frozen concentrated orange juice is most important, and beginning with the selection of

fruit it continues through all operations. Care

must be taken to see that fruit of proper maturity is being purchased to satisfy the requirements of grade standards and any special

standards of the processor or purchaser. The

usual tests for maturity include determination

of soluble solids and citric acid and calculation

of the Brix-acid ratio. Current United States

standards for fancy, unsweetened, frozen concentrated orange juice specify that this ratio

must be not less than 11.5:1 nor more than

18:1; these standards also specify a minimum

and maximum citric acid content {HI). In

Florida a State law prohibits the addition of

sugar to this product, and therefore the fruit

must produce a concentrate within the above

ranges to receive this grade. Fruit is sometimes purchased on the basis of soluble-solids

content, because juice of higher soluble solids

requires less evaporation of water to reach the

desired concentration, and manufacturing costs

are reduced. In addition to reduction of such
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 43

FIGURE 15.—Air-blast freezing tunnel for freezing concentrated orange juice.

Corp., Orlando, Fla.)

(Courtesy of American Machinery

costs, a higher yield of concentrate is obtained

from a given number of gallons of fresh juice

U70).

Generally the field buyer has a good idea of

the condition of the fruit, and often "grab

samples" may be taken from the top of the load

in the yard to check its suitability. A larger

and more representative sample is automatically taken as the fruit is conveyed from the

trucks to the storage bins. Solids and acid content and juice yields are determined. Advantage is taken of the information regarding the

analysis of fruit in the bins in order to maintain by blending a Brix-acid ratio within as

close limits as possible.

The next point of sampling is the blending

tank, and here again there is opportunity to

mix juices of high and low ratio.

Concentration at each stage of evaporation

is checked periodically to see that the plant is

operating properly. A Brix hydrometer is used

to determine the soluble-solids content of fresh

juice and a refractometer is used for concentrates. Special care is taken to check the Brix

of the concentrate leaving the evaporator and

after blending with the cutback juice. Samples

for analysis of recoverable oil are taken after

the cutback juice has been added. Oil levels

are usually maintained at about 0.025 ml. per

100 gm. of concentrate. Maintaining such

levels can be controlled to a large extent by

the method of extraction of the cutback juice,

and can be supplemented by the addition of

cold-pressed orange oil. Further tests may be

made to determine the tendency to clarify and

the possibility of gel formation {J^J^} during

storage.

Sanitation is an important factor in quality

control (27, iS) (see also p. 45). Great care

must be taken to insure that the fruit carries

a minimum number of organisms into the

plant, that decayed or bruised fruit is removed

at the inspection table, that equipment is kept

clean and sanitary, and that micro-organisms

are not allowed to develop in appreciable numbers at any stage during processing (112, Í69,
44 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

U79, U82, Í83). A close check should be kept

of the concentrate being produced, and microbial counts should be made regularly. The frequency of such checks depends on the size of

the operation and specifications of the manufacturer, but several samples should be taken in

each shift. Periodically surface counts should

be made of the unwashed and washed fruit,

conveyors, and belts to check the effectiveness

of sanitary procedures in use.

Since agar-plate counts require 2 to 3 days

before the number of colonies can be determined, this method has been supplemented by

more rapid direct counts H80), Although

these direct counts are not so accurate as plate

counts, they provide an immediate check on

plant conditions.

Organisms have been found that are capable

of developing in the first stage of evaporation,

and they cause disagreeable off-flavors (113,

161, 162, 305). At times the incidence of organisms of this type has been discovered by

direct count or by detection of diacetyl (182,

Jf^69) before the numbers reached serious proportions.

Each plant establishes a regular cleanup

schedule, based on experience in the plant and

especially on results of routine bacteriological

tests (27, i3). The schedule varies with the

arrangement of the equipment and the condition of the fresh fruit, and is more frequent in

warm weather than in cool.

Another problem associated with extended

operation is the formation of a film of hesperidin (p. 61) on the tube surfaces of the fallingfilm evaporators. Not only does this film interfere with the rate of heat exchange (slowing

the rate of production) but if allowed to form

in sufficient thickness it will flake off and appear in the finished product as harmless but

unsightly white scales. Formation of hesperidin scales has at times forced shutdowns and

necessitated doubling the frequency of cleaning.

Cleaning an evaporator begins with washing

with warm water to remove adhering concentrate, followed by treatment with warm, dilute

caustic soda to remove hesperidin and other

films, a thorough scrubbing by hand of any

remaining unclean surfaces, and final rinsing

with clean water. A detailed inspection is

made to check the eflficiency of the cleaning.

Juice extractors are cleaned more frequently,

once each 8-hour shift or more often. Some

plants have an extra row of extractors, and

one row at a time may be out of service for

cleaning. Most extractors are equipped with

spray nozzles to enable rinsing with highpressure, chlorinated-water sprays during brief

periods while juicing is stopped. Belts and

other conveyors are kept wet with chlorinated

water to prevent growth of micro-organisms.

A frozen concentrate will maintain its quality during storage at 0^ F. for a year or more.

At 5° changes are slow but perceptible. At

10° or above the rate of change increases considerably, and if the concentrate is allowed to

remain for long at this temperature, deterioration becomes significant (101, 297), The first

evidence of deterioration is a loss in the

amount of suspended matter or cloud. As deterioration proceeds the upper portion of the

reconstituted juice will become completely clear

in 5 to 10 minutes after mixing. The concentrate may also exhibit gelation in the can.

This is noted first by a slight lumpiness or

weak gel, but in extreme cases the concentrate

will retain its form even after having been removed from the can (365, í71). Off-flavors

also develop under conditions permitting these

physical changes, but are not evident until

clarification is well advanced.

As previously pointed out (p. 6) gelation

and clarification are considered to be associated

with the action of the enzyme pectinesterase.

Methyl groups are split off from the pectin

molecule to produce low-methoxyl pectin, which

in turn reacts with normally occurring calcium

or magnesium ions to form a gel. In concentrated orange juice favorable pH conditions

exist for this type of gel formation.

If the product is kept at 0° F. or below, gelation and clarification do not take place. The

rate of deterioration at higher temperatures

depends on whether or not a heat treatment has

been used, the variety of fruit (a concentrate

from seedy varieties deteriorates more rapidly), and probably its maturity and environm.ent

during growth.

Since storage of frozen concentrated orange

juice at 0° F. or below is recognized as highly

desirable, it is advisable to provide additional

stability as insurance against inadequate refrigeration for brief periods during distribution or while the product is in the hands of

consumers. One aid is the avoidance of excessive pressures during extraction and finishing

of the juice (323), Most processors apply heat

to evaporator feed juice to inactivate at least a

portion of the pectinesterase present. Temperatures of 145° to 160° for 5 to 30 seconds have

been used commercially. This materially delays clarification and gelation under adverse

storage conditions. Heat has been applied by

steam injection (Í5), as well as by tubular and

plate-type heat exchangers (fig. 16). Cutback

juice is not heated. Considerable information

on the eflfects of various heat treatments on

pectinesterase and concentrate stability is

available (33, 219, 338, 367, 368),
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 45

FIGURE 16.—Plate-type heat exchanger.

Opinions differ on the advisability of partial

enzyme inactivation to provide increased stability. Some claim that this technique will increase the tendency of handlers to use lower

quality fruit and to abuse the product. Some

think that heating causes significant flavor

damage, whereas others believe that any heat

damage either is too small to be recognized by

the average consumer {35A) or is overshadowed by the better average condition of the

product as purchased.

It has been observed {33, 70, 219) that increasing the concentration from the usual 42°

Brix to 51° or 59° decreases the tendency of a

concentrate to clarify. A high-density, frozen

concentrated orange juice that is full flavored

can be prepared by flavor fortification with

cold-pressed peel oil {355). Developments in the

field of low-temperature evaporation have also

made possible the production of higher density

products by the cutback process. Experimental

studies have indicated that the higher density

concentrates not only have more cloud stability

but flavor stability tends to increase with concentration {102, 355).

Microbiology of Frozen Concentrated Orange

Juice

Bacteriological control problems assunied

real proportions when a rapidly expanding

frozen-concentrate industry was started in

1945. The production of an ever-growing volume of unheated concentrate was accompanied

by its quota of microbiological problems, which

have reached the attention of both research

and quality-control laboratories.

Although quality-control procedures vary

from plant to plant, it is a common practice to

determine the total microbial count of the finished product, and other determinations are

included according to the dictates of the plant

management. Bacteriological control methods

used in one plant have been described {27),

and the role of sanitation in the quality control

of frozen citrus concentrates has been further

discussed in a survey of methods used by 15

processing plants in Florida {AS).

Total Counts.—Investigations of methods for

the detection of total numbers of micro-organisms in citrus products have followed several

difl'erent lines, including the development of

suitable plating media, adaptation of the Frost

little-plate method, a polaroscopic method, and

the adaptation and development of direct microscopic methods.

The need for a standard plating medium for

the examination of citrus products was predicated by the fact that difl'erent media were

used by bacteriologists to obtain total counts

{181, Suit). Different incubation temperatures

were used, which varied from room temperature to 86.0°-89.6° and even 98.6° F. This

diversity of media and incubation temperatures

was unfortunate, as it precluded uniform results from one laboratory to another. In selecting a medium for the enumeration of microorganisms in citrus products, due consideration

must be given to the fact that a diversity of

microbial types may be found in the products

and that many of them have rather fastidious

nutrient requirements. It was concluded that

no single medium would equally support the

growth of all types of micro-organisms found

in citrus products. The possibility exists that

abnormally high counts resulting from a buildup of contamination in plant equipment might

not be reflected in the counts obtained during

routine examinations if a single medium were

used {181).

The development of orange serum agar resulted in a medium that not only gives consistently high counts but favors the growth of

certain organisms capable of multiplying in

orange juice and causing undesirable changes

{28, 162). A comparison of 18 different media

has demonstrated that orange serum agar gives

some degree of differentiation among yeasts,

Leuconostoc, and lactobacilli organisms, and

maximum counts are obtained {30J^). Orange

serum agar is now available in a stable, dehydrated form {Al2).
46 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

Plate counts have the disadvantage that an

incubation period of at least 2 days is required

before many of the colonies develop to detectable size. Microbial buildups may occur in

plant equipment during this incubation period,

and the product may be damaged before the

abnormal conditions are detected. This situation has given impetus to a number of investigations aimed at shortening the time necessary

to detect abnormal microbial populations in the

juice. One of these investigations was concerned with the possibility of using the oxygen

uptake of ora ige juice as a means of estimating its microbial population (172). Although

early results showed promise, subsequent experience with the method found little correlation between plate counts and oxygen uptake.

It was believed that lack of correlation was due

to differences in oxygen requirements by the

different types of micro-organisms found in

orange juice, and to the fact that these microorganisms are not always present in the same

proportions.

An adaption of the Frost little-plate method

for the enumeration of micro-organisms in

orange juice has reduced to 6 or 7 hours the

time required to obtain a viable count. In this

method inoculated orange serum agar is applied

in a measured volume of the material to a definite area of a microscope slide. After incubation takes place, the slide is heated to dry the

agar. It is then stained and examined under

the microscope. Good correlation has been

found between this method and regular plate

counts {S69).

Many investigations have been made to develop a satisfactory method for the direct microscopic examination of orange juice. One

problem is that the juice may be washed from

the slide during the staining process. The combination of a bacteria-free gelatin solution

mixed with the juice helped to cement the film

to the slide and made possible a more dependable enumeration of micro-organisms in the

juice (J^IA),

In the direct microscopic examination of

frozen concentrated orange juice, a modification of North's aniline oil-methylene blue stain

has been used by a number of bacteriologists

(162). Two later methods recommend the mixing of a dilute stain solution with the juice and

drying 0.01 ml. of the mixture on a microscope

slide (182, 480). Slides prepared in this manner require no rinsing, and there is no danger

of washing off the microbial cells. The principal difference between the last two methods is

that one (182) uses 0.075-percent crystal violet

stain solution and the other (Í80) uses a mixture of 10 ml. of North's aniline oil-methylene

blue stain and 2 ml. of a 1-percent alcoholic

solution of basic fuchsin, mixed and diluted to

100 ml. The latter method produces a preparation with greater color contrast and is preferred by many workers.

The phase-contrast microscope has been used

for the direct microscopic examination of

orange juice, but its use has been largely discontinued. The alinement of the phase microscope with its light source is very critical, and

often considerable time is required to adjust

the instrument properly. Also, particles of

orange tissue of similar size and shape may

be difficult to distinguish from microbial cells.

At least one plant in Florida has a phase-contrast microscope and light source clamped into

permanent alinement, which has been satisfactory for estimating the microbial quality of

orange juice. However, most workers prefer

to examine a well-stained slide under the conventional bright-field microscope.

All these methods have been used to obtain

estimates of the microbial content of citrus

products or to gain information regarding the

bacteriological quality of the products. Fruit

quality is of prime importance in the production of jui<îe having good bacteriological quality. Orange juice aseptically produced from

''soft rot" fruit had an initial viable count of

2,500 times that of juice prepared from apparently sound oranges (483). The necessity of

thorough sorting and eliminating unsound fruit

has been indicated by reports of high plate

counts from juice aseptically extracted from

oranges that appear normal on casual examination but upon more careful observation were

found to have soft spots (162). Other work

has shown that certain acid-tolerant bacteria

will multiply when inoculated into oranges on

the tree, and that juice with high microbial

content may be obtained from these oranges

even though they appear to be sound, normal

fruit (113).

Although the fruit itself may be largely responsible for the wide variation in microbial

numbers often found during the course of a

day's operation, the role of adequate sanitizing

of plant equipment in the production of concentrates of good microbial quality cannot be

neglected (27, 162) (also see p. 43).

Off-Odor and Off-Flavor' Spoilage.—A microbial spoilage characterized by a ''buttermilk" odor and flavor has proven troublesome

at times in the manufacture of frozen concentrated orange juice. It is caused by the growth

of certain lactic acid bacteria in the juice. The

lactic acid and diacetyl produced by these organisms are responsible for the buttermilk

flavor. A method for detecting this type of

spoilage is based on a colorimetric estimation

of diacetyl (182, 469). The bacteria involved
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 47

have been identified as Lactobacillus brevis, L.

Vldnatarum var. mobilis, Leiiconostoc clextranicum, and L. mesenteroides (162), Rapid

means for generic identification of the organisms causing buttermilk flavor in frozen concentrated orange juice have been investigated

(29).

Control measures consist of thorough washing and sorting of the fruit, thorough cleaning

of fruit- and juice-handling equipment, and

frequent sanitizing of the evaporators. Much

of the trouble from these bacteria comes from

their growth in adhering material in the evaporators, and when careful attention to plant

sanitation and fruit sorting does not eliminate

the source of the trouble, it may be necessary

to flash-heat the evaporator feed juice.

Bacteria of Sanitary or- Public-Health Significance.—Man's concern with his physical

well-being has led to investigations of the survival of pathogenic bacteria in orange juice.

Survival of enteric pathogens in orange juice

was found to be dependent on the temperature

at which the juice is stored. At low temperatures (25° F.) certain of the enteric pathogens

survived for several days in single-strength

juice, w^hereas at normal temperatures the hydrogen ion concentration (pH) of the juice

proved to be rapidly lethal to the micro-organisms (22). When concentrated orange juice

was inoculated with pathogens and with fecal

material and then frozen for 48 hours, Escheri'

chia coliy Salmonelkt typhosa, and Shigella

paradysenteriae could not be isolated (138).

Enterococci were found to survive in frozen

orange concentrate longer than the other enteric organisms. However, the micrococci

found in uninoculated concentrate were not

predominantly of the type common to the intestinal tract (211).

Because coliform organisms are used by

public-health workers as an index of human

contamination, a number of investigations have

been made of the incidence, significance, and

survival of coliforms in frozen citrus juices

(27, 28, 112, 138, 139, 26J,, 27U. 320, 327-329,

Í30, 479, J,81-i83). Several papers have appeared on methods of isolating these organisms, especially E. coli, from frozen concentrates (28, 27Jf, U81, U83). The most satisfactory method uses boric acid broth as the initial

enrichment medium (hU9).

All types of coliform organisms have been

found in frozen single-strength and concentrated orange juices, but the Aerobacter types

were more common than was E. coli (320, 328,

Í81, 483). They are also associated with unsound or damaged fruit (329, 483). In one

study on the sources of coliform organisms

in citrus products it was established that the

bacteria may be associated with scale infestations, fruit with slightly decomposed areas

ruptured fruit, and fruit flies (328).

Coliform organisms have been found to survive in frozen concentrated orange juice for

extended periods when the storage temperatures are maintained at 0° F. or lower. When

subjected to fluctuating temperatures encountered in the distribution and marketing of the

product, the coliforms die off more rapidly and

should be largely eliminated by the time the

juice is consumed (27). Also, the flash-heating

of the evaporator feed juice, which is now

widely practiced, acts as a further safeguard

against coliform contamination of frozen concentrate.

FROZEN CONCENTRATED LEMON JUICE

This is a pure-juice product made by concentrating single-strength juice in a low-temperature evaporator (p. 40), chilling to around

30^ F. by passage through a heat exchanger,

filling into drums lined with polyethylene bags,

and freezing at —10^. Sometimes the chilled

concentrate is filled into cans and frozen for

institutional uses.

FROZEN CONCENTRATE FOR LEMONADE

Frozen concentrate for lemonade is rapidly

becoming a leading food item. Unlike orange

juice, lemon juice is always diluted with water

and sweetened before drinking as a beverage.

Frozen concentrate for lemonade has the proper amount of added sugar to reconstitute this

product as a ready-to-drink lemonade. The

frozen concentrate is principally a singlestrength lemon juice with sugar added. However, since most people prefer a tartness in

lemonade, the acid content of the product is

adjusted by the addition of a small amount

(approximately 10 percent) of concentrated

lemon juice to give the proper balance of sugar

and citric acid.

A typical frozen concentrate for lemonade

M^ould be prepared as follows: Add sufficient

concentrated lemon juice to 280 gallons of

single-strength lemon juice and 2,800 pounds

of granulated sugar so that each 100 gm. of

the final product will contain from 3 to 3.5 gm.

of citric acid. This will make approximately

500 gallons of 55^-Brix concentrate. The concentrate is reconstituted to lemonade by adding

4 volumes of water to each volume of concentrate.

It is desirable that some juice cells should

be added to this concentrate to improve the

appearance of the reconstituted lemonade. This

is done by screening juice cells from juice after
48 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

extraction and mixing them into the concentrate.

FROZEN CONCENTRATE FOR LIMEADE

Frozen concentrate for limeade was introduced to the trade in 1951. It consists of Hme

juice to which sucrose has been added to raise

the Brix to about 48"^. Some processors apply

a mild heat treatment during preparation, and

subsequently freeze and store the product at

0° F. or lower. On reconstitution with 4 to

41/2 parts of water, an excellent limeade is obtained. As limes vary somewhat in solids and

acid characteristics, some processors concentrate batches of juice for blending to maintain

a uniform acid content in the finished product.

Then by the addition of sugar to a 48°-Brix

level, a uniform composition is attained. When

properly prepared the product is excellent and

has been well received by the public. Two additional lime-juice products were developed by

the United States Department of Agriculture

in 1954 (35), at least one of which is now in

commercial production.

During concentration of lime juice much of

its characteristic aroma and flavor are lost.

Efforts to regain the original flavor through

the addition of fresh juice, as is the practice in

the production of frozen concentrated orange

juice, or through the use of emulsified lime oil

were unsatisfactory. The use of lime puree

(p. 49) in amounts sufficient to give an oil

content of 0.003 percent in the reconstituted

limeade proved most satisfactory. The two

products prepared by this method (35) include

an 8-fold sweetened concentrate, which requires only the addition of 7 parts of water to

prepare the beverage, and a more concentrated

product, which is diluted to 35 times its volume

by the addition of sugar and water. Preparation of the products is as follows :

To make 100 gallons of an 8-fold sweetened superconcentrate, use 6 gallons of lime

puree, 42 gallons of a 2.06-fold concentrated

lime juice, and 691 pounds of beet or cane

sugar. For the finished beverage add 7 volumes

of water to 1 volume of the sweetened superconcentrate.

To make 100 gallons of an unsweetened superconcentrate, use 26.3 gallons of lime puree

and 73.7 gallons of a 5.12-fold concentrated

lime juice. For the finished product add 301/4

pounds of sugar to 1 gallon of the superconcentrate and dilute to 35 gallons.

FROZEN CONCENTRATED TANGERINE JUICE

Tangerines are handled in the same manner

as described under canning of tangerine juice

(p. 29). Adjustment of extraction machines is

necessary to accommodate tangerines, and the

yield of juice is only about half that from oranges. To keep evaporators running at full capacity it is therefore necessary to have about

double the number of extractors used for oranges. Frozen concentrated tangerine juice

shows little tendency to gel, and it is unnecessary to heat-inactivate enzymes prior to concentrating.

FROZEN CONCENTRATED GRAPEFRUIT JUICE

Production is carried out with essentially the

same equipment as for frozen concentrated

orange juice. Frozen concentrated grapefruit

juice has a tendency to gel or clarify because

of the presence of pectinesterase (p. 6), and

therefore the juice is heated to 150^-180'^ F.

for a few seconds before it goes to the evaporator. Both sweetened and unsweetened frozen

concentrates are produced commercially.

Frozen Purees

Frozen fruit purees for use in frozen desserts, baked goods, and other foods have been

in commercial production since 1937. Citrus

fruits, however, have not been included among

such products, largely because they were

thought to be too strongly flavored, would not

keep well in storage, and would develop a

'^terpeny" off-flavor because of the high oil

content resulting from the use of whole fruit

in the pureeing process. However, it has been

shown that frozen citrus purees, if properly

prepared, will not develop off-flavors (23, 31),

and such products are now being produced

commercially in California and Florida.

Freezing preservation of citrus-fruit purees

has proved to be a highly efficient and economical method for the preparation of fruit bases

possessing natural flavors, color, and nutritive

value. These sweetened or unsweetened fruit

bases can be kept in good condition at 0° F. for

more than a year, with very little, if any,

loss of the original vitamin C content. The

frozen puree can be defrosted, some of it used,

and the rest refrozen without injury to color

and flavor, provided it is not exposed to the

air for long periods and is not allowed to remain at room temperature too long.

The processing method developed for the

preparation of frozen citrus purees is a comparatively simple but effective means of retaining color and flavor in the finished product.

Sound, fully mature fruit is first thoroughly

washed, preferably with a good detergent, and

rinsed well with clean cold water to reduce

microbial contamination to a minimum. With

certain citrus varieties purees made from immature fruit have a tendency to be bitter.
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS

Medium to small fruit is preferred because of

the better yield and quality of the juice. After

the fruit is washed, the stem end may be cut

off and discolored spots removed so that no

dark specks will be mixed in with the brightcolored puree. If Washington Navel oranges

are used, the navel end should first be removed.

After the whole fruit has been trimmed, it

is either crushed in a machine, such as an apricot pitter, or sliced by means of circular saws.

The crushed or sliced fruit is then put through

a rotary {2Í7) or tapered screw press fitted

with stainless-steel screens of appropriate

mesh, so that most of the fruit is reduced to

the form of a puree. Other machines may be

available to accomplish the same purpose.

Screen openings of 0.027 to 0.044 inch in diameter are usually employed, depending on the

end use of the puree. Screen sizes of 0.027 and

0.033 inch are preferable when purees are intended for sherbets, ices, pies, and beverages,

but larger sizes are better for purees intended

for marmalades, jams, cake, and sundae toppings.

Yield of puree from the whole fruit is approximately 50 to 60 percent, and should contain 0.40 to 0.75 percent of peel oil depending

on the variety of fruit. With some lots of fruit

the peel-oil content of the puree may be considerably higher and too strong for most uses.

Then the oil content of the puree can be controlled if various proportions of the fresh fruit

are passed through an abrasive machine prior

to crushing to remove most of the oil sacs or

ñavedo. Another method of standardizing the

oil content of purees is by adding different proportions of single-strength juice.

After a puree of the required oil content is

obtained, it is run into a stainless-steel tank,

and dry sugar gradually added with thorough

mixing in the proportion of 1 part of sugar to

5 parts of puree. Purees may be packed without the addition of sugar. The sweetened or

unsweetened puree is filled into enameled tin

containers of 1 to 21/2 gallons' capacity, the cans

are hermetically sealed or closed with slip tops,

and the contents are frozen at subzero temperatures. The product is stored at 0° to —10° F.

The time of freezing can be reduced if the

puree is first frozen to a slush in a mechanically agitated heat exchanger before filling into

cans.

Orange, tangerine, lemon, and lime purees

have been successfully used in the commercial

preparation of milk sherbets and water ices.

However, taste tests have indicated that milk

sherbets that contain approximately 2.5 percent of butterfat have a more pleasing flavor

and texture than water ices. In sherbets and

ices made from citrus purees, minute pieces of

49

orange, lemon, and lime peel are detectable,

which provide visual evidence to the consumer

that the products are flavored with natural

iruits.

Lemon puree is particularly useful as a flavor base for lemon pies. Frozen purees also

have been used m the preparation of puddings

and cakes. Lemon and lime purees have been

used by the beverage trade for the manufacture

oí ades and similar fruit drinks.

Powdered Citrus Juices

The dehydration, or drying, of citrus juices

to a powdered form has been of considerable

interest for many years because of the numerous advantages of such a product. It has been

accomplished by several methods, but it appears that in addition to the quality of the

powder the economics of the drying process

will dictate the commercial success of any process or combination of processes. Since citrus

juices consist largely of water, it is not economical to attempt to dry them directly to a

powder. A more economical procedure is first

to concentrate the juice in a vacuum evaporator, then remove the remaining water by a

vacuum drying process to avoid heat damage

to the flavor. The sixfold concentrate now

being produced by low-temperature evaporation for the manufacture of frozen citrus concentrates provides an excellent starting material for the preparation of a powder {262),

Citrus-juice concentrates have been converted to powdered form by spray-, drum-, and

freeze-drying. However, dehydration was accomplished by the addition of drying aids to

promote the rate of drying, to counteract hygroscopicity of the dried product, and to reduce

the tendency of the powder to cake during

storage. Although drying aids, such as carboxymethylcellulose {105) and glyceryl monostearate {íl6)y in concentrations as low as 1

percent on a total-solids basis have been used

experimentally, the results were characterized

by low yields, flavor changes, and a powder

difficult to reconstitute. Other drying aids,

such as corn-sirup solids, pectin, and whey,

have been used, but in such large amounts that

they altered the nature of the powder.

During World War II lemon juice mixed

with corn-sirup solids was spray dried commercially, and a powder was produced that

contained about 20 percent of lemon-juice

solids and 80 percent of corn-sirup solids. In

addition, some lemon oil and crystalline ascorbic acid were added to the dry powder before packaging. This product is still being

produced as a flavor base for the confectionery

and soft-drink trades. Later an orangeade
50 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

powder was prepared by spray-drying, which

contained 25 percent of orange-juice solids and

75 percent of corn-sirup soHds (188).

Drum-drying has been judged unsuitable for

preparing acceptable citrus-juice powders,

since such powders are too difficult to reconstitute (295). High production costs and large

capital investments in equipment have deterred

the commercial exploitation of freeze-drying

citrus juices to a powder (56, 120). Although

powders prepared experimentally by freezedrying were readily reconstituted, they were

reported to show a tendency to cake and develop off-flavors during storage (119).

In 1943 a method was developed for the

preparation of powders made from fruit juices

mixed with corn sirup (175). This method

was the forerunner for the development of several processes in which citrus juices were dried

in a puffed, or expanded, form (51, 159). One

of these methods consisted of spraying 50^-

Brix orange concentrate on the walls of a jacketed, cylindrical, vertical drier under a vacuum

(71, 379, 382). The film of concentrate on the

walls was of a thickness representing a yield

of 0.26 pound of dried powder per square foot.

Water at approximately 125^ F. was circulated

through the jacket during drying. The concentrate was allowed to dry until it reached the

desired moisture content, then it was scraped

off the walls and removed through an air lock

into an airtight dolly. For a given cycle time

a single drier could be duplicated to give a

sufficient number of units so that the concentrate could be charged to the system and the

powder removed continuously.

A commercial plant was constructed with a

capacity of 5,000 pounds of powder per 24

hours, in which the normal operation was to

charge each drier in succession, dry under an

absolute pressure of 400 to 600 microns for 2

to 6 hours until it reached the desired moisture

content, discharge the driers, and repeat the

cycle. The powder was usually removed at 1.5-

to 2.5-percent moisture levels. In-package desiccation was used to increase stability (71), a

technique previously described by Howard

(190).

In 1947 a continuous belt-type vacuum drier

was specially designed for drying orange juice

in a puffed form (399).

A process employing a vacuum shelf drier

has been described for the preparation of a

dehydrated orangeade powder (3^0,416). The

juice was first concentrated in a vacuum evaporator to remove 97 percent of the water, then

mixed with 2 to 21/2 times its weight of dry

sugar to form a moist granular mass. The addition of calcium carbonate in a ratio of 1 part

to 8 parts of acid in the juice aided drying.

Sufficient ascorbic acid was added to the mixture to provide a total of 0.05 percent in the

finished beverage. This mixture was dried in

shallow pans in a vacuum shelf drier at 122° F.

at about 20 mm. of pressure. Its moisture content was reduced to 0.5 percent. Sufficient anhydrous citric acid was added to give a citric

acid content of 3.5 to 4.0 percent.

The addition of 0.006 percent of fivefold concentrated orange oil in the form of a 10-percent

extract gave an acceptable flavor with good

stability. The dried powder was reconstituted

by adding 6 or 7 parts of water to 1 part of

powder. Samples showed an almost complete

absence of darkening during storage for 5

months at 98.6° F.

Later a method was developed by the United

States Department of Agriculture for puff-drying liquid materials in a vacuum shelf drier at

approximately 1 mm. of mercury pressure in

such a manner that the product puffs during

drying to form an expanded honeycombed or

spongy structure (1,17, Ifl8) (fig. 17). The

feed juice consists of 58°- to 60°-Brix orange

concentrate produced by low-temperature evaporation. The concentrate is spread evenly over

the surface of shallow trays to obtain a film

approximately X\Q inch deep, or a tray loading

of 0.5 pound per square foot. The shelf drier

is so constructed that steam or hot water can

be circulated through the shelves to supply heat

to the trays of the concentrate. Temperatures

encountered in a typical drying cycle in which

the shelves were preheated to 200° F. and

maintained at that temperature during the

early stages are shown in figure 18.

After the product is dried for 90 to 100 minutes, the moisture content is reduced to about

3 percent, and then the product is cooled to

below 90° F., the vacuum is released, and the

trays are removed from the drier. During the

drying process the original i/ig-inch layer of

concentrate has been expanded 15 to 20 times

in volume. After the product is cooled, it is

quite friable and is easily removed from the

trays. It is very hygroscopic, and handling and

packaging must be carried out in a room maintained at not more than 15-percent relative

humidity to prevent uptake of moisture.

As previously pointed out practically all the

characteristic flavor and aroma of citrus juices,

particularly orange, are derived from volatile

oils, most of which come from the peel (flavedo) of the fruit during extraction of the juice.

These volatile oils are largely lost during concentration of the juice in the preparation of a

powder, which is one of the principal reasons

why most citrus powders prepared to date have

been lacking in natural flavor and aroma. The

success of the United States Department of
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 5|

FIGURE 17.—Orange juice is dried in a vacuum shelf drier under a high vacuum so that the product puffs to form

an expanded spongy structure.

Agriculture method for preparing orange powder is largely dependent on the manner of returning to the powder essential flavoring oils

lost during processing.

A process for incorporating fruit flavors and

flavoring oils into sorbitol has been patented

{13U). In this process orange oil is added slowly to molten sorbitol, which has been heated to

about 392° F. to remove water and then supercooled to around 194°. The emulsion of oil in

hquid sorbitol is then allowed to crystallize, and

the mass crushed to granules 10 to 20 mesh in

size. The sorbitol-oil granules are then mixed

with the orange powder before packaging.

Further research by the United States Department of Agriculture has shown that glucose or

sucrose could be used in place of sorbitol.

In-package desiccation appears to be indispensable to the preparation of a stable oraujgejuice powder. It is accomplished by seahng

orange powder of 3- to 4-percent moisture as it

comes from the drier in a hermetic container

with a sufficient amount of desiccant to reduce

the residual moisture in the powder to 1 percent or lower. The desiccant, calcium oxide or

calcined lime, is enclosed in a siftproof, moisture-permeable paper container, which permits

transfer of water vapor from the product to

the desiccant without contaminating the product. This desiccant will react with 82 percent

of its weight of water during conversion to calcium hydroxide and will maintain virtually

zero humidity until completely reacted. Since

calcined lime will increase in volume on rehydration, the container must be so constructed

that it will not burst with the expanding lime

and contaminate the product (166).

Experiments have indicated that puffed orange powders could be produced with a continuous belt-type vacuum drier, and a commercial plant was built that utilized this type of

equipment. Orange powders, with and without

the addition of drying aids, were produced with

both shelf and belt-type vacuum driers under

conditions that caused the concentrate to puff

during drying (A).

Samples of orange powder prepared by puffdrying and containing 40 percent of added

corn-sirup solids were stored at 70°, 90°, and

100° F., and assayed periodically for chemical
200

160

\ ^SHELF TEMPERATURE

\

PRODUCT TEMPERATURE

^ INCH ABOVE TRAY

20 40 60 80

TIME IN MINUTES

100 120

FIGURE 18. Time-temperature cycle for converting ßO^-Brix orange concentrate into orange powder of 3-percent

moisture content, with a tray loading of 0.5 pound per square foot and at 2 mm. of pressure.
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 53

and organoleptic changes (306), In-package

desiccation that reduced the moisture content

of the powder to 1 percent or lower was very

effective in reducing flavor deterioration, undesirable color changes, and loss of ascorbic

acid during storage. Incorporation of the orange oil in sorbitol was effective in stabilizing

the added oil during storage. Samples of

orange powder were still acceptable after 6

months' storage at 100° and had changed very

little after 1 year at 70°.

CITRUS BYPRODUCTS

Pectin

PECTIN MANUFACTURE

Over 2 million tons of citrus peel, containing

about 3 percent of pectin on a fresh-weight

basis, is produced each year in the United

States as a byproduct of the citrus-processing

industry. Approximately 5 million pounds of

citrus pectin is produced yearly from this

source, mainly in California.

Pectin manufacture is an expensive and complicated process, consisting of numerous batchtype operations, use of corrosive chemicals and

expensive solvents, and filtration of exceedingly

viscous solutions. The principal steps include

preparation of the peel for extraction or storage for later use; removal of bitter glycosides

and crude sugars ; conversion of protopectin in

the peel to soluble pectin ; filtration of extracted

pectin ; and precipitation, purification, and drying of the prepared pectin.

Citrus peel employed for pectin production is

usually that from which peel oil has previously

been removed. Peel is first comminuted or

minced to facilitate washing and extraction.

If the peel is not extracted immediately, it is

heated to 104°-108° F. for about 10 minutes to

inactivate pectic enzymes, and then dried for

storage. Otherwise, inactivation of pectic enzymes is accomplished during the extraction

step. The minced or dry, stored peel is washed

copiously with water until all glycosides and

sugars are removed. Some water-soluble pectin

already in the peel may be extracted during

washing, but it is of little consequence as this

pectin is usually of a low jelly grade.

Pectin extraction is performed by placing

the prepared peel in vats of water, where the

slurry is brought to a boil, and concentrated

hydrochloric or sulfuric acid, previously diluted

to prevent local overheating, is added to adjust

to about pH 2.0. Because of the strong binding

power of pectin for polyvalent metal ions, extreme care must be exercised in later purifications when sulfuric acid has been employed in

order to remove the traces of lead, which are

always found m this technical reagent

The acid slurry is heated at 200° to 212"' F

for 45 to 60 minutes. During this time protopectm IS converted by ''restricted hydrolysis^'

to pectin, which is extracted into the solution.

Too high an acidity or too long an extraction

at these temperatures may result in an undesirable degradation of the pectin. In general,

several extractions of a given lot of peel under

mild conditions are preferred to a single extraction under severe conditions of time, temperature, and acidity.

The extract obtained may contain approximately 1 percent of pectin. If the concentration of pectin in the extract is low, it may be

evaporated under low-temperature conditions

to a pectin content of 3 to 4 percent to facihtate subsequent precipitation.

Filtration of the extract is a tedious process,

because the extract is corrosive and the pectin

solutions are viscous. Some type of filter aid

must be employed, which usually consists of

shredded paper or diatomaceous earth added

during the extraction treatment or before filtering. Filtration is carried out in two stages.

An initial coarse filtration removes the large

pieces of pulp and other coarse material, after

which a fine filtration produces a clear filtrate.

Continuous drum and plate-and-frame filters

(fig. 19), in conjunction with vacuum and pressure, are ordinarily used in the filtration procedures. The corrosive nature of the acid extracts necessitates the use of inert structural

materials, such as stainless steel for the equipment and Orion and Dynel fabrics for the filter

cloths.

The extracted pectin is next purified by precipitation from solution and careful washing.

It is sprayed or blended into tanks containing

any one of several organic solvents, such as

ethanol, acetone, isobutanol, or isopropanol,

where the pectin precipitates as a gelatinous

mass. The amount of solvent employed is such

that there will be a final concentration of 50-

to 70-percent solvent in the tank after the pectin extract is added.

The precipitated pectin is freed from the solvent by pressing (fig. 20) and/or by the action

of drain screws or roll presses used to convey

the pectin precipitate to a series of washing

vats. After several washings in 50- to 70-percent solvent-water mixtures, the pectin is given

a final dehydrating wash in 80- to 90-percent

solvent. The washed pectin is drained and

pressed to remove up to 50 percent of the solvent and water present, and it is then dried in

warm air or on a heated drum to a moisture

content of 6 to 10 percent. The dried pectin
54 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

FIGURE 19.—Filtering pectin extract in plate-and-frame filters. (Courtesy of Sunkist Growers, Inc., Los Angeles,

Calif.)

is ground to pass a 60-mesh screen, and packaged for marketing (fig. 21). Some citrus pectin is sold as a liquid product in solution with

added citric acid.

In an efficiently operated plant the apparent

high cost of solvent is considerably reduced by

employing solvent recovery and repurification

throughout all stages of precipitation, washing,

and drying. An excellent and detailed description of this process for pectin manufacture has

been given by Hull et al. {192). Additional information has been reported by McCready and

Owens (259).

Salting-out with high concentrations of polyvalent metal salts is an alternate method sometimes employed for precipitating pectin from

its extracts. In this process a mixture of aluminum hydroxide and chloride and ammonium

sulfate is added to the pectin extract, causing

the pectin to precipitate as a gel. When this

process is employed, there is a greater need for

careful washing with acidulated alcohol to remove the strongly bound aluminum ion and the

ammonium sulfate. This method of pectin manufacture has been fully described elsewhere

(209).

Because of natural variations in the peel of

different citrus fruits, the process of pectin

manufacture is made flexible, so that by employing different conditions of extraction and

by blending various batches a uniform grade of

product may be obtained. Pectins normally obtained by the described process are the so-called

rapid-set pectins, which gel very quickly. This

type of pectin is desirable in the manufacture

of jams and preserves containing pieces of

whole fruit that might collect at the top of the

jam or preserve if it gels slowly. However, for

other types of jams and jellies manufacturers

prefer a pectin that will not gel until the bottled products are in storage and will not have

to withstand the shock of too much handling.

The so-called slow-set pectins are made by a

slight variation of the normal manufacturing

process for pectin. The pectin extract obtained

in the standard manner is allowed to stand 10

to 20 hours at 30° to 40° F. in a strong acid

bath before precipitation and purification. This

treatment is believed to result in a slight

demethylation of the pectin and perhaps some

reduction in molecular weight, which gives the

pectin the slow-setting properties desired.
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 55

FIGURE 20.—Pressing solvent from precipitated pectin. (Courtesy of Sunkist Growers, Inc., Los Angeles, Calif.)

The value of commercial pectins is determined principally by their ability to form jellies. Pectins are therefore standardized before

marketing with respect to their jelly-forming

ability. This standard is called jelly grade, and

is a number that represents the pounds of

sugar that 1 pound of pectin will support in an

approximately 65-percent sugar jelly containing small amounts of acid. Pectin grades are

determined by preparing a series of 65-percent

sugar jellies under carefully defined conditions

and rigidly defined composition {222). Different proportions of the pectin are used in the

series of jellies, which are later compared visually with a standard jelly. From the minimum

amount of pectin employed in producing a jelly

essentially like the standard jelly, the jelly

grade is computed to pounds of sugar per

pound of pectin. Thus the expression 200-

grade pectin refers to the fact that 1 pound of

200-grade pectin will support 200 pounds of

sugar in a 65-percent sugar-solids jelly of

standard quality. One pound of a 100-grade

pectin will support only 100 pounds of sugar.

A liquid pectin of 50 grade means that 1 gallon

will support 50 pounds of sugar in a 65-percent

sugar-solids jelly of standard quality.
56 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

FIGURE 21.—Packaging dried citrus pectin. (Courtesy

of Sunkist Growers, Inc., Los Angeles, Calif.)

USE OF PECTIC SUBSTANCES

The most important use of pectic substances

is in the commercial and home production, of

jams and jelUes. Approximately 6 million

pounds of dry pectin is produced yearly for

this purpose alone, of which over three-fourths

is derived from citrus fruit. Apple pectin accounts for approximately one-fourth of the

total production.

Pectins have many other uses in small

amounts, which depend on their remarkable

abilities to form and stabilize emulsions, in

addition to the fact that they are a normal constituent of foods and may therefore be safely

ingested. Pectins have been used as thickeners

and stabilizers in milk products, as chocolate

milk, and as emulsifiers in cosmetics, cold

creams, and soaps (222). Pectin has also been

suggested in some cases as a carrier for drugs

that are injected intramuscularly. The function

of pectin is to delay absorption of the drug to

enable a more prolonged effect, such as in insulin-pectin preparations and pectin-penicillin

preparations (222).

Pectin and its derivatives appear also to have

certain inherent therapeutic properties, particularly in certain cases of malfunction of the

digestive tract. Pectin preparations have been

employed as a supportive measure in cases of

infant diarrhea and other alimentary-tract disorders U78). During World War II it was

found that dressings of 2-percent pectin solution were one of the most effective means of

treating tropical sores of vague etiology, and

very satisfactory results have also been reported (160) in the treatment of deep and

superficial wounds, where pectin-solution treatments appeared to accelerate healing by promoting epithelization. Other medicinal applications of pectin or its derivatives, particularly

in Germany, have included use as a hemostatic

agent and as a blood-plasma substitute (222).

LOW-METHOXYL PECTINS

These pectins contain from 2.5 to 5.0 percent

of methoxyl groups by weight. Low-methoxyl

pectins and pectic acid may be obtained by

modifying the regular process of pectin manufacture. The modifications consist principally

of treating the pectin at some stage of manufacture with hot concentrated acid, cold alkali,

or the enzyme pectinesterase. The pectin may

be demethylated while it is still in the peel in

the form of protopectin by extraction with alkali in place of acid or by directly treating the

extract containing pectin with acid, alkali, or

pectinesterase. Usually demethylation is accomplished by treating the pectin extract with

hot acid or cold alkali. Demethylation at this

stage has the advantage that the low-ester pectin or pectic acid, depending on the extent of

demethylation, will spontaneously precipitate

in the acid solution or can be precipitated from

the alkali-treated solution by reacidifying and

adding traces of calcium or other polyvalent

cations. The low-ester pectin precipitate may

then be washed with acidulated water or very

dilute acidulated alcohol solutions instead of

large amounts of alcohol as required in the

purification of fully methylated pectins. A final

washing with acidulated 70-percent alcohol is

usually sufficient to remove the traces of cation

employed to aid precipitation of the low-ester

pectins.

Commercially employed processes of manufacturing low-ester pectins still involve many

trade secrets, and careful control of times and

temperatures as well as concentrations of acid

or base must be exercised to obtain the desired

degree of demethylation without otherwise degrading the anhydrogalacturonide chain. Pectic acid is, of course, produced by the same

process simply by allowing demethylation to

proceed to completion. Excellent descriptions

of procedures for the manufacture of low-ester

pectins are available (133, 259).

Low-methoxyl, or low-ester, pectins are becoming increasingly important owing to their

ability to form low-solids jellies, such as tomato

aspic, by the addition of traces of calcium or
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 57

other polyvalent cation. Besides their uses in

low-solids jellies, low-ester pectins are useful

in compounding cold puddings and in forming

films for coating nuts, candies, and other food

products. Pectic acid is sometimes used in the

pharmaceutical industry for compounding

quick-dissolving tablets or pills.

PECTATE PULP

Although not a pure pectin preparation, pectate pulp should be mentioned. This product is

a form of leached citrus peel, containing principally low-ester pectins and pectic acids, crude

cellulose, hemicelluloses, and other peel constituents of high molecular weight. Pectate

pulp is prepared by treating citrus peel with

soda ash, after which it is washed, dried, and

ground. Pectate pulp may be dispersed in

water by adding phosphate salts, and may be

caused to flocculate under a wide variety of

conditions, and thus it is made useful for recovering or removing suspended matter from

solutions. In addition, pectate pulp may be

used in adjusting the viscosity of quenching

baths for tempering special steels, as an aid

in creaming rubber latex, and in formulating

oil-well drilling muds (192),

Citric Acid

Although all varieties of citrus fruits contain citric acid, only lemons are used in its

manufacture in the United States. Citric acid

is found free in the juice, but it cannot be separated directly. Attempts have been made to

do this, but all have been unsuccessful (66,

333),

The manufacture of citric acid from lemon

juice has been practically replaced in the

United States by a mycological process (2H,

Í55), beet molasses being utilized chiefly as the

raw material. Of a total United States output

of 50 to 60 miUion pounds annually, approximately 2,000,000 pounds of citric acid and

3,330,000 pounds of calcium citrate are still

produced from lemons in California (187).

The process is based on the precipitation of

citric acid as a calcium salt and decomposition

of the calcium citrate with sulfuric acid. The

various steps in the production of citric acid

are shown in figure 22.

Undergrade, or cull, lemons from the inspection tables of citrus-processing plants or surplus lemons during a season of overproduction

comprise the raw material. This material is

conveyed to lemon grinders (fig. 23), where the

juice is separated from the chopped peel. The

rag, seeds, and pulp are removed in perforated

rotary reels. The rag, wet with juice, is extracted further in a finisher, and this juice is

combined with the reel juice. The combined

juices are then partially deoiled in centrifugal

separators.

A specified amount of juice discharged from

the centrifugals is pumped to a still, where oil

is removed by steam distillation. The deoiled

juice is cooled, combined with the juice that

has not been treated by steam distillation, and

pumped to storage tanks as the raw material

for citric acid recovery. This liquor is held in

storage tanks (fig. 24) for about a week, during which time enzymes present degrade the

pectin and allow the pulp particles to separate

and congeal.

The clarified portion of the aged juice is

pumped from the storage tanks to neutralizing

tanks for further treatment. The pulpy scum

from the surface of the tanks is pumped to special filter tanks, where it is boiled to bring

about a coagulation of the pulp particles, and

then filtered, the filtered juice being combined

with juice in the neutralizing tanks.

To this juice filtered ''C liquor (see below)

is added to fortify the acidity, since acid recovery will be higher and handling more efiicient when the juice is of 6-percent acid content. The mixture is next heated and agitated

to promote coagulation of the remaining fine

pulp. An aqueous lime slurry is then added to

precipitate citric acid as calcium citrate. The

liming is carefully controlled to avoid overneutralization, a residual acidity of 0.05 to 0.2

percent being desirable. The slurry is filtered,

the filtrate discarded, and the filter cake

washed with hot water.

The calcium citrate can be made into citric

acid immediately, stored for later manufacture

into citric acid, or packaged for shipment as

citrate. If it is to be stored or packaged, it is

dried in a two-stage rotary drier and packaged

in bags or stored in steel silos.

Citrate to be made into citric acid is conveyed to a decomposing tank containing wash

water from a previous filtration. The slurry

of calcium citrate obtained is carefully treated

with sulfuric acid. As the calcium citrate is

decomposed, citric acid is liberated, and the

calcium combines with the sulfuric acid to form

insoluble calcium sulfate (gypsum).

Filter aid is added and the slurry is filtered

to yield a clear citric acid liquor containing

about 25 percent of citric acid ; this concentration is reduced to approximately 15 percent

after washing the filter cake. The filter cake

of calcium sulfate is discarded. . The weak citric acid liquor is fed to a stain-
JUICE-FERMENTING

TANK

LIME SLURRY

TANK

CALCIUM CITRATE

STORAGE

JUICE

PRESS

NEUTRALIZIN

TANK

VACUUM EVAPORATORS

^

CENTRIFUGE MIXER

SULFURIC ACID

I

^ O

'—mr

GYPSUM

RESS

V-1Í

SETTLING

TANK

1±Í3 DECOMPOSING

TANK

FERROCYANIOE

CARBON

BROWN

FILTER

DECOLORIZING

TANK

SCREENS

WHITE

FILTER

4

CRYSTALLIZER

^WW\/V\

LZ"

FIGURE 22.—Flowsheet of citric acid process.

PACKAGING

GO

>

H

O

S

n

a
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS

FIGURE 23.—Lemon grinders used in the preparation of citric acid. (Courtesy of Sunkist Growers, Inc., Los Angeles, Calif.)

FIGURE 24.—Juice storage and fermenting tanks. (Courtesy of Sunkist Growers, Inc., Los Angeles, Calif.)

less-steel vacuum evaporator (fig. 25), where

it is concentrated at 122° to 130° F. to about

30° Baume. During concentration the calcium

sulfate remaining in solution will precipitate.

The concentrated acid liquor is pumped to leadlined tanks, where the precipitated calcium

sulfate is allowed to settle out. Clear concentrated liquor is drawn from this settling tank

and fed to a stainless-steel, calandria-heated

evaporator fitted with an impeller. Concentration of the acid is continued under vacuum at

a temperature of 113° to 136° F. until a concentration is reached where, upon seeding with

citric acid crystals, the concentrated acid is induced to crystallize as a crude brown grain of

suitable size. This magma is discharged to a

stainless-steel basket centrifugal and centri-
60 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

FIGURE 25.—Evaporators for concentrating citric acid

liquor. (Courtesy of Sunkist Growers, Inc., Los

Angeles, Calif.)

fuged partially dry. This crude acid is called

"A" crystals.

The mother liquor from the "A" crystals is

diluted to about 30° Baume, and the vacuumevaporation steps are repeated to obtain a concentrate from which "B" crystals are recovered. This second mother liquor is also diluted,

settled, concentrated, and crystallized to obtain

"C" crystals. The "C" liquor is too impure for

further refining and is returned to the neutralizing step for recycling. The "C" crystals are

dissolved in the weak liquor used for the first

concentration, and recycled through the "A,"

"B," and "C" crystallization steps.

The "A" and "B" crystals are combined in

a rubber-lined tank with white magma mother

liquor and redissolved. After the mixture is

adjusted to 30° Baume, carbon is added for

decolorizing and calcium ferrocyanide for removal of iron. The mixture is warmed and filtered to give a clear, colorless citric acid liquor,

which is concentrated at 140° F. in a stainlesssteel vacuum evaporator to 38° Baume, mixed

with filter aid, and filtered again so that the

acid solution has a sparkling clarity.

The warm acid liquor is then pumped to

open, horizontal, semicylindrical, jacketed crystallizers equipped with agitators. Cool water is

circulated through the jacket at a slow rate to

^^^BB Í •iP^

FIGURE 26. — Centrifuging white citric acid crystals.

(Courtesy of Sunkist Growers, Inc., Los Angeles,

Calif.)

reduce the temperature of the acid solution.

During this period a heavy batch of citric acid

crystals grows in the liquor. This white magma is then centrifuged (fig. 26) and rinsed.

The centrifuged crystals are dried in a rotary drier with a stream of warm air. They

are then screened into coarse granular, granular, or fine granular sizes, and packed into kegs

and barrels for shipment.

Citric acid is the most commonly used organic acid in the food and pharmaceutical industries. Its salts, especially sodium salt, are

widely used in medicines and foods as buffers

and stabilizers. It is used extensively in effervescent salts, candy, soft drinks, metal pickling, water conditioning, and silvering of mirrors. The acid is also used as an ink ingredient

in engraving, dyeing, and calico printing, and

also in the preparation of paints and alkyd

resins. The plastics industry uses large

amounts of its esters as plasticizers.

Citrus-Seed Oils

It has been estimated that if all the seeds

from citrus-processing plants in Florida alone

were used for the extraction of oil, around 9 or

10 million pounds of oil would be obtained annually. However, only a small fraction of the

seeds are so used, as most of them are dried

with the waste cannery peel and rag to produce

dried citrus-pulp stock feed (197) ( see p. 73).

The seedy varieties of citrus fruit, such as

seedling and Pineapple oranges and Duncan
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 61

grapefruit, contain about 3.5 percent of seeds, The press cake is used as a protein supplewhich in turn contain from 55 to 60 percent of ment in cattle feed, but is not suitable for

moisture and about 15 percent of oil. The aver- swine (131), It is toxic to chicks, probably

age proximate composition of air-dried citrus because of its limonin content, but the toxic

seeds, compiled from various published data, is principle can be removed by solvent extraction

as follows: {99^ lOO),

Constituent Percent -, ^^^ expressed oil is run into tanks, where

Ash 2 48- 3 57 ^^ allowed to settle briefly, then pumped

Crude fiber ] 5Al-ll'.79 through filters and into storage tanks, where

Crude protein 6Í94-15Í94 it Undergoes a final settling before use. SubseEther extract 21.88-34.44 quent treatment of the oil depends on the use

Nom^itrogenous substances ^6'82lll*86 ^^^ ^'^^^^ ^^ ^^ destined. For edible purposes

^^^ ~ • • it is alkali refined. This treatment removes the

Citrus seeds are obtamed from citrus-proc- bitter principle, probably limonin (318), and

essmg plants, where large quantities of fruit any free fatty acid. The losses in this step

have been collected and utilized primarily for usually do not exceed 2 percent. The bland,

juice or sections. The seeds are obtained in- neutral oil resulting may then be hydrogenated

timately mixed with rag and pulp but separate for use in shortening preparations or, if necesfrom the peel. Separation of seeds from the sary, may be winterized if intended for a salad

rag and pulp may be accomplished in several oil or liquid shortening. This treatment is acways, but the difficulty of so doing is one of the complished by chilling until the higher melting

chief deterrents to increased production of glycerides crystallize and then removing them

seed oil. Separation may be accomplished with in a filter press. Crude grapefruit-seed oil

a paddle-type finisher or similar equipment. will solidify around 10"^ to W F.

Another procedure is to lime the mixture of Crude citrus-seed oils are in general suitable

seeds and wet pulp, as in the preparation of for the usual nonedible uses to which semidried citrus feed, and dry in rotary driers. The drying oils are put. They include soap making,

seeds can then be separated from the dried pulp sulfonation and sulfation for detergents, and

by screening and winnowing. The separated the preparation of fatty acid derivatives,

pulp is of value as feed. Grapefruit-seed oil has found some commercial

Dried seeds from which the hulls may or application in the treatment of textiles (213)

may not be removed are passed through crack- and leather (212),

ing rolls, and the oil is extracted in screw ex- For the physical and chemical constants of

pellers of the type commonly used in preparing citrus-seed oils, see page 17.

cottonseed oil and tung oil (197, 318, H7).

These machines produce a press cake with an Flavonoids

oil content ranging from 14 to 16 percent. The

oil content of the press cake can be further re- The commercial production oí flavonoids

duced in these machines but at the expense of from citrus wastes was stimulated initially m

heat damage to the oil. More modern machines 1936 by reports of the vitammlike (vitamin 1^)

have been developed that will reduce the resid- activity of extracts of lemon peel presumed to

ual oil to 7 percent without heat damage, but contain physiologically important flavonoid

these machines are not yet in general use. Sol- compounds. Although the so-called vitamin ±^

vent extraction would reduce the residual oil has lost favor as a vitamin, reports continue to

even further, but so far as is known this has suggest that it may have some important pharnot been attempted commercially. The compo- macological properties. There has thereiore

sition of a sample of press cake after extrae- been a small but steady production ol several

tion of oil from grapefruit seeds (318) is as citrus ñavonoid preparations. Citrus and othei

follows • flavonoid compounds and their derivatives have

^ ' been suggested as antioxidants (356), as dyes

Constituent Percent ^226 U65-Uß7), and for miscellaneous physaMoisture . 3.43 ological applications (U20, h6U, U77), The reNitrog-en as ammonia 4.21 \ , r^ j.- i fl^TT^-r^r^'/qa -fnv fvncith fp Nitrolen as protein 21.60 ported effectiveness of flavonoids tor tiostmte

Crude fat (ether extract) 13.95 (12U) and radiation injury (6h^ 115, W¿, 4Uö,

Crude fiber 26.50 J^QS) remains controversial (73, 88, HO)-

Ash 4.04 Hesperidin is the oldest commercially availCalcium .o5^^ ^^j^ flavonoid produced from citrus fruits

Magne¡rum "::::::::::::::::::::::::::::::: .39 (192). it is generally prepared from heaviiy

Phosphates .55 limed, ground orange peel. At pH 11 hesperiSilica ___ .081 ¿.^ jg soluble in aqueous solution as its yellowSodmm and potassium ^hlorides__--_-__------ 2.48^ ^^^^^^^^^ chalcone. The lime coagulates the pec-
62 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

tic materials in the peel, and the extract containing hesperidin chalcone can be squeezed

from the peel residue by powerful presses. The

exúdate is filtered, pumped to a glass-lined

tank, and brought to pH 6 with hydrochloric

acid. The acidulated solution is heated and

allowed to stand overnight, during which time

the chalcone is reconverted to hesperidin, which

crystallizes out within a few hours at room

temperature. The mother liquor is removed

from the hesperidin in plate-and-frame filters

and discarded, and the filter cake of hesperidin

is dried in a tray drier.

Patents have been issued on processes for

the preparation of hesperidin (17, 178, 180)

and naringin (16), All processes are similar

in principle to that previously described. A

solvent-extraction procedure has been patented

for the purification of crude flavonoid preparations {2J^5).

Hesperidin methyl chalcone is also produced

commercially. This yellow amorphous material

is soluble and stable in aqueous solution. The

unmethylated chalcone, formed by the addition

of alkali to hesperidin, is water soluble, but

reverts to insoluble hesperidin within a few

hours at room temperature. Several patents

(185, 186, 216) have been issued on processes

for solubilizing flavonoids without chemical

modification.

Naringin, a flavanone glycoside extracted

from gi'apefruit peel, is used commercially to

impart a bitter flavor to beverages, confections,

and marmalade made from sweet oranges (192,

207, 226), It has no known therapeutic properties. Naringin has also been suggested as a

raw material for the production of rhamnose,

p-coumaric acid, phloroglucinol, and dyes (226,

An aqueous extract of lemon peel that is prepared in a manner to eliminate pectins and oils

and is vacuum concentrated and dried is marketed under the name Lemon Peel Infusion,

Dried. Another product, called Lemon Peel

Extract, Dried, is prepared by drying an alcoholic extract of lemon peel. The two products

are essentially the same, but the latter has

more desirable physical properties (192), Both

are prepared for pharmaceutical purposes.

Essential Oils

Since 1920 the production of citrus oils in

the United States has increased to such volume

that they are now exported to foreign markets.

Production could be further increased if the

demand justified, because processing plants do

not extract the maximum amount of oil present

in the fruit.

Current methods used in this country for

manufacturing cold-pressed citrus oils may be

divided as follows: (1) Those in which oil is

recovered from the peel after extraction of the

juice, (2) simultaneous extraction of juice and

oil emulsion from whole fruit, and (3) removal

of the oil-bearing flavedo (thin outer layer)

from whole fruit by abrasion.

Most of the citrus oils are produced by the

first method, and at least three types of machines are used. With one process the peel remaining after juice extraction is conveyed to

tapered screw presses, in which the screws

rotate in conical sections with i^-inch perforations. As peel is fed into the machine, these

revolving screws press it tighter and tighter

against the perforations. This lacerates and

twists the peel and causes oil to be released

from the flavedo. The resulting oil-juice emulsion and particles of peel are washed away

with water sprays. The macerated residual

peel may be used for the production of pectin

(p. 53).

The oil emulsion is next put through a revolving reel or other screening device to remove

the larger particles of peel. It then goes

through a high-force, disk-type centrifuge

known as a sludger, which removes more particles and most of the water. The discharge

from the sludger contains 15 to 30 percent of

oil. The oil cream next passes through a solidbowl centrifuge, which yields a 99-percent oil

discharge. Most of the remaining water is

then removed by a third continuous disk-type

centrifuge (fig. 27), which yields an oil of

greater than 99.9-percent purity. The oil is

transferred to tin-lined or stainless-steel drums,

and held in cold storag3 to precipitate sterols

and waxes, which are separated by décantation. The decanted oil is given a final polishing

by centrifugation, after which it is barreled

and stored. The wax may be recovered and

used as an ingredient in a compound for waxtreating fresh fruit in packinghouses.

Another machine, which simultaneously extracts juice and shaves off the flavedo, consists

of two heavy rotating cylinders of stainless

steel, one of which revolves against a perforated grid that presses out the juice after the

cut fruit has been flattened by being passed

through the rolls. This extractor is equipped

with a peel shaver, which removes the thin

flavedo from the flattened peel after the juice

has been pressed and just before it is discharged from the machine. The shaved flavedo

is finely shredded, mixed with water to form a

slurry, and the oil separated as previously

described.

Most of the oils produced in Florida are extracted from peel with a machine made of two

closely set, heavy, stainless-steel cylinders,
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 63

FIGURE 27.—Centrifuges used to refine citrus-oil emulsion. (Courtesy of Sunkist Growers, Inc., Los Angeles, Calif.)

which rotate slowly and are adjustable to control pressure on the peel. Sufficient pressure is

maintained to just puncture the oil sacs in the

flavedo without macerating the peel. The cylinders are machined with capillary grooves

running around their circumference. These

grooves are just deep enough to receive oil

from the punctured sacs, but still keep it from

being reabsorbed by the spongelike albedo of

the peel as the drums revolve. As no water

sprays are used, loss of water-soluble constituents is kept at a minimum. Liquid from the

peel and small quantities of juice adhering to

the peel act as the carrying medium for the oil.

This oil emulsion is screened, and the oil separated in high-speed centrifuges as previously

described.

Another method, not extensively used by the

industry, consists of removing the oil-bearing

flavedo from the whole fruit by abrasion. Fruit

is automatically conveyed through a tunnel

made up of horizontal, carborundum-covered

rolls revolving at high speed. As fruit passes

through this tunnel it is subjected to sufficient

abrasive action to remove the flavedo to a depth

of about %4 inch. At the same time the fruit is

sprayed with water, which removes the oil and

peel debris as a slurry. This slurry is screened

to remove solids, and oil is recovered from the

emulsion as previously described. The whole

fruit, minus the flavedo, is then conveyed to

juice extractors. It is reported that this machine will recover more oil from a given lot of

fruit than other machines used for this purpose. For example, an average yield of 9.7

pounds of orange oil per ton of peel has been

reported (227), which is about twice the yield

obtained with screw presses.

Another type of abrasive peeler used in

California consists of a continuous spiral about

1 foot high, with the inner stainless-steel surface partially punched with holes to form a

sharp grating surface. The spiral grater,

which is stationary, is suspended over a revolving table covered with the same grating

surface as the spiral. Two spiral graters of the
64 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

same size are used. Whole fruit is fed into the

center of one spiral and emerges from the center of the second spiral. While the fruit is

being grated through the two spirals, it is continuously sprayed with water. The resulting

effluent is screened, and the oil recovered by

centrifugation as previously described. The

whole fruit, minus the flavedo, is then conveyed

to juice extractors. This machine cannot be

used to recover oil from lemons because of

their elongated shape.

The yield of oil obtained under commercial

conditions depends on a number of factors

other than the type of machine employed.

Thickskin fruit yields less oil than thinskin

fruit because of the tendency for the thicker

spongy albedo to absorb more oil. Fruit that

has stood for some time so that the peel has

become flaccid yields less oil than corresponding fruit with firm peel. Actual yields of oil

per ton of fruit vary from 2 to 7 pounds for

lemon oil, 1 to 8 pounds for orange oil, 1 to 2

pounds for grapefruit oil, 0.1 to 0.3 pound for

lime oil, and 1 to 2 pounds for tangerine oil.

These yields are far short of the actual amounts

present in the peel of the respective fruits.

Cold-pressed citrus oils may be concentrated

by vacuum distillation to reduce their limonene

content from around 90 percent to 50 percent.

These oils, referred to as ''fold oils,'' are used

mainly in beverages because of their greater

storage stability.

The accumulated discharges from the coldpressed oil centrifuges contain, on an average,

0.1 percent of oil. This liquid may be steam

distilled, the vapors condensed, and the condensed water and oil collected in drums, from

which the oil is decanted from the top of the

water layer. The peel may also be steamed

distilled, after being pressed, to obtain oil.

Steam-distilled citrus oils are inferior to coldpressed oils both in flavor and in keeping

qualities.

''Stripper oil'' (90 percent or more of limonene) is a byproduct in the production of citrus

molasses (p. 77). This oil has been used in

certain plastics and in the preparation of isoprene. A patent (380) has been issued for its

use in preparing penetrating oils.

Oil obtained in deoiling of canned singlestrength orange juice (p. 27) is also of commercial importance. This product approaches

cold-pressed oil in quality, and sometimes retains a yellow color. It is of better quality than

distilled or "stripper oil."

Although not manufactured by the citrus industry, terpeneless and sequiterpeneless citrus

oils are available for special uses. These are

oils from which practically all the terpene

hydrocarbons have been removed and the more

flavorful oxygenated fractions are left. Methods for their preparation include vacuum distillation, solvent extraction, or a combination

of the two. A Chromatographie method for

preparing terpeneless oils has been reported,

and shows possibility for commercial adaption

(232).

The various types of citrus oils described are

widely used in the flavoring of food products,

confections, beverages, and liqueurs, and for

the manufacture of pharmaceuticals, soaps,

perfumes, and other cosmetics. Commercial

methods for the manufacture of citrus oils in

the United States and other countries, preparation of modified oils, and the uses and properties of citrus oils have been described in detail

U2, 135, 192, 229).

The composition of American citrus oils has

been the subject of considerable research. Table

19 lists some of the volatile constituents found

in domestic citrus oils.

TABLE 19.—Some volatile constituents of

American citrus oils (30, 122, 312, 314,

315, 334, 357)

Constituent

Tangerine

oil

Orange

oil

Lemon

oil

Grapefruit

oil

Acetaldehyde _ _ _ _ +

+

+

4-

+

+

+

+

+

+

+

Acetic acid -f

CioHisO Alcohol

Bisabolene __ -f

Cadinene _ +

+

+

+

-1-

Capric acid

Caprylic acid

Citral

Citronellol _ _ _ _ _

4-

-h

+

+

Decyl aldehyde

Decylic acid _ _

+ +

Formic acid. _ _ +

Geraniol _ -f

Geranyl acetate _ -f-

(¿-Limonene

Linalool

Linalyl acetate

+

+

+ +

+

-h

-h

-f

Methyl alcohol _ +

Methyl anthranilate. +

Nonyl alcohol + 4-

Octyl acetate -f

Octyl alcohol. +

"'"+""" +

+

-1-

Octyl aldehyde

Octylic acid

+

a-Pinene +

+

+

/3-Pinene_ _

7-Terpinene

The purity of citrus oils is determined by

measuring their specific gravity, refractive index, optical rotation, aldehydes, and esters, and

the amount of residue on evaporation (tables

20 and 21). Sometimes, however, it is extremely difficult to detect possible adulteration by
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 65

TABLE 20.—Approximate physical constants of domestic cold-pressed citrus oils (458)

Specific

gravity

(25° C.)

Refractive

index

(nz>, 20° C.)

Optical

rotation

(aD, 25° C.)

10-percent distillate

Residue Source

Refractive

index

{no, 20° C.)

Optical

rotation

(aD, 25° C.)

on

evaporation

Grapefruit :

Florida

Texas, Marsh seedless

0.8508-0.8532

1.856

.8475-.8525

1.881

.843-.846

.8440

.8445

.8416-.8458

1.846

.8456

1.4746-1.4761

1.4769

1.4738-1.4749

1.4849

1.4731-1.4742

1.4735

1.4738

1.4718-1.4734

1.4742

1.4734

+91.19°-92.96°

1+96.45°

+52.71°-70.18°

1+40.55°

+98.33°-94.0°

+97.78°

+96.93°

+95.16°-97.76°

1+97.10°

+91.18°

1.4698-1.4712 +95.81°-98.14°

Percent

6.02-8.02

Lemon, California

Lime, Florida

1.4726-1.4737 +46.47°-65.74° 2.01-4.52

13 74

Orange:

California 3 5-5 1

California, Valencia..

California,

Washington Navel.

Florida

Texas, Valencia

1.4723

1.4724

1.4703-1.4715

+99.21°

+98.71°

+96.81°-98.70°

3.6

4.53

1.07-4.93

Tangerine, Florida 1.4711 +92.68° 4.53

1 At 20° C.

TABLE 21.—Approximate physical characteristics of domestic steam-distilled citrus oils (169)

Source

Specific

gravitv

(20° C.)

Refractive

index

{nD, 20° C.)

Optical

rotation

(aD, 20°C.)

Aldehydes Esters

Residue

on evaporation

California orange 10.840-0.842

.8415-.8439

.8556-.8579

.8407

1.4717-1.4730

1.4714-1.4746

1.4743-1.4751

1.4720

1+99.1°

+91.50°-96.50°

+46.84°-50.52°

+93.67°

Percent Percent Percent

0.4-1.0

Florida grapefruit 2

Florida Persian lime

Florida tangerine

2.30-4.06

1.61-2.71

1.24

0.08-2.52

2.41-3.49

.25

.19-3.66

.18-1.23

.20

1 At 25° C.

2 Vacuum steam distilled.

these methods alone. Only cold-pressed orange

and lemon oils are listed in the United States

Pharmacopoeia.

Citrus oils should be stored in well-filled bottles or cans in a cool place. The oils are subject to oxidation in the presence of moist air,

during which limonene is oxidized to caryone

and carveol, and isocitral in lemon oil is

changed to p-cymene H19).

Citrus Vinegar

Vinegar made from citrus fruits, especially

orange vinegar, has a fine ñavor and would be

an easily marketable product if it could compete economically with apple vinegar. As a

specialty product orange vinegar is produced

chiefly in countries outside the United States.

The chemical mechanisms by which citrus

vinegar is made are the same as those used for

other fruit vinegars. Sugars present in citrus

juices are first fermented to ethanol by the

action of yeasts. The alcohol is then oxidized

to acetic acid by Acetobacter organisms.

In the United States vinegar is required by

the Federal Food, Drug, and Cosmetic Act to

contain not less than 4 gm. of acetic acid per

100 ml. at 68° F. The sugar content of juice

from mature oranges may vary from 8 to 11

percent, with an average of around 9.5 percent.

In general, any fruit containing more than 9

percent of sugar can be converted into a vinegar, which will contain more than the legal

limit of 4 gm. of acetic acid per 100 ml. Since

a good orange vinegar should contain approximately 5 percent of volatile acid, as acetic (50

grain), and 1 percent of fixed acid, as citric,

low-sugar citrus juices should be concentrated

to increase their sugar content before being

converted to vinegar. Before vinegar is manufactured from citrus juices, the regulations of

the Alcohol and Tobacco Tax Division, Internal

Revenue Service, covering this subject should

be consulted.

Vinegar can be made from citrus juices by

two methods—a slow or so-called roller process

and a rapid or generator process.

The roller process is a modification of an old
66 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

French method called the Orleans process. A

suitable generator consists of a 50-gallon oak

barrel, fitted with a wood rack the full length

of the barrel about 5 inches below the bung,

which is filled with beechwood shavings or

corncobs. A sufficient number of holes are bored

at each end of the barrel just below the rack to

allow a free flow of air. The shavings or corncobs provide a greatly increased exposed surface to bring the alcohol in closer contact with

atmospheric oxygen.

If the generator is new or has not been used

for some time, it should be steam scalded or

thoroughly washed with hot water. After all

water has been drained out, the inside of the

barrel and the shavings are wetted with unpasteurized vinegar of good quality. This is

the source of the Acetobacter organisms.

The orange juice is first fermented in a separate container by adding 1 pint of brewer's

or baker's yeast to 40 gallons of juice or by

using 6 cakes of compressed yeast that have

been softened in orange juice. Active dry

yeast, which is sold in envelopes holding 7 gm.,

may also be used. After fermentation has

ceased, usually in 3 to 5 days, the juice is

strained to remove all the yeast. The temperature of fermentation should not be higher than

85° F.

Approximately 40 gallons of this fermented

orange juice is poured into the generator

through the bung, which is then closed, and the

holes at each end are plugged with cotton. Several times, but at least once each day, the cotton plugs are replaced with wood plugs, the

barrel is turned, so the bung is at the bottom,

and shaken several times to bring the juice in

contact with the shavings or cobs. After the

barrel is returned to its original position, the

cotton plugs are reinserted. An excellent vinegar is produced by repeating this treatment

daily for 60 to 90 days at temperatures around

80° to 85° F.

The rapid generator or German process

allows the fermented juice to trickle down

through a tall wood tank or series of tanks

loosely packed with pumice, beechwood shavings, or corncobs, and thus is a continuous

process (58, 332, 336). The pumice, shavings,

or corncobs are raised off the bottom of the

generator by means of a false wood bottom

perforated with holes to allow a free flow of

air. A similar perforated wood disk is placed

on the top of the pumice or shavings.

The generator is first seeded with acetic acid

bacteria by trickling warm unpasteurized vinegar through the pumice or shavings bed until

it flows out at the bottom. Filtered fermented

juice is then evenly distributed over the top

disk and allowed to trickle down through the

packed generator. A little experimenting with

the air supply and flow of juice will soon establish the proper amount of air and feed to obtain

efliicient operation. The temperature should be

kept between 80° and 85° F., and any deviation

can be corrected by increasing or decreasing

the flow of juice. The vinegar drawn off at the

bottom will not be completely acidified, and it

must be recirculated through the same generator or another generator prepared in the same

manner as the first.

Orange vinegar produced by this process

should be aged, which may require a year. It

should then be filtered, filled at 140° F. into

sterile bottles, capped, and pasteurized at 140°

to 150° for about 30 minutes. The bottles are

allowed to cool in air.

Care should be taken to avoid all contact

with corrosive metals, such as iron, which in

combination with tannins may form a black

color rendering the vinegar unsalable.

The appropriate food regulatory authorities

should be consulted relative to the proper labeling of orange vinegar.

Marmalades

Some dictionaries define marmalade as a preserve made by boiling the pulp of bitter or acid

fruits with sugar to the consistency of jam.

Modern usage usually limits the term to a

citrus-fruit jelly, in which sliced or chopped

portions of the peel are embedded. Federal

grades and standards for orange marmalade

have been established {ÍU2).

In making marmalade the ingredients are

concentrated by cooking to such a point that

the soluble-solids content of the finished marmalade is not less than 68 percent. Marmalade

containing sweet orange should be composed of

not less than 30 parts by weight of fruit to 70

parts by weight of sugar, whereas bitter orange marmalade should contain not less than

25 parts of fruit to 75 parts of sugar {H2),

Marmalades contain peel in amount and

physical condition according to the desire of

the manufacturer. Published formulas (55, p.

105; 75, p. 398) call for as little as 1 pound of

peel to 20 pounds of fruit pulp in tangerine

marmalade and for as much as 1 pound of peel

to 9 pounds of fruit pulp in sweet orange marmalade. Grade standards specify only that the

peel be evenly distributed, of uniform size, and

in substantial but not excessive amount. Fancy

marmalades contain sliced peel 1/30 to y^^ inch

thick or peel chopped to small fairly uniform

size.

Peel may be separated from the fruit either

by reaming the juice and removing the adhering rag or by peeling the whole fruit. A con-
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 67

venient method of peeling is to heat the whole

fruit for 1 to 3 minutes in boiling water to

puff the peel, score it in 3 or 4 places, then pull

it off by hand. Peel so obtained should be

quickly and thoroughly cooled in water to prevent development of excessive toughness. It

may be cooked in water to tenderness either

before or after slicing or chopping. It is important that peel be completely tenderized prior

to final processing, as cooking with sugar does

not increase tenderness. The time required for

cooking to tenderness depends on the condition

of the peel, and may vary from 3 to 30 minutes

at a full boil. Upon reaching the desired texture, the peel should be quickly cooled to prevent mushiness.

Sometimes cooking the peel in water serves

another purpose in addition to tenderizing it.

Marmalades containing two or more kinds of

citrus fruit, such as orange and grapefruit,

have a more appealing taste than those containing single varieties, but it is often considered advisable to reduce the bitterness of

the grapefruit peel by cooking in 2 or 3 changes

of water. Other strongly flavored citrus varieties, such as tangerines and calamondins, are

also improved by this method of reducing flavor

intensity.

Most British marmalades contain all the interior fruit juice and tissue and are semitranslucent or opaque, whereas American preference is for peel suspended in a clear jelly.

The former type is prepared by cooking the

peeled fruit in a small amount of water, removing the seed and undisintegrated membrane with a brush-type finisher, and adjusting

the acidity with lemon juice or citric acid to

pH 3.0-3.4. After the tenderized peel is added,

the mixture is converted into marmalade by

further cooking with sugar. Marmalades prepared in this manner, unless made from very

ripe fruit, do not require the use of added

pectin.

Clear-type marmalades are usually made

from reamed or pressed juice. Complete clarity

of the juice is not essential, as its normal

cloudiness disappears upon the addition of

sugar. Passing the juice through a fine mesh

screen and then straining through muslin will

provide the base for a very clear jelly. The use

of such juice will require the addition of pectin

to obtain jelly of the proper strength. Although the natural pectin content of the fruit

is thus wasted, commercial manufacturers consider that the small cost of prepared pectin is

httle enough to pay for insurance that each

batch will turn out to be of the proper consistency. Variations in the pectin content of citrus

fruits are so great that laboratory tests to determine proper proportions of fruit and sugar

must be run on each batch when the natural

pectin content of the fruit is utilized.

For either cloudy or clear marmalade the

jelly base or juice should be adjusted to the

proper pH (3.0-3.4) and combined with the

peel before sugar is added. An advantage from

having the acid present during cooking is that

some inversion of sugar takes place, which reduces the possibility of sugar crystalhzation.

When commercial pectin is used, it is generally

mixed with a small amount of sugar and added

slowly with vigorous stirring to the hot but not

boiling mixture. Addition of the total required

sugar in small lots permits better penetration

of sugar into the peel than if the whole amount

IS added at once. After all sugar is added, boiling is continued until the proper concentration

is reached, which may best be determined by a

refractometer but may be approximated bv

finishing at a temperature of 222^ F. at sea

level.

After boiling is completed, the marmalade

should be allowed to stand until it cools to

approximately 185° F. for skimming. Containers should be filled and sealed hot and inverted to sterilize the lids. If peel has been

properly tenderized and is present in sufficient

quantity, it should have no tendency to separate or float. If floating is apparent, the fault

may be remedied by inverting the containers

several times while cooling.

Several methods are available for preserving

citrus pulp for subsequent conversion into marmalade. Much of the British production of

marmalade is from imported pulp preserved

with sulfur dioxide. After the cooked, sliced

peel is combined with the screened pulp, the

mixture is partially cooled and filled into barrels with enough sulfur dioxide solution to provide about 2,000 p. p. m. of sulfur dioxide after

thorough mixing. Such a mixture may also be

preserved by sealing it hot in tin cans (38i)

or by holding it under refrigeration.

Several methods for dehydrating citrus for

use in marmalades have been developed. Thin

slices of oranges may be dehydrated at 150° F.

on monel screen trays until the fruit is brittle.

This reduces the fresh fruit to 20 percent of its

original weight, and marmalade prepared from

such a dehydrated product is considered satisfactory (76).

Another method involves comminuting citrus

fruit, adjusting to a pH of 2.5 to 4.5, spreading

in thin layers, and drying at a low temperature

(37J,).

Orange concentrate has been suggested for

making marmalade (386), It is claimed that

such a product has a vitamin C content of 17.5

to 29.2 mg. per 100 gm., or a retention of 80

percent of the original vitamin content. Com-
68 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

mercial orange marmalades generally contain

2.1 to 6.3 mg. of ascorbic acid per 100 gm.

{S86),

Brined Citrus

GRAPEFRUIT, LEMON, AND ORANGE PEEL

The peel of each of these fruits is preserved

in brine and subsequently used in the preparation of candied products and mincemeat. The

brining technique has long been an established

practice on the Continent, but is a comparatively new industry in the United States. In the

citrus regions of Italy lemons are halved and

the pulp scooped out, the cleaned peels then

being placed in brine. The brine is usually sea

water, and salt is added to raise its density to

7"^ Baume. The material remains in this solution for 4 or 5 days, and is then removed and

packed in casks. The halves are packed one

within another in layers, and salt is sprinkled

between every second layer. The cask is then

filled with 10°-Baumé brine, and the material

stored for 2 months before shipment. Fresh

brine is added from time to time during the

storage period to keep the volume constant and

the peel covered. Before shipment the brine is

drawn off and replaced with sea water or its

equivalent.

Sometimes the halved fruit is packed with

the pulp intact (called ''salati ordinari''), but a

stronger brine is used than when the cleaned

halves are packed, and the product is shipped

at once.

Peel from which the oil has been wholly or

partly removed is not considered suitable for

brined peel.

In the brining of peel it is desirable to select

material free from scale or melanose. The fruit

used is generally the ''cups" left after reaming

to obtain juice for canning. The cups thus obtained still retain considerable quantities of

rag, and this must be removed by hand if

cleaned cups are to be brined. It is claimed

(373) that there is now a machine available to

accomphsh this, and that it is being used by a

concern in southern California. Cleaned cups

yield a cleaner looking product than those containing rag.

The cleaned cups should be washed with

water, and then packed into tight 52-gallon fir

barrels, which are paraffin lined and from

which the head has been removed. The cups

are packed one inside the other, i.e., nested,

and in rows starting at the outer periphery of

the barrel. The peel should be completely covered with a 10-percent salt solution. The material is then allowed to stand until curing is

completed. It is necessary to maintain the

brine at 10-percent salt content by the addition

of dry salt. Complete curing is judged by the

appearance of the peel, which should be translucent but not mealy or mushy. Greater turgidity can be imparted to the peel by adding small

amounts of calcium hydroxide, about 1/2 ounce

per gallon of brine.

After the peel has been cured, the brine is

drawn off and fresh 15-percent brine added.

The barrel is headed up and stored for shipment. Stored barrels should be inspected frequently, and fresh 15-percent brine added

through the bung to maintain full volume. Sulfur dioxide is sometimes added to give from

500 to 600 p. p. m. Addition of sulfur dioxide

must be declared on the label of the barrel.

LIMES

Pickled limes were formerly rather popular

in the Northeastern United States, especially at

beach resorts where they could be seen displayed in fancy glass jars. The fruit used was

the small Key or West Indian lime.

The following method of preparing this type

of product is offered as a suggestion for further experimentation. The fruit should be

yellow, because in that stage the peel is more

tender. Green-colored fruit will sometimes become yellow or olive green when brined, but

the product is not so tender as the yellow fruit.

The whole fruit is gently boiled for 15 minutes

in water containing 0.5 percent of sodium carbonate to remove part of the bitterness. It is

then washed in clean cold water.

The limes thus prepared are placed in barrels or other suitable containers and covered

with 10-percent brine. To keep the fruit submerged, it is necessary to weight it down with

a lattice frame that will fit inside the barrel.

The strength of the brine should be closelv

checked and maintained at a concentration of

between 10 and 15 percent. If more salt is

necessary, it may be sprinkled on the lattice so

that it will not fall to the bottom of the container and remain undissolved. A scum of wild

yeast and mold may form on the surface of the

brine, and this should be removed by skimming; otherwise the acidity of the brine will

be reduced.

The temperature during curing should be between 80° and 85° F. At the end of 30 days

the brme should be discarded and fresh brine

added. The finished product should not be

mealy ; if mealy, it indicates improper curing.

At the end of the curing period the brine is

discarded and the fruit processed by covering

with water and maintaining at 120° F. for 3 to

4 hours. The limes are then removed from the

water, rinsed, packed into the final container,
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 69

and covered with 5-percent brine to which

vinegar has been added to bring the acidity to

2 percent. If a more sour product is desired,

the acidity may be increased to 4 percent.

CITRON

For commercial purposes citron {Citrus

medica) 5 or 6 inches long and 3 or 4 inches in

diameter is generally used. It should be picked

green but full grown. Yellow well-ripened fruit

is not suitable for brining. Satisfactory fruit

will exhibit a certain oiliness on the surface,

and the oil sacs will be well developed.

The washed fruit is cut in half lengthwise,

but the seeds and pulp generally are not removed. It is packed into crocks or barrels and

covered with brine. The brine may be either

natural or artificial sea water of the following

approximate composition :

Sodium chloride 28 pounds

Potassium chloride 7 pounds

Magnesium chloride 3 pounds

Water 100 gallons

If curing temperatures run higher than 60*^ F.,

salt must be added to raise the concentration ^o

7-10 percent.

During brining the tissues of the fruit become translucent, and a vigorous growth of

yeast takes place. The length of time for curing will vary with the size of the fruit, its maturity, and temperature during curing. In general, the period varies from 16 to 30 days.

During brining it is necessary to keep the solution at the proper strength by the addition of

salt. After curing is completed, the brine is removed and replaced with a 6- to 10-percent salt

solution.

Candied Citrus

GRAPEFRUIT, LEMON, AND ORANGE PEEL

The raw material used for candying the peels

of these fruits {37, 57, 106) may be either

fresh or brined. However, brine must be removed by soaking the peel in cold or hot water.

Peel for candying should be free of blemishes

and reasonably thick. The flavedo, or oil-bearing surface, should be scoured with a wire

brush or roughened with a cheese grater in

order to release part of the peel oil. Adhering

membrane should be removed from the peel,

which should then be cooked until tender. As

many changes of water should be used in cooking as considered necessary to reduce the

strong flavor of the peel. Grapefruit peel is

normally quite bitter, and several changes of

water are required to reduce the bitterness to

an acceptable level. However, all bitterness

should not be removed, as it is a distinguishing

characteristic of the fruit. Nonbitter candied

peel may be prepared from oranges or lemons

After the peel is cooked, it should be drained

and pressed as dry as possible. Although it

may be cut into strips or pieces as desired before cooking, circulation and drainage of hot

water are more rapid and handling is easier if

half or quarter peels are used.

Since the peel of juice types of citrus is more

porous than citron, sugar will saturate it, so

that the tedious process required for citron

will not be necessary. The tenderized peel

may be boiled slowly for about an hour in a

sugar solution consisting of equal parts of

sugar and water and 0.05 percent of citric acid.

The peel is allowed to remain in this solution

overnight. It is then boiled again until the temperature reaches 219° F., and drained while

still hot. Overcooking should be avoided to prevent hardness in the finished product. Stickiness may be reduced by dipping the hot peel

quickly into boiling water, draining, and drying

on wire trays in a current of warm air. Dusting with powdered sugar prior to complete dryness will also tend to keep the pieces from

sticking together. Candied peel should be

stored in moistureproof containers.

KUMQUATS

Kumquats are grown extensively in China

and Japan. The fruit is also grown in Florida,

where the Nagami {Fortunella margarita) and

the Marumi (F. japónica) species are found.

The plant is a shrub, reaching a height of 10

to 15 feet. Its fruit is oblong in shape, varying

from IVo to 1% inches in length. It is much

used for holiday decorative packing, being harvested along with a few branches and leaves.

The bright-orange outer rind has a spicy

flavor, the inner rind is sweet and granular,

and the juice has an acid taste.

Kumquats are consumed chiefly after being

candied. Considerable difficulty is experienced

in candying them, because the fruit generally

collapses and the product does not have a pleasing appearance.

The fruit is first washed and then passed on

a belt beneath a spiked roll to puncture the

peel. The belt can be either leather, rubber, or

stitched canvas, through which copper tacks

are driven and held in place with a suitable

backing. The roll and belt should be so adjusted that the fruit is not torn or impaled on

the spikes.

After the fruit is punctured, it is gently

boiled in water for about 30 minutes, but the

time of boiling will depend on the condition of
70 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

the fruit. Boiling should not be so prolonged

that the fruit disintegrates, but merely sufficient to render the kumquat tender. The fruit

IS then drained and boiled for 30 minutes in a

20-percent sugar sirup. Additional sugar is

added to increase the sugar content to 40 percent and boiling continued for another 30 minutes. It is then allowed to remain in this solution overnight. The next day sufficient sugar

is added to increase the sucrose content to 55

percent, and the fruit is boiled for 1 hour and

allowed to remain in this sirup overnight. It

is then drained and any collapsed fruit is

culled. The remaining fruit is packed into jai-s

and covered with boiling 60-percent sirup. The

jars are sealed and allowed to cool in air, and

no further processing is necessary.

CITRON PEEL

Brined citron should be boiled in several

changes of water until tender, or until a needle

will easily penetrate. The boiling time can be

reduced by cutting the peel into small pieces

of the desired shape. Both salt and sulfur dioxide are substantially removed in this tenderizing process. To restore crispness the freshened

peel should be soaked in cold water.

Sirup used in candying must contain some

invert sugar in order to prevent crystallization

of sugar in the finished product. The original

sirup may be made up to contain either 60 percent of sucrose and 40 percent of dextrose, or

corn sirup, or all sucrose plus approximately

0.05 percent of citric acid. Candying procedures vary, but all consist of heating the cured

citron in increasingly concentrated sirups over

an extended period of time.

The following procedure has been found to

produce an attractive stable product. Prepare

a 20-percent sirup, heat to boiling, and add the

tenderized peel. Set aside overnight. Add 1

pound of sugar for each pound used in the

original sirup, reheat to boiling, and set aside

overnight. Repeat this procedure until 4

pounds of sugar have been added for each

pound used originally. After the peel has set

overnight in the final sirup, remove and drain

the peel, then dip quickly into boiling water

twice, and dry the pieces on wire trays under a

current of warm air.

For use in baking, the candied citron may be

stored indefinitely in sealed containers without

further treatment. For use as a confection, the

pieces may be rolled in powdered sugar while

still damp, or they may be glazed by dipping in

a hot saturated sugar solution containing 3

parts of sucrose to 1 part of corn sirup, draining, and drying in a current of warm air.

WASTE DISPOSAL

Dilute Liquid Waste

Great strides toward the elimination of

wastes created during citrus processing have

been made by utilizing some of the solid and

concentrated wastes for feed and molasses

manufacture (p. 73). However, there remains

the problem of disposing of large volumes of

process water containing small amounts of

citrus juice.

When the processing plants were small, their

dilute waste could be dumped into streams,

lakes, or tide waters without upsetting the

biological balance of the receiving waters.

However, citrus processing has become a major

industry, and organic solids carried by dilute

waste are now calculated in tons rather than

in pounds. When these materials decompose,

oxygen in the water is used up, fish and plants

die, and anaerobic or putrefactive reactions set

in with the consequent creation of a major

nuisance.

Factors affecting the magnitude of the wastedisposal problem for each plant include the size

of the plant, the products being manufactured,

and the efficiency of plant sanitation. Large

plants, of course, use more water, most of

which becomes contaminated to some extent.

Plants producing single-strength juices use

large quantities of water to wash the incoming

fruit, to cool the canned juices, and to wash

processing equipment. Plants canning fruit

sections use water for the same purposes, and

in addition use hot lye on the peeled fruit and

copious quantities of fresh water to rinse off

the lye solution. Concentrate plants must exercise greater sanitary precautions because their

product is unpasteurized, and they must continuously use large amounts of chlorinated

water to keep conveyors and fruit clean. They

also use large quantities of water in their vapor

condensers. Surface condenser water may not

become contaminated, but barometric condenser water contains some peel oil and picks up a

trace of juice by entrainment. Plants manufacturing citrus pulp and molasses may have

wastes very high in organic material, although

the volume should be much lower than that

from food-processing plants. Effluents from

peel-oil mills are high in organic materials and

are very diflRcult to treat.

Waste waters can be grouped roughly into

three classes according to their soluble-solids

content. First, there is the very dilute waste

containing little more than a trace of organic

matter, such as fruit-rinsing water, surface

condenser water, and water from barometric

condensers of evaporators. Usually these
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 7|

waters can be discharged directly into lakes,

streams, or bays without ill effect." Next comes

the waste of intermediate concentration, such

as floor washings, equipment cleanup water,

and sectionizing waste. These waters range

from a trace to about 2 percent in soluble solids,

and need some treatment unless the receiving

body of water is exceptionally well situated.

The third classification ranges roughly from 2

percent to 6 percent in soluble solids, and includes drippings from can-closing and filling

machines, effluent from peel-oil centrifugals,

and waste alkali from sectionizing or evaporator cleaning. This waste generally requires

treatment of some kind. With the exception of

waste alkali, this waste is such that concentration into molasses may be considered as a

method of treatment. FIGURE 28.—Lagooning of liquid citrus waste.

METHODS FOR DISPOSAL

The organic matter of dilute citrus waste is

high in carbohydrates and low in nitrogen in

contrast to domestic sewage, which is low in

carbohydrates and high in nitrogen. Methods

commonly used for the treatment of domestic

sewage are, therefore, not adaptable without

modification for treatment of citrus waste.

Wastes from a small processing plant and from

a fairly large city might profitably be treated

together (5), since the combined wastes would

have all the nutrients needed for rapid microbial action and consequent reduction of the biochemical oxygen demand (B. 0. D.). A disadvantage to such a plan is that citrus processing

is seasonal, and the peak of the season coincides with an annual influx of winter visitors.

These circumstances necessitate the installation

of oversize treatment plants at considerable

cost. Processing plants have sometimes been

called on to bear a major share of the cost of

such plants.

Plants located near large areas of deep sand,

away from population centers, are fortunate in

having an inexpensive and satisfactory means

of disposal at hand. Dilute waste may be conducted into a series of basins, where the liquid

percolates into the soil and solids are dried by

the sun. Sufficient land must be available to

permit rotation of disposal areas so that surfaces which become clogged may be allowed to

dry and may be cultivated for renewal of their

absorptive capacity {199). One large plant in

California {255) has used this method for a

number of years. Some income is derived by

periodically withholding treatment from a por-

' Under no circumstances can effluents from canneries

or retention tanks be discharged into a stream without

permission from the proper Federal, State, or local

authorities.

tion of the disposal area and by using it to produce farm crops.

Spray irrigation of pasture and wooded land

in Florida with citrus waste {3, 32k) proved

destructive to vegetation, but might prove successful if the waste were applied less heavily

on pasture land bearing a leguminous cover

crop {376).

Pumping waste water into wells was tried in

Florida and in Texas, but was abandoned because of contamination of underground water

supplies. Excessive pressures were developed

by anaerobic fermentation, and the wells blew

back, with damage from ensuing fire in at least

two instances {267).

The lagooning of wastes (fig. 28) has been

tried for a wide variety of cannery effluents.

Only under very special conditions has this

method met with success. Unless expensively

treated with supplemental nitrogen, extensive

aeration, and chemical flocculation, lagoons

soon became offensive {199, 267, 375, h57).

Chemical flocculation and precipitation of

rather heavily contaminated cannery effluents

have been attempted, but have not shown sufficient merit to warrant widespread use. In

Texas several thousand tests with lime and

alum as précipitants indicated an average reduction of 30 percent in B. 0. D. {k31). _ In

Florida the addition of lime for flocculation,

with aeration to promote floating of solids, resulted in a 64-percent reduction m suspended

solids {3, 32U, U63). Ground phosphate rock

has also been used to promote flocculation. Although chemical flocculation can be accomplished at small expense for personnel and

equipment, the cost of chemicals is high. Such

treatment does not significantly reduce the

oxygen demand of the wastes, but is helpiul m

regulating the pH and clarifying the wastes

for subsequent treatment.
72 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

BIOLOGICAL TREATMENTS

An important factor contributing to the

difficulty of biological treatment is the seasonal

and sometimes intermittent supply of citrus

waste. Biological filters and digesters rely for

proper operation on a continuous feed supply of

fairly uniform composition. Biological treatments investigated include simple yeast fermentation, methane production, trickling filters, and an activated sludge process.

Aerated fermentation by yeast is a simple

and rapid method of removing from dilute

citrus waste the sugars, which comprise about

two-thirds of the organic matter present. This

procedure was demonstrated in experiments on

the production of feed yeast (p. 78). Remaining decomposable organic matter includes alcohols, esters, pectins, pectin-degradation products, glycosides, and salts of citric acid (4^50).

Application of yeast fermentation has been

made as a preliminary step in methane fermentation and in activated sludge digestion.

Methane production from fairly concentrated

citrus waste has been extensively investigated

(268-270). Substrates were diluted orange

juice and effluent from a peel-oil mill. Initial

production of methane was satisfactory, but

progressive inhibition of fermentation occurred. It was found that citrus-peel oil contributed the inhibiting factor. Even when a

substrate was prepared from citrus molasses

from which peel oil had presumably been removed during evaporation, there remained a

decidedly inhibitory effect. Complete removal

of peel oil was accomplished by a preliminary

treatment of the waste, consisting of aerobic

yeast fermentation with vigorous aeration

(270, 48i). Waste containing at least 0.5 percent of solids is needed for satisfactory digestion. With waste containing less than 1 percent of solids, lime must be used to maintain a

pH around 7, and nitrogen must be added as a

supplemental nutrient. The principal difficulty

is in retaining sufficient bacterial sludge in the

digester. Complicated sludge manipulation and

the preliminary aerobic treatment make the

value of the process rather limited (269).

Biological oxidation of citrus waste on trickling filters has been extensively considered.

The earliest reported pilot-plant investigations

(Í61) indicated that trickling filters could be

used to eliminate up to 80 percent of the oxygen

demand if the weather remained warm. Later

investigations (324-, 431, 463) showed that with

a high ratio of recirculation (6:1 to 8:1) and

added nutrients (50 p. p. m. of nitrogen and

20 p. p. m. of phosphates) it is possible to operate trickling filters on citrus waste having a

B. 0. D. as high as 2,000 p. p. m. and to produce an effluent with a B. 0. D. of less than

500 p. p. m. The chief difficulty in maintainmg

filter efficiency was due to the intermittent incorporation of strong alkali (used to clean

evaporators and juice lines) in the waste

waters, with resulting destruction of biological

growth on the filters. It has been demonstrated

(230) that segregation of alkaline wash water

for use in citrus-pulp manufacture is economically advantageous. Removal of this adverse

factor would make trickling-filter operation

feasible.

The biological disposal method that has received rather widespread attention is an activated-sludge treatment. The primary reason

for failure of early activated-sludge experiments was the low nitrogen content of citrus

waste. A series of experiments at the Southwest Foundation for Research and Education,

San Antonio, Tex., showed that it may be possible to operate an activated-sludge system

without adding chemical nitrogen (266). Under certain conditions nitrogen bacteria when

inoculated into activated sludge continue to

propagate and supply sufficient nitrogen to permit digestion in a 23-hour cycle to the extent

of 99.9-percent reduction of B. 0. D. of orange

juice diluted 1:49.

Investigations by the Texas State Department of Health have shown that an activatedsludge pilot plant could be operated successfully on wastes from both citrus- and tomatocanning plants (378). This plant operated in

three stages—8 hours' storage, 8 hours' aeration, and 3 hours' final settling. The 8 hours

of storage, in which fermentation was started

with brewer's yeast, was necessary to condition

the citrus waste for digestion in the second

stage by organisms originally obtained from a

municipal activated-sludge plant. Part of the

sludge from the settling tank was returned to

the aeration tank and the balance discharged.

When dilute waste with a B. 0. D. of less than

1,000 p. p. m. was used, reductions in oxygen

demand Avere obtained to as low as 1 p. p. m. ;

when waste with a B. 0. D. up to 2,000 p. p. m.

was used, effluents below 500 p. p. m. were obtained. No mention of the use of added nutrients was made.

In Florida, experiments (96) were run on

orange juice diluted to a B. 0. D. of about

2,000 p. p. m., with results similar to those reported above. Other experiments in Florida

(463) indicate increased efficiency of activatedsludge treatment when 50 p. p. m. of nitrogen

and 20 p. p. m. of phosphates were added as

nutrients.

At least one processing plant in Florida handles its disposal problem by emphasizing in-
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 73

plant sanitation (273, 1^85). An analysis was

inade of each plant operation, and its contribution to waste actually measured. It was always

found that waste could be reduced, and sometimes it was turned into profit. Such concentrated waste as bin drippings and cleanout

water from evaporators, tanks, and homogenizers was pumped into storage tanks for conversion into feed molasses. Settling tanks were

installed in condensate lines from molasses

evaporators to permit recovery of distilled oil,

which not only returned a profit but also removed an unexpectedly potent pollution factor.

A program of employee education was instituted, emphasizing the elimination and reporting of leaks, the conservation of water, and

general good housekeeping. As a result of this

overall program dilute waste from the plant

was being emptied directly into a salt-water

bay without treatment and without creation

of a nuisance.

Citrus Feed

When citrus fruits are processed for juice or

sections, there remains for disposal or utilization 45 to 60 percent of their weight in the

form of peel, rag, and seeds. This material was

originally considered as waste, and for lack of

a better name is still so called. An analysis of

grapefruit cannery waste is reported in table

22.

TABLE 22.—Analysis of g rapefmit cannery

waste (335)

Florida

grapefruit

California

grapefruit

Constituent

Peel Rag

Peel and

rag

combined

Total solids _

Percent

16.71

.74

.43

.74

1.71

1.18

.28

6.35

.83

3.10

.40

Percent

15.60

75

''"''63'

1.44

1.06

.16

6.30

.44

3.56

.10

Percent

22.02

Ash

Volatile oil

.70

.56

Acid, as citric

Crude fiber

.43

2.00

Crude protein (NX6.25)--

Crude fat (ether extract)

Total sugar (as invert)

Pentosans

Pectin (calcium pectate)

Naringin

1.63

.23

8.68

1.31

3.93

.63

In the 1920's and early 1930's the quantity of

such waste was insufficient to cause concern to

the citrus-canning industry H57, Jf58), Sometimes the material was returned to groves for

its fertilizer value, and disked in or covered

with soil to discourage fly breeding. Soil-building value of the waste is largely limited to its

humus value. Its nitrogen content (about 0.14

percent) is insufficient to support the bacterial

action of decomposition, and it will therefore

rob the soil of existing nitrogen unless supplemented by chemical fertilizers. One method of

overcoming this deficiency was to add cyanamide to the ground material. Between 200 and

400 pounds of calcium cyanamide were added

to each ton of ground waste, mixed thoroughly,

and allowed to stand until the mass became dry

and crumbly. There was a gradual loss of ammonia from the mixture, and no guaranteed

analysis could be safely given. Sometimes nitrates, ammonium sulfate, and superphosphate

were added along with the cyanamide {217,

398).

Feeding value of cannery waste was early

recognized by dairymen and cattlemen located

in the vicinity of citrus canneries {1, 285, 353).

Because of the high water content and perishable nature of the waste, it could not be transported economically for feeding purposes. The

fresh material was difficult to handle, fermented

rapidly, and soured, and became a ñy-breeding

and odoriferous nuisance. Its preservation by

ensiling has been reported {25, 38), but the

method has not been widely adopted. Stabilization of the waste by drying seemed the logical

manner for preservation to permit distribution

and storage. However, direct drying could not

be successfully practiced because of the high

moisture content (80 to 85 percent) and the

slimy consistency of the waste.

It was found that the hydrophilic nature of

the pectin in the waste could be destroyed by

adding lime {67, 250), and its moisture content

reduced by draining and/or pressing. A number of patents {J,8, U9, 65, 67, 21^3, 250-252,

309, U52-U5U) covering the drying of citrus

waste have been issued. . As early as 1927 orange pulp was being dried

in California for use as a livestock feed. Dried

grapefruit pulp was available in Florida m

about 1932, and in 1938 Texas producers were

shipping as far as New England. Considerable

experimental work has been carried out on the

feeding of dried waste to both dairy and beet

cattle (7, 69, 89, 206, 236, 285, 310, m) • In

general, it has been found to be similar m leed

value to beet pulp, being low in crude protein,

fiber, and fat, but high in nitrogen-free extract,

or carbohydrate, which is 88 to 92 percent digestible. It is classed as a high carbohydrate
74 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

FIGURE 29.—Loading citrus waste onto a dragline for grinding and liming. (Courtesy of Florida Citrus Canners

Coop., Lake Wales, Fla.)

concentrate. A typical analysis of dried citrus

pulp (169) is as follows:

Constituent Percent

Ash 4.3

Ci-ude fat 3.5

Crude fiber 13.0

Ci-ude protein 6.2

Dry matter 92.0

Nitrogen-free extract 63.0

Modern methods of extraction and finishing

have increased juice recovery, reducing the

percentage of waste in 10 years from about

65 percent of the fruit entering the processing

plant to an average of less than 50 percent.

Citrus waste in 1955 probably amounted to

about 2 million tons, most of which was converted to cattle feed, with a production in

Florida at that time of some 300,000 tons.

There is a great deal of variation in plant

design and in methods used for the manufacture of cattle feed from citrus waste. Detailed

descriptions of individual plants have been published elsewhere (150, 163, 169, 192, 389), and

therefore a general description of the major

processing problems will suffice here.

Figure 29 shows citrus waste being loaded

onto a dragline for grinding and liming. Figure 30 shows the sequence of operation in a

feed and molasses mill and gives performance

examples. The peel, rag, and seeds are disintegrated in hammer mills or shredders, then 0.3

to 0.6 percent of lime is added, either dry or

as a slurry. Sometimes the lime is added before

or during shredding. Some operators add lime

at a constant rate, whereas others vary its addition according to color changes or the pH of

the aging mixture.

Pug mills, which in their simplest form are

large screw conveyors with nights interrupted

to promote mixing, are used for rapid curing.

Constant agitation and remixing of the pulp

and released liquor provide intimate and renewed contact between reacting constituents.

When bins are used, a minimum of i/> hour is

required for curing.

After proper curing the waste is in satisfactory condition to be pressed. Pressing is facilitated if the material is heated. Continuous

presses are used to reduce the moisture content

from 81-83 percent to 70-75 percent, during
CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS 75

PEEL, RAG, SEEDS

2,000 lbs.

LIME-IO lbs.

^^^^^^^^^^^^^^

^VLVkikVk.Vi^^^

DISINTEGRATOR

DELAY SCREW OR

PUG MILL PRESS

PRESS CAKE

900 lbs.

DR I ER

PRESS

LIQUOR

130 GALS,

VAPOR CONDENSER

PREHEATER

ST RIPPER OIL

l'/4 QTS.

MEAL

28 lbs.

PULP

250 lbs.

FINES"

»/2 lb.

CITRUS

MOLASSES

13 GALS.

SLUDGE

SETTLING

TANK

/^r^rKV

\¿V=J¿

EVAPORATORS

FIGURE 30.—Flowsheet showing steps in the production of citrus feed and citrus molasses.

which operation more than half of the original

weight of the waste is removed in the form of

press liquor containing 10 to 15 percent of dissolved solids. Treatment of the press liquor is

discussed on page 77. After the material is

dewatered, either by drainage or pressing, it

is ready for drying.

A single-shell, direct-fired rotary kiln is the

simplest form of drier (fig. 31). Cured waste,

obtained as described, is dropped into a blast of

air and furnace gases at the end of the kiln

next to the firebox, and is lifted and dropped

repeatedly through the hot air stream by staggered oblique vanes attached to the inside of

the rotating kiln. Progress of the feed through

the kiln is enforced by proper placement of the

vanes. Also as the material dries and becomes

lighter, it becomes more susceptible to the action of the air draft induced by suction fans,

and the very small pieces, which dry first, are

drawn out and collected in a cyclone separator.

This material, called ^'fines,'' is sold as a fertilizer extender. The heavier pulp leaves the kiln

at temperatures above 200° F., and is cooled by
76 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

FIGURE 31.—Rotary driers for making citrus waste into feed. (Courtesy of Florida Citrus Canners Coop.,

Wales, Fla.)

Lake

being dropped through a current of fresh air

in a rotating unheated cooler. The last few feet

of the cooling cylinder may be composed of

wire netting, which screens out the smaller

pulp pieces. This material, called "meal," is

bagged separately and sold for mixing in prepared feeds, or may be pelleted for range

feeding. Fines and meal amount to less than

10 percent of feed-mill production.

Some of the larger plants having several

kilns group them and use a 2-stage process.

Several kilns at high temperatures can be used

for primary driers, reducing the moisture content of the pulp to 35-40 percent. Combined

output of 3 or 4 primary driers can be finished

to 10-percent or lower moisture in a single

secondary kiln maintained at lower temperatures. Heavier loading of the secondary kiln

with a lower ratio of air to product is possible,

because surface moisture of the pulp has already been removed and drying can be no

faster than the rate of migration of the moisture from inside the pulp pieces. Air warmed

sufficiently to maintain an outlet temperature

of 210° to 230° F. serves to remove moisture

vapor as fast as it is released, and is not hot

enough to cause scorching of the product.

Variations in design of direct-fired driers include single-shell kilns 4 to 6 feet in diameter

and 30 to 60 feet long and double- or triplepass kilns, which have concentric cylinders

with the product passing first through the center of the smaller cylinder and then consecutively through the annular spaces between

them. Steam tube driers produce minimum

heat damage to the product, but they remove

moisture so slowly that multiple units are

usually required and the investment in equipment is extensive. For good economy, exhaust

steam should be used to heat the driers.

Pressing of cured pulp is generally practiced

in Florida because of high fuel costs, but in

Texas cheap fuel is available and pressing has

been omitted. During aging in bins, where free

drainage of released moisture occurs, the moisture content is reduced by only about 2 percent.
^^^,_^^ vMlns handling unpressed cured waste

'ii'Ulï'^Vîaporate more than twice as much water

per unit of waste as those handling press cake

(163). Cost of kilns to provide extra capacity^

is less than the cost of presses and multipleeffect evaporators required to handle large

amounts of press liquor. The lower efficiency

of fuel utilization in kilns as compared with

that in multiple-effect evaporators should be

studied before the type of operation is finally

determined.

Some plants in Florida eliminate the need

for pressing by returning partially dried feed

to the pug mill to absorb moisture as it is released from freshly limed and ground peel.

Such a scheme of operation results in eliminating the problem of disposition of the press

juice and drainage liquors. Plants allowing

free drainage from limed peel in bins must handle or dispose of drainage liquor, which

amounts to as much as 20 percent of the weight

of waste received (389). Plants using presses

must handle in the form of press liquor more

than half the waste received.

Manufacturers using presses may produce

feed containing the full complement of carbohydrates of the whole waste by concentrating

their press liquor in multiple-effect evaporators

and returning it to the feed line at any one of

four places : To the press cake as it enters the

primary driers (2^3), to the partially dried

feed between stages, to the hot dried feed, or

even to the feed brought from the warehouse

for mixing. Controlled tests indicate that pulp

containing up to 50 percent of added molasses

tends to pick up little, if any, more moisture

than does dried pulp alone, when stored at a

relative humidity of 70 percent or less. All

samples, with or without molasses, will mold

when stored at a relative humidity of 80 percent or higher (32).

From 8 to 12 tons of fresh waste are required to yield 1 ton of dry feed.

Citrus Molasses

Liquid obtained from cured citrus waste

contains from 10 to 15 percent of soluble solids,

of which from 50 to 70 percent are sugars

(322, Í51 ). This material, which may amount

to more than half the total weight of cannery

refuse available to the feed mill when presses

are used, is far too valuable to discard. Furthermore, disposal itself would be rather expensive. The liquor has a B. 0. D. of between

40,000 and 100,000 p. p. m. (-457), and creates

an excessive nuisance when dumped into lakes

or streams. This carbohydrate-rich material

may be used for making feed molasses, as it

can be mixed with citrus pulp or sold separately to cattle feeders or to manufacturers of

mixed feeds.

For conversion to molasses the press liquor

is passed through vibrating screens to remove

as much of the suspended solids as possible,

then flash heated to reduce subsequent scaling

in the evaporator tubes used to concentrate to

72° Brix. Scaling can be further reduced by

using waste alkali from sectionizing plants

and evaporator cleanup operations (230). If

the liquor is superheated (220°-260° F.) in a

tubular heat exchanger, then is flashed into a

vapor-liquid separator, and the vapors are conducted to a surface condenser, an appreciable

ducted to a surface condenser, an appreciable

ered as a byproduct (3^5), Some clarification

of the press liquor can be obtained by holding

it in settling tanks after heating, then drawing

off the evaporator feed material from a point

below the surface and above the settled solids.

The sludge, composed of both floating and

settled solids, may be disposed of by spreading

it on sandy soil. Heating alone apparently modifies the material so that scaling in evaporator

tubes is reduced, but clarification prior to concentration gives a molasses that is better appearing, less viscous, and more stable and that

has higher sugar and lower ash than molasses

obtained from unclarified liquor (167).

Clarification has been obtained in Texas by

heating with submerged combustion burners

(Jf8, U9, 389). In this method carbon dioxide

comes into intimate contact with the liquor

constituents and promotes defecation by carbonation, in addition to the effect of temperature. In practice the Hquor is slightly concentrated by a submerged burner to about 15""

Brix, then passed through a clarifier, or continuous settler, and into a second submerged

burner, where it is further concentrated to

about 22° Brix. The discharge from the second

burner goes into additional settling tanks,

thence to a multiple-effect evaporator.

At least one feed mill produces citrus molasses from press liquor by use of falling-film,

low-temperature evaporators similar to those

used for the production of frozen citrus concentrates (p. 40). With this equipment scaling

of evaporator tubes is apparently not a problem The molasses produced is light m color

and, although unclarified, is not exceptionally

viscous.

Citrus molasses is normally a thick viscous

liquid, which is dark brown to almost black

and has an extremely bitter taste. Florida

standards require it to contain at least 45 percent of total sugars, as invert, and to test not

less than 35.5° Brix after dilution with an

equal weight of water (169). During storage

there is a small decrease in pH and m total
^ö AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

sugars and a small increase in invert sugar and tions. The production of feed yeast has been

in viscosity (168), considered, because it is rich in protein and BA typical analysis of Florida citrus molasses ^ vitamins. Since fermentation and production

(i^9) is as follows: It has a concentration of of this type of yeast are very rapid, opportuni72 Brix, a pH of 5, and a viscosity of 2,000 ties appear favorable for utilization of press

centipoises at 25° C. In addition to 35 p. p. m. liquors on a large scale.

of niacin, 11 p. p. m. of riboflavin, and 10 The potential supply of fermentation sugars

p. p. m. of pantothenic acid, its composition from such liquor is over 100 million pounds a

includes— year considering the liquor that could be made

Constituent Percent from all the fruit used in citrus-processing

Nitrogen-free extract 62.0 plants. The potential yield of dry yeast from

Total sugars 45.0 this liquor would be about 50 million pounds.

KuS sùïaVs" 23*5 Strains of Torulopsis utilis have generally

Sucrose IIIIIIIIIIIIIIII 20Í5 heen used for feed yeast, because they can be

Carbonate ash 4.7 propagated on a wide variety of materials and

Acid, as anhydrous citric 4.5 are rapid growers.

G£S^^*^^ 3'o The crude protein content of dried Torula

Pentosans i I'.e yeast from citrus press liquor ranges from 45

Pectin 1.0 to 55 percent (9^^ 322), and compares favor-

^^f -2 ably with other proteins in digestibility. AlPotassium^ !_III 1 1 1 11 ill though studies have not been conducted on the

Calcium .8 amino acids present in yeast from citrus waste.

Sodium .3 the composition of yeast from other sources

Magnesium .1 ^jji serve as a guide. Torula yeast has been

ChlorineVJT" 07 found to contain arginine, cystine, glutamic

Phosphoinîs IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII !o7 â,cid, glycine, histidine, isoleucine, lysine, methiSilica .01 onine, phenylalanine, threonine, tryptophane,

Manganese .008 tyrosine, and valine. All amino acids that have

Copper .003 been found indispensable to humans, rats, and

Citrus molasses resembles cane molasses and Clicks have been found in reasonable quantity.

is similarly used for feeding both dairy and Feedmg tests {9i, 237 238, m) have mdirange cattle (n, 189, 359, 360). Its extreme cated a deficiency of the essential ammo acid

bitterness makes it unsuitable for human con^ methionine, but this is not serious as the shortsumption unless treated for removal of narin- f^^. ^.^^ be compensated for by including a magin {50, ÍOA), but the bitterness does not terial containing this animo acid such as a

affect its usefulness in cattle feeding. Citrus T^^^^J u "^ *^^ f^^^'"^Ç l^^.^î'u ^'? methionine. It

molasses may be admixed with pressed pulp should be pointed out that the inclusion of yeast

just prior to being placed in the drying kilns, ^^ ^^e human diet is based chiefly on its value

and thus the ratio of total digestible nutrients of supplementing the ammo acid and vitamm

to crude fiber is increased in the dried product deficiency of cereal grains. It should not be

without destroying the keeping quality of the f^P^Í^-H^ ^^ ^^^ ^^ ^^^ ^^'^ ^^^^^^^ ^^ protein m

pulp {32). It may also be used in the prepara- ^^e diet of either animals or humans.

tion of balanced prepared rations. In Florida The mtrogen-contammg substances m yeast

large quantities are fed directly to cattle from have been designated as crude protein m rectroughs supplied automatically with molasses ognition of the fact that other nitrogenous confrom storage tanks. As much as 6 pounds, or stituents are present. About 80 percent of the

0.5 gallon, may be consumed per head per day. nitrogen is as ammo acids {Wl ). On the basis

Citrus molasses may be used for feeding swine of the total nitrogen, nonprotem constituents

{77, 78), but is not so readily accepted by have been found as follows : Purines 8 percent,

swine as by cattle. pyrimidines 4 percent, choline 0.5 percent, and

The feeding value of citrus molasses is some- glucosamine 0.5 percent {286),

times enhanced by ammoniation {283, 292) or Phosphorus is another important constituent

by the addition of urea to increase the crude of yeast, and averaged about 1.4 percent in 17

protein content. samples of dried yeast prepared from citrus

waste.'' This value is in agreement with that

P . YAiict obtained from yeast from other sources, but is

reea leasr j^j^j^ j^ comparison with most feeds. Much of

Press liquor obtained during the manufacture of dried citrus pulp contains sugars avail- . Unpublished data from the Citrus Products station

able as a raw material for industrial fermenta- (u. S. Dept. Agr.), Winter Haven, Fla., 1955.
FIGURE 32.—Plant for producing akohol from citrus

waste.

CHExMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS

the phosphorus is in the form of nucleic acids,

and the presence of these compounds is considered as a complicating factor in evaluating

yeast as a source of protein.

Yeasts are regarded as good sources of Bvitamins. In one study (45Í ) a sample of

T. utilis produced from citrus press liquor gave

the following values : Thiamine 2.8 to 2.9, riboflavin 3.6 to 4.2, niacin 50.4, pantothenic acid

8.4, all expressed as milligrams per 100 gm. ;

and ergosterol 0.5 percent.

In earlier work on the production of yeast

from citrus press liquor, a batch method was

used {322). Later a continuous process was

developed (-451), in which a nutrient solution

prepared from citrus press liquor was fed continuously into an aerated propagator and an

equal volume of yeast slurry simultaneously

harvested. The average time in the propagator

was somewhat less than 3 hours. During fermentation practically all the sugars are consumed. The yield of yeast will vary with the

sugar content of the mash, lower sugar contents favoring higher percentage yields based

on the amount of sugar present. A 2-percent

sugar mash will give a yield of about 40 percent based on the sugar consumed.

Although citrus press liquor is rich in carbohydrates, certain nutrients need to be supplied,

particularly nitrogen and phosphorus. Nitrogen can be supplied in the form of ammonia

and ammonium sulfate. When the organisms

have utilized the ammonia from ammonium

sulfate, sulfuric acid remains and serves to control the pH. By varying the ratio of ammonia

to ammonium sulfate, the pH of the fermentation can be controlled within reasonable limits.

A pH of about 4.0 appears to be the most

favorable for growing the yeast. Phosphorus

may be supplied as trisodium phosphate. Practically all the nitrogen and most of the phosphorus to be found later in the yeast must be

supplied. Approximately 0.36 pound of ammonium sulfate and 0.18 pound of trisodium

phosphate must be added for each pound of

yeast produced.^

Air must be supplied in generous quantities

in order to promote rapid yeast growth and

minimum alcohol production. Air is introduced

at the bottom of the fermenter and passes up

through the liquid as fine bubbles. A method

commonly used in the baker's yeast industry is

to place coils with i/ig-inch holes at the bottom

of the fermenter and connect an air-pressure

line to the coil. Another method is to introduce

air through porous ceramic material in the

form of candles or a false bottom in the yeast

propagator. In still another procedure air is

dispersed by a Waldhof aeration wheel {201)

79

or arrowhead turboimpeller {352). Less air is

required if the bubbles are finely dispersed.

The approximate range is from 200 to 500

cubic feet of air per pound of yeast produced.

Industrial Alcohol ^

Oranges, grapefruit, and tangerines contain

considerable amounts of sugar, which can be

converted to ethyl alcohol by yeast fermentation. Citrus juices have been used for the preparation of alcoholic beverages, such as wines,

brandies, and cordials {i60). Because a slight

bitterness remains after sugar removal, dry

wines have not found favor, and sweet citrus

wines are preferred.

Industrial alcohol can be made from the

press liquor obtained in drying citrus waste for

feed or from citrus molasses (-457). Citrus

press liquor is too dilute in fermentable sugars

to provide a reasonable concentration of alcohol

for economical recovery. Citrus molasses may

be added to the press liquor until the sugar

content is 10 to 12 percent. Frequently citrus

molasses is merely diluted with water to the

proper concentration. Figure 32 shows a plant

where alcohol is made from citrus waste.

No particular difficulties are encountered in

the fermentation by yeasts (preferably Torwlopsis utilis), but the pH of the mash should be

adjusted to about 4.0 by the addition of sulfuric acid. Fermentation is complete in 3 days,

and about 90 percent of the theoretical yield

(45 percent of sugars) has been reported

{322). Dried yeast can be obtained as a byproduct, or the still residues can be concentrated for stock feed. If press juice is used,

some peel oils will be found in the alcohol from

the rectifying column. Special equipment is

needed for removal of this oil prior to fermen-

" Anyone contemplating the production of alcohol

should first consult the proper authorities in the Alcohol and Tobacco Tax Division, Internal Revenue Serv-

' See footnote 7.
80 AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE

tation. Use of citrus molasses eliminates this

problem.

Lactic Acid

Lactic acid has been considered from time to

time as a possible fermentation product from

citrus juices, citrus press liquor, or citrus molasses. Lactic acid is used in certain food products and in the preparation of plastics. Fermentation is easily controlled, because it is conducted at IIS"^ to 131° F., which is too warm

for most other fermentations. Lactobacillus

delbrueckn is a common organism used for

lactic acid production. The chief difficulties in

fermentation are in the recovery and purification of the acid. Fractional distillation is not

possible because of the high boiling point of

the acid, and other means of recovery are

necessary.

Experiments have been conducted on the production of lactic acid from grapefruit juice

(319y 321). A starter of naturally occurring

organisms was prepared by neutralizing the

juice with calcium carbonate and incubating at

122"^ F. for 2 to 3 days. Approximately 5 percent of the starter was used to inoculate the

main batch. The main fermentation was conducted at 122° to 130°, the mash being neutralized periodically with calcium carbonate to

keep the pH between 4,0 and 6.5. Fermentation

required 6 to 8 days for completion. Sugar

utilization was 81 to 92 percent, and the yield

of lactic acid amounted to 71 to 84 percent of

the sugar fermented.

At the conclusion of the fermentation calcium hydroxide was added until the batch was

alkaline to phenolphthalein. The precipitated

calcium citrate was removed by filtration, and

the filter cake washed with hot water. Combined filtrate and washings were treated with

activated carbon, and the mixture again filtered. The filtrate was concentrated to 21°

Baume, and upon standing calcium lactate was

crystallized. Additional crystals were obtained

by further concentration, and purification was

accomplished by recrystallization. Lactic acid

was obtained by adding sulfuric acid to the

calcium lactate and separating the precipitated

calcium sulfate.

Other methods for the recovery and purification of lactic acid have been considered because

of the losses incurred in the preceding method.

One approach to the problem consists of using

ammonia instead of calcium carbonate or hydroxide to neutralize the acid as it is formed

during fermentation {116, 117). Ammonium

lactate remains in solution during fermentation, clarification, and concentration, and is

converted into butyl lactate, which is sufficiently volatile to be purified by fractional distillation. In the laboratory an impure solution was

evaporated at a pressure of 30 mm. of mercury

to a concentration of 71 to 73 percent of ammonium lactate, then butyl alcohol added, and

the mixture refluxed for 6V2 hours. The vapors

escaping were fractionated to recover ammonia,

butyl lactate, and alcohol. The alcohol was returned to the system, and the ammonia could be

recovered for use in the next fermentation.

The butyl lactate was of high purity, and could

be marketed as such or hydrolyzed. This method has been investigated on a pilot-plant scale

at Lake Alfred, Fla.
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1948. THE INFLUENCE OF HESPERIDIN-C ON ABNORMAL CAPILLARY FRAGILITY IN RHEUMATOID ARTHRITIS PATIENTS. Del.

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1951. GELATION AND CLARIFICATION IN CONCENTRATED CITRUS JUICES, INTRODUCTION

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1945. ASCORBIC ACID RETENTION IN FLORIDA

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INDEX

Acetaldehyde in—

Lemon pulp, 13

Orange juice, 14

Acetvlesterase, 7

Acids, in citrus, 7-8, 9, 14, 17, 18

Changes in, during ripening, 8, 15

Estimation of, 8

Addback juice, 42

Alcohol :

In orange juice, 14

Preparation of, from citrus waste, 79-80

Amino acids:

Distribution of, in Valencia oranges, 5

Effect of heat and storage on, 4

In different citrus juices, 5

Anthocyanin, in blood oranges, 15

Ascorbic acid. (See Vitamin C.)

Auranetin, 10

Aurantamarin, 10

Bacteria in citrus, 45-47

Bacterial counts, methods for, 45-46

Beverage(s) :

Analyses of, 37-38

Bases, 36-38

Bioflavanoids. (See Flavonoid(s).)

Biotin, 22

Bitter principles of oranges, 11-12

Chemistry of, 12

In relation to rootstock, 12, 13

Removal of, 12

Seasonal variation in, 12

Time of disappearance of, 12

Blended citrus juices, 28-29

Bottler's bases, 37-38

Pasteurized, 37

Preserved with sulfur dioxide, 37

Brined citrus, 68-69

Browning reaction, 3, 4

Candied citrus, 69-70

Cannery waste, grapefruit, analysis of, 73

Canning:

Grapefruit juice, 28

Sections, 30-32

Lemon juice, 29-30

Frozen, single strength, 30

Lime juice, 29

Mandarin sections, 32

Orange juice, 24-28

Retention of vitamin C during, 21, 22, 28

/3-Carotene, 15, 21, 22

Chilled orange juice, 33-34

Choline, 22, 78

Citric acid:

In citrus juices, 8

Preparation of, from lemons, 57-60

Citron :

Brining of, 69

Candying of, 70

Peel, candying of, 69

Citronetin, 10

Citronin, 10

Citrus—

Feed:

Analysis of, 74

Feed value of, 73

Citrus—Continued

Feed—Continued

Preparation of, 73-77

Yield of, 77

Molasses:

Addition of, to citrus feed, 77

Alcohol from, 79-80

Ammoniation of, 78

Analysis of, 78

FeedValue of, 78

Florida standards for, 77

Preparation of, 75, 77-78

Viscosity of, 78

Yield of, 75

Cloud loss in citrus juices, 6-7, 19, 27, 28, 44

Concentrates:

By freezing, 42

Frozen grapefruit, 48

Lemon, 47

Lemonade, 47

Limeade, 48

Orange, 38-48

Evaporators for, 40

Gelation in, 7, 43, 44

Microbiology of, 45-47

Quality control of, 42-45, 46

Storage of, 44

Survival of coliforms in, 47

Vitamin C in, 21, 24

Tangerine, 48

Pasteurized, 35-36

Lemon, 36

Lemonade, 36

Cutback juice, 40

Cytochrome oxidase, 7

Deaeration, 27

Decarboxylases, 7

Dehydrated citrus juices.

Deoiling, 27

Enzymes, 6-7, 20

Classification of, 6

Inactivation of, 6

Pectic, 6-7, 20

Ergosterol, in feed yeast, 79

Eriodictyol glucoside, 10

Essential oils: . _

Addition of, to frozen concentrated juice, 4ö

To powdered orange juice, 51

Composition of, 65

Concentrated, 64

Fold, 64

Preparation of, 62-65

Storage of, 65

Stripper, 64, 77

Terpeneless, 64

Volatile constituents of, 64

Yields of, 64

Fatty acids in—

Citrus seed oils, 17

Orange juice, 18

Pulp, 18

Rind, 18

Flavanones, in citrus fruits, 11

Flavonoid(s), 9, 10-11, 61-62

Function of, in citrus, 9

(See Powdered citrus juices.)

97
98 AGRICULTURE HANDBOOK 98, if ^ J^ DEPARTMENT OF AGRICULTURE

Flavonoid ( s) —Conti nued

Glycosides. (See Flavonoid(s).)

Preparation of, 61-62

Tests for, 9

Uses for, 61

Fold oils, 64

Folie acid, 22

Frozen concentrates. (See Concentrates and under individual fruits.)

Gases in citrus juices, 13

Gelation, in frozen concentrates, 7, 43, 44

Grapefruit :

Cannery waste, analysis of, 73

Juice:

Acids in, 8

Ash constituents of, 24

Canning of, 28

Flavanones in,11

Flavonoids in, 10

Nitrogenous constituents in, 4, 5

Sugars in, 16

Vitamins in, 22

Volatile flavoring constituents of, 14

Pigments in, 15

Sections, canning of, 30-32

Hesperetin, 10

Hesperidin, 9, 10, 16, 44, 61, 62

Chalcone, 10

In lemon juice, 11

Oranges, 11, 44

Methyl chalcone, 62

Preparation of, 61-62

Hydrogen sulñde in citrus, 14

Inorganic constituents, 24

Inositol, 21, 22

In-package desiccation, 51

Isohesperidin. (See Naringin.)

Isolimonin, 11, 12

Juice, chilled, 33-34

Sacs, oil in, 3, 13

Kumquats, candied, 69

Lactic acid, from citrus waste, 80

Lemon juice:

Acids in, 8

Bottled, 30

Canning of, 29-30

Dried, 49

Flavanones in, 11

Flavonoids in, 9

Frozen concentrated, 47

Single strength, 30

Nitrogenous constituents of, 5

Products from, 30

Proteinases in, 7

Sugars in, 16

Vitamins in, 22

Lemonade, frozen concentrate for, 47

Lime juice:

Canning of, 29

Flavonoids in, 10

Frozen concentrate from, 48

Nitrogenous constituents of, 5

Sugars in, 16

Types of, 29, 37

Vitamins in, 22

Limeade, 48

Limes, pickled, 68-69

Limonexic acid, 12

Limonin, 11,12, 61

Lipids, 17-18

Classes of, 17

Lipids—Continued

Effect of pasteurization on, 18

Extraction of, 17

In Florida Valencia orange juice, 18

Rind and pulp of oranges, 18

Storage changes in, 18

Lycopene, in grapefruit, 15

Mandarin sections, canning of, 32-33

Marmalades, 66-68

Maturity index, 8

Methane, from citrus waste, 72

Molasses. (See Citrus molasses.)

Naringenin, 9, 10

Naringin, 9, 10, 11, 16

Change of, in grapefruit during maturation, 9, 11

Content of, in grapefruit, 11

Preparation of, 11, 62

Neohesperidin, 10

Niacin, 6, 22, 78, 79

Nicotinic acid. (See Niacin.)

Nitrogenous constituents, 3-6

Nobiletin, 10

Nomilin, 12

Off-flavor:

Buttermilk, 46-47

By amino acid-sugar interaction, 3

Lipid rancidity, 18

Micro-organisms, 44, 46-47

Volatile constituents, 13, 14

Oil:

Essential:

In relation to flavor in canned juice, 27

Preparation of, 62-65

Stripper, 64, 77

Fatty, 17-18, 60-61

Preparation of, from seeds, 60-61

In juice sacs, 3, 13

Orange juice:

Acetaldehyde in, 14

Acids in, 8, 14

Amino acids in, 4-5

Ash of, 24

Bacteria in, 46-47

Bitter principles in, 11-13

Canning of, 24-28

Composition of, 2

Concentration of, 35-36, 38-42

Drying of, 49-53

Flavanones in,11

Flavonoids in, 10

Furfural in, 14

Gases in, 13

Hesperidin in, 11, 44

Lipids in, 18

Proteins of, 5

Sugars in, 16

Vitamins in, 22, 23

Volatile flavoring constituents of, 14, 27

Orangeade powder, 50

Pantothenic acid, 22, 78, 79

Para-aminobenzoic acid, 22

Pathogens in orange juice, 47

Pectate pulp, 57

Pectic—

Acid depolymerase, 7

Substances, 3, 18-20

Changes in, during ripening, 20

Determination of, 19-20

Nomenclature of, 19

Properties of, 19

Sources of, 20
INDEX 99

Pectin :

Low-methoxyl, 56

Preparation of, 53-57

Standardization of, 55

Uses of, 55

Pectinesterase, 6-7, 20, 44

Assay for, 6

Cause of cloud loss, 6-7

Index of pasteurization, 28

Occurrence of, in citrus, 7

Peel:

Brining of, 68

Candying of, 69

Pigments in, 15

Sugars in, 16

Peroxidase, 7

Phosphatase, 7

Index of pasteurization, 7

Phosphoribiosomerase, 7

Pickled limes, 68-69

Pigments, 1, 15

Isolation and analysis of, 15

Ponciridin, 10

Poncirin, 10

Ponkanetin, 10

Powdered citrus juices, 49-53

Press cake from grapefruit seeds, analysis of, 61

Proteinase, 7

Protopectinase, 7

Provitamin A. (Seefi-Cavotene.)

Puif drying, 50, 51

Purees, frozen, 48-49

Quality control, 42-45

Rhoifolin, 10

Riboflavin, 22, 78, 79

Rootstock, effect of, on bitterness in Navel oranges, 12,

13

Rutin, 10

Seed(s) :

Composition of, 61

Oil from, 60-61

Composition of, 17

Preparation of, 60-61

Press cake from grapefruit, analysis of, 61

Sorbitol, for incorporating flavoring oils, 51

Storage of—

Canned orange juice, 28

Loss of vitamin C, 23

Frozen concentrates, 44

Loss of vitamin C, 24

In refrigerated juice, 21

Pasteurized concentrates, 36

Stripper oil, 64, 77

Structure of citrus fruits, 1-3

Sugars :

In citrus juices, 15-16

Rind of grapefruit, lemons, and oranges, 16

Increase in, during maturation, 15

Kinds of, in citrus, 16

Sulfured citrus juices, 34-35, 37

Methods used, 35

Tangeretin, 10, 15

Tangerine juice:

Canning of, 29

Flavanones in, 11

Flavonoids in, 10

Frozen concentrated, 48

Nitrogenous constituents of, 5

Vitamins in, 22

Terpeneless citrus oils, 64

Thiamine, 22

Tocopherols, 22

Tricin, 10

Vinegar, citrus, 65-66

Vitamin(s), 20-24

B,. {See Thiamine.)

B,. {See Riboflavin.)

Be, 22

Bn, 22

C—

In citrus juices, 21, 22

Beverages, 37-38

Frozen concentrated juice, 21, 24

Loss of, during storage of, 24

Retention of, during canning, 21, 28

Storage of canned juices, 21, 23

In refrigerated fresh juice, 21

Determination of, 21

In citrus juices, 21, 22, 24

Feed veast, 79

E. {See Tocopherols.)

K, 22

P, 61

Volatile flavoring constituents, 13-14, 62-65

Addition of, to frozen concentrate, 45

Amounts present, 13, 62-65

In canned orange juice, 27

Fresh orange juice, 14

Relation to ofl'-flavor, 13, 14

Incorporation of, in powdered orange juice, 51

Water-insoluble in grapefruit juice, 14

-Soluble in grapefruit juice, 14

Waste:

Alcohol from, 79-80

Analysis of grapefruit cannery, 73

As fertilizer, 73

Citrus molasses from, 75, 77-78

Disposal of, 70-73

Biological treatments, 72-73

Chemical flocculation, 71

Flooding wasteland, 71

Lagooning, 71

Spray irrigation, 71

Drying of, 73-77

Feed from, 73-77

Lactic acid from, 80

Yeast from, 78-79

Yeast, from citrus waste, 78-79

Ergosterol in, 79

Vitamins in, 79

Yields of—

Citrus feed, 77

Molasses, 75

Essential oils, 64

Feed yeast, 79

Ùx U. S. GOVERNMENT PRINTING OFFICE: 1956-381654