20 CIRCULAR 183, 'U. 8. DEPARTMENT OF AGRICULTURE

(72) Waverman, H. C,, and Jonus, D. B.
1921, STUDIES ON THE DIGKSTIBIVITY OF PROTEINS 1N VITRO, 1, THE
RELAVIVE DIGESHIKILITY OF VARIOUS PREPARATIONS OF TILE
PROTBINS WROM THM CHINESE AND GRORGTA Vei.VET BEANS. Jour.
Biol, Chem, 47; 285-205
(73) Wnras, H. G., Lewis, J. H., and Jones, D, B,
1927, IMMUNGLOGIC REACTIONS OF Tih GLOBULINS FROM TUR SEEDS OF
LEGUMINOUS VIANTS. THE HIOLOGIC MWACTTONS OF THE VROB-
TABLE PROTEINS, IX. Jour, Infect. Diseases 40: 326-342, illus,

PUBLICATIONS FROM VARIOUS SOURCES

(74) Avwarer, W. O., and Bryant, A, P,
1906, THE CHEMICAL COMPOSITION OF AMERICAN FOOD MATERIALS, U. 9.
Dept. Agr. Of. Expt, Stas, Bul. 28, 87 p., illus. (Rev.)
(75) Brisx, M. J.
1916. ON THE CHEMICAL CONSTITUTION OF THE PLOTEINS OF WHEAT FLOUR
AND ITS RELATION TO BAKING STRENGTH, Jour. Indus, and
ngin, Chem, 8: 138-144,
76) ——— and Pinckney, A. J.
1924, THE IDENTITY OF GLUTIN PROTEINS FROM VARIOUS WHEAT FLOURS.
Cereal Chem, 1: 309-316.
(77) ——— and Sannsrnpa, R.M,
1926, AN IMPROVED METHOD FOR THE PREPARATION OF Witmat GLIADIN.
Cereal Chem, 3: 144-149,
(78) Cross, R. J., and Swain, R. E.
1924. TH AMINO ACID DISTRIBUTION IN PROTEINS OF WIEAT. FLOURS,
Tadus. and Mngin. Chem, 16; 49-52,
79) Diun, D. B., and Arseera,
1925, PREPARATION, SOLUBILITY, AND SPECIFIC ROTATLUN OF WHEAT
entapin, “Jour, Biol, Chem. 65: 279-304, illus.

Ero, 1.
1923-24. BrITRAGE ZUR KENNTNIS DES GEIADINS. Jour, Biochem, 3:
873-392, illus.
(81) Gren, J., and Friepn, G.

1914, BHITRAGE ZU DN PHYSIKALISCH-CHEMISCIIEN WIGENSCHAPTEN DER
ALKONOLLOSLICREN PROYEINE DES WEIZENS UND RoGriExs, Bioe
chem. Ztschr, 66: [154}- 164, ills.

(82)7 Gunspeun, HH. 8,
A861. UBER DIE IM WassHR LUSLICHEN BESTANDIWILE DES WHIZEN-
pape Sitzber, Akad. Wien, Ber. (Abt, 2) 44: 429-444,
2. TL

(88) Kivpaut, J. G.
1892, OPTICAL SEHAVIOUR OF SOmB VEGETABLE pRoTETs. Forhandi,
Skand, Naturfors, 1892: 385-390, [Abstract Jour, Chem, Soe.
70 (pt. 1): 583. 18964
iG, J. and Rivtainy, P
1904, UBEX DIE PROTRINSTOFFE DES WEIZENKEDDERS UND BEIND BEAE-
HUNGEN ZUR BACKPAHIGKEIY DKS WEZBNMBHLES. 1, DIR PROM
THINSTOFFE DKS WEIZENKLEVEHS, Ztschr. Untersucb, Nahr, a.
Genus:
(85) Lamour, RoW.
1927, A COMPARATIVE STUDY OF Tis GLUTBLINS OF THE CRRHAL GRAINS
Jour, Agr, Tescarch 85: 1091-1120.
(86) Leavenwores, C,8., Waxman, A, J., and Osnonve, T. B.
1923. SOME HASIC SUBSTANCKS PROM TNH Tacw OF YE ALPALYA PLANT.
Jour, Biol, Chem. 58: 209-214.

(87). Ltmuns,
1919. BEITRAGE ZUR KOLLOIMCHEMIE DES KKOTHS. Tit. KOLLOMICHEMI-
SOHM STUDIEW AM ROGGEN- UND WHIZKNGLIADIN MOT RESONDERER
BERUOKSICMTIGLNG DES KUEBER- UND BACKPAIIGK HITSCRORLEMS,
Kolioid Zischr, 25: (1774-196, illus.
(88) Matuewso, Wb.
1906. THY OPTICAL ROTATION OF GLIADIN 1Y CERTAIN ORGANIC SOLYENTS
Jour. Aimer. Chem, Soc, 28: 1482-1185.
swrtu, GG.
1901, THE cHmaistey_of WHEAT cLUTEN. Roy, Canad. Inst. Trans,
(1902/3) 7: 497-515,

(89) X.



14 CIRCULAR 183, U. S DEPARTMENT OF AGRICULTU

MEAT AND EGGS

The factor 6.25 is used in estimating the proteins of eggs and meat.
Most of the proteins that have been isolated from animul tissue con-
tain about 16 percent nitrogen.

PROTEINS OF FRUITS AND VEGETABLES

There is not sufficient knowledge regarding the proteins of fruits
and vegetables to justify the calculation of # special factor. Until
more is known regarding the proteins of this class of foods the con-
ventional factor 6.25 should be used.

Tn table 5 are given the nitrogen-conversion factors for some food
substances concerning the protems of which, it is believed, there is
sullicient knowledge to justify the use of special factors for convert-
ing the percentages of nitrogen into that of protein. Although it
should be understood that these factors will not give absolutely
correct values, for the reusons that have been already discussed, it
is believed that they will give the true protein content more aceu-
rately than will the mdiseriminate use of the conventional factor 6.25.

TABLE

Factors suggested for use in converting percentages of nitragen in various
substances into terms of protein !

Factor |: Factor

Substance Substance suggested

Coreal grain
Wheat, endosperm.
Wheat? embryo
Wheat, bran.
Wheat, whole kernel

d
Squash sco
Hrurmpsin seod

Minonsis
Cocomat.

T
| H
| saeested |

1 Ofiseods and nuts—Continued.
Bragil nut.

Hazelant.~

Walnut.

|
|
i
|
i

Castor heat 7

}eslunces of anitnal origin:
Milk,

Eges
Meats.
Yelvtine

Legurainous sds:

Adaulcl bean,
Jack bean

tt
638; wea

For comparison, in table 6 are shown the percentages
(NX6.23) in some commen food mai i

genorally used im ealeulating

eeption af whaat, milk ual gelatin, for whieh the following fuctar

motein pereeninges of all the 3 His

wials as given

this

fare sometimes nse:

of protein
n standard

food-composition tables and the percentages obtnined for the same
material us calculated on the basis of the conversion factors given in

table 5. There
most cases.

Nuts present dilferences of from 2 to 4 percent.

s a marked differenee in the protein percentages in
For geintin, the difference is more than 10 pere

nt.


PROTEIN CONVERSION FACTORS 5

atically investignted, hus been shown to coptain a number of nitrog-
enous compounds which are not even related to protein (86, 99, 100,
101, 102, 108, 104)2 The following are some nonprotein compounds
containing nitrogen that have been isolated from alfalfa leaves:
Stachydrine, asparagine, choline, adenine, trimethylamine, betaine,
arginine, lysine, tyrosine aspartic acid, alanine, valine, leucine,
phenylalanine, serine, and glutamic acid. Unquestionably there are
other nonprotein compounds in alfalfa that h: not yet been dis-
covered, Here again as in the case of potatoes it is evident that the
protein percentage calculated on the basis of total nitrogen must be
far too high.

The statement is sometimes made that, from a nutritional stand-
point, the nonprotcin nitrogenous compounds in food materials may
have a value quite equal to that of protein itself, and that, for this
reason, it is immaterial whether the protein content is based on the
total nitrogen or on the actual protein present. This might be true,
if amino acids were the only nonprotein compounds involved. But,
as is well known, a large number of nitrogenous compounds exist,
both in plant and animal material, which can not replace protein in
the dict.

Tt is not the purpose of this discussion to convey the idea that the
method of estimating protcins by basing it on the total nitrogen
should be abandoned, but to point out some of its limitations so that
those nsing if may be cognizant of its inaccuracics and can be guided
accordingly.

At present one great obstacle to progress in the estimation of pro-
tein in food motcrial is the lack of information regarding the non-
protein nitrogenous constituents of food material, With one or two
exceptions, no plant or animal material has been systematically
studied with the object of learning the nature of its total nitrogenous
constituents. Most investigators have been content merely to isolate
and identify individual compounds. Knowledge of the nature of the
total nitrogenous constituents of food material is essential, not only
for estimating their protcin content but also for ascertaining their
food valuc.

NITROGEN CONTENT OF DIFFERENT PROTEINS

As pointed out in the preceding paragraphs, the crror involved in
estimating protein upon the basis of total nitrogen unfortunately
ean not be corrected until much more mformation is available regard-
ing the nenprotein constituents. With reference, however, to the
second sou of error in estimating protcin, namely, that arising
from the indiscriminate use of the conversion factor 6.25, we do
have data which can be gpplicd for the calculation of factors that
will give results representing more nearly the true protein content.
The chief protems in most of our common food materials have been
isolated and studied. The percentage of nitrogen in these proteins
is definitely known.

In tablo 1 is given a list of different protcins that have been
isolated from plant and animal sourees, and the percentages of
nitrogen which these proteins contain. Most of the proteins listed

2 Ttulle numbers in parentheses refer to Bibliography, p. 16.



PROTEIN CONVERSION FACTORS 21

ud CampBeLt, G. I

180G, CONGLETIS ANN virEELin. Jour. Amer, Chem. Soe. 18: 609-623

nid Campana, G. b.

THE NUCLEIC ACID OF THE EMRRSO OF WHEAT AND Urs PROTEIN
comronsps. Jour. Amer. Chen Soe, 22

and Harms, LP.

37

1903, NITROGEN IN pRovKEIN RoDIES. Jour, Amer. Chem. Soe, 323
853.
(93) — and Hanrts, 1. FL
1908, THE SPECIFIC ROTATION OF SOME VEG TABLE PROTEINS. Jour. Amer,

Chem. Soe. 25:
(94) - . ~~ and MenpeL, L, B.
1919, THE NUTRIT(VR VALUR OF THE WHRAT KERNEL AND IS MILLING

812-848,

propucts. Jour. Biol. Chem. 37: 557-601.
(95) - ----- and Voonnrns, ©. G.
1893, TRE PROTEIDS OF THE WrtaT KexNEL, Amer, Chem. Jour. 15:
392-471,

(90) Rerreause, H.
1862. TmeR Dim ZUSAMAUENSEWZUNG DBS PFLANZENDELMS UND DAS VER-
HALTEN DESSELBEN zi; wassmr, Jour. Prakt. Chem, 86:.257-
265.

1872. pIy ELWEISSKORPER DER GETREIDWARTEN, UCESENFROOIIE UND
OusAMEN. 252 p. Bonn,
(98) Tacuw, B. L.
1925, TNE 1S0-ELECTRIC POINTS OF GLIADIN AND GLUTELAN. Jour. Amer.
Chem. Soc. 47; 418-422, ilius.
(99) Vickery, H. B.
1024, SOMD NITROGENOUS CONSTITUENTS OF THE JUICE OF THE ALFALPA
PLANT. I, THK AMIDE AND AMINO ACID NITROGEN. Jour, Biol.
Chem. 60: 647-655,

(100) -———
1924, SOME NITROGHNOUS CONSTITTENTS OF TH
PLANT, Il, THE BASIC NITROGEN. Jot

127,

JUICE OF THY ALPALEA
. Biol, Chem. 61: L17--

012) ——
1925. SOME NITROGENOUS CONSTITUENTS OF THE JUICE OF THD ALFALFA
PCANT, IV, THE BRTAINE FRAcTION. Jour. Biol. Chem. 65:
81-89,

(102)
1925, soME NITROGENOUS CON 8 OF THE JUICE OF THE ALFATYA
PLANT. VI. ASPARAGINE AND AMINO Acibs IN ALFALFA. Jour.
Biol. Chem. 65: 657-664.
(103) ~— . .- and Luavaxwonrn, C. 8.
1925, SOME NITROGKNOUS CONSTILUENTS OF TRE JUICE OF THE ALFALTA
PLANT, Ut ADENINE IN ALWALKA, Jour. Biol, Chem. 63: 379-
583.
(104) —— and Vinson, ©. G
1925, SOMF NITROGENOUS CONSTITURNTS OF THE JUICE OF THE ALFALPA
TLANT. V. THY BASIC LEAD ACETATE pRnciprEate. Jour, Biol.
Chem. 65: 91-95,
(105) Woonman, H. E.
1922. THE cumausTKY OF THE SrAENGTR OF WHeaT FLOUR. Jour. Agr.
Sei. [England] 12: [231] 243,



18

(36)

(39)
(40)

an

(12)

(8)

(49)

(30)

CIRCULAR 183, U.S. DEPARY:

MENT OF AGRICULTURE

Jones, D, B.
1922. THE PEANUT—AN KXCELLENT FOOD Hi ’ NOT A MAGIC HAIR RE-
storen, Peanut Promoter &: 50, 52.

1925, THE CuRMISTRY OF PROTEINS AND LYS RELATION TO NUTRITION.
Amer. Jour, Pub, Health 15: 953-957.

1927, rRorbiNs IN PREDSruFrs VARY MUON. U.S. Dept. Agr. Yearbook
1926: 612-637, illus.

— -—— and Csoxra, F. AL

1925. Proveins OF THE COTTONSEED, Jour. Biol. Chem. 64: 673-683.
and Csoxra, PLA.

1927. srupiEs oN GLUTELINE
Jour. Biol. Chem. 7.

and Csoxka, F. Al

1928. STUDIES ON GLUTELINS. IV. THK GLUPELINS OF COUN (ZEA MAYS).
Jour. Biol, Chen. 78: 289 202.

— and Csonna, F. AL

1980, TLE PROLAMINS OF DWARF YELLOW MULO AND EELERITA, TWO HOW
CULTURAL VARIKTIES OF HOLCUS soRGHEY. Jour. Biol, Chem.
88: 305-309.

-~-— Tinks, ALJ. and Guasporrr, C. E, P.

1922. 4 CHRAUCAL STUDY OF THY PROTEINS OF THY ADZUKL BEAN, PIASKO-
Lis anauianis, Jour. Biol, Chem. 51: 103-115.

—-— Finns, A.J. and Warermay, I.
1922. A Nove ON THE NUTRITIONAL ADEQUACY OF THE PHOTEINS OF THE
CHINESE AND GWORGIA VELVET DEANS WITH REFERENCE TO AMINO-
act compossrion. Jour. Biol. Chem. 52: 209-210,

Gresporrr, C.F. F., Jouns, C. O., and Finns, A. J.

1922. VROTEINS OF THE LIMA BEAN, PUASEOLNS LUNATUS. Jour, Biol.
Chom. 83: 231-240.

and Grrspoker, C. EP.

1923, PROTEINS OF WHEAT BRAN. I. ISOLATION AND ELEMENTARY ANALY-
SES OF A GLORULIN, ALKUMIN, AND PROLAMINE. Jour, Biol
Chem, 58: 117-181.

~- and Guasporrr, C. BF,

1923, PROVKINS OF THE CANTALOUPE SEED, CUCUMIS MILO. ISOLATION

OF A CRYSTAMIINE GLOBULIN, AND A COMPARATIVE STUDY -ov
THIS GLONULIN WITH THD CRYSTALLIN GLOBULIN OF THE SQUASH
SEND, CUePRuIA MAXIMA. Jour. Biol. Chem. 56: 79-96, illus.

pasporre, C. Ii. F., and Moxrumn, O.

1924, TRE TRYPTOPRANH AND CYSTINE CONTENT OF VARIOUS PROTEINS.
Tour. Biol, Chem. 62: 183-195.

—— and Gersporer, C, ELF.

1925. PROTEINS OF WHEAT BRAN, IL DISTRIBUTION OF NITROGEN, PER
CENTAGES OF AMINO ACIDS AND OF FREE AMINO NITROGHN: A
COMPARISON OF ‘TH BRAN PROTEINS WITH THE CORRESPONDING
PROVEINS OF WHWAT ENDOSPERM AND EMBRYO. Jour, Biol.
Chem, 61: 241-251.

m= - Grraponer, CE. F., and Monier, 0.

PROVEINS OF THE BARK OF THE COMMON LOCUST TREK, ROBINTA PSEU-

1. ENZYMES ASSOCIATED WITT ‘HE PROTEINS! ‘tHE
COMPOSTHION, PROVERTIDS, NTVROGEN DISTRIBUTION, AND SOME
OF THE AMINO acivs oF THE aLBUMN. Jour. Biol, Chem. 64:
655~671,

~ —— and Gunepoxrr, C.K. P.

1927. THE GLORLLINS OF RICK, ORYZA SATIVA, Jour. Biol. Chem. 74: 415—
126.

and Gerspuner, C, BE. F.

1927. PROTEINS OF SPSAME SEED, SESAMUM INDICUM. Jour. Biol. Chem.
75: 213-225, ius.

225,
—— > and Gmrsvorrr, C.F. FP,
1929, Prov
Cr

UL, THE GLUTELMIN OF RICE (ORYZA SATIVA)
427-

NA MILL). Jour. Biol.

5 OF THE AVOCADO (VERSES AMER
em. 81: 533-539.



410 CIRCULAR 183, U, 8, DEPARTMENT OF AGRICULTULE

to be due to the fact that bran contains a higher percentage of pro-
tein than the endosperm. Work done in this Bureau on the proteins
of wheat bran has shown that this is only one cause. The other cause
of the lower protein content of the flour as compared with that of the
wheat from which it is made is that the proteins of bran have uot the
same nitrogen percentage as the proteins of four. Two of them have a
lower pereentage. The nitrogen of bran represents approximately 22
percent of the total nitrogen of wh From a commercial wheat
bran from which nearly all the adhering parts of the starchy cndo-
sperm had been removed by special treatment, three proteins were
i ed, an albumin, a globulin, anda prolamin. These proteins were
obtained in percentages of 2.87, 2.85, and 3.35, respectively, which
amounted collectively to 10.57 percent of the bran. They have been
studied extensively with regard both io their chemical composition
and properties and to their nutritive value.

The results of these studics have raised the question whether or
not a factor based not only on the nitrogen content of gliadin and
glutenin, but also taking info account the proteins of the bran and
the embryo, would give the protein content of wheat more accurately
than the customary factor 5.7. Special interest lends itself to this
matter in view of the current practice of buying wheat upon analysis
and determination of its protein content. Accordingly, the factors
have been calculated for converting the percentages of nitrogen in,
the endosperm, embryo, and bran into terms of protein. On the
basis of the nitrogen percentages in these different parts of the secd
and of their relative distribution in the wheat kernel, a factor has
been obtained for the conversion of the nitrogen of the whole kernel
into terms of protein,

ENDOSPERM

The proteins of the endosperm consist chiefly of gliadin and
glutenin, which oceur in wheat in approximately equal quantities,
Although traces of a globulin and albumin have also been isolated
from flour, il is questionable whether these small quantities found
in the best flour may not be due to the presence of small quantities
of embryo that escape separation in the milling proe
rate, the quantities are too small to affect the resul nificantly
when considering the conversion factors for the endosperm nitrogen.
Gliadin contains about 17.6 percent of nitrogen, and glutenin about
17.5. The endosperm proteins collectively, therefore, contain 17.55
percent of nitrogen, which, divided into 100, gives the conversion
factor 5.698, or 5.7, which is the factor generally used for the con-
version of nitrogen into protein percentage not only in flour, but
also in whole wheat.

BRAN

The proteins of bran have been shown (46) to consist essentially
of an albumin, a globulin, and a prolamin. The proportions in
which these proteins have been isolated from bran and the per-
centages of nitrogen which they contain are shown in table 4.


PROTEIN CONVERSION FACTORS 7

ERRORS ARISING FROM THE USE OF WRONG CONVERSION FACTORS

s shown in table 1, the nitrogen content of different proteins
varies within a rather wide range. Some proteins that have been
isolated contain as little as 13 pereent nitrogen; others contain more
than 19 pent As an illustration of a possible error in cileulat-
ing the ein content of a single food material by using the fac-
tor 6.25, Make the case of almonds. The protein content of the edible
portion of almonds is usunlly give. in tables showing the compo
tion of foods as 21 percent. are is obtained in the conven-
tional manner by multiplyin , nitrogen content of almonds, 3.36
percent, by the factor 6.2 35. However, the protein of nimonds eon-
sists of a globulin named. amaniin, containing 19.3 pereent of nitro-
gen. ‘This protein has b soluted in crystalline form (90) and has
No other protein has been found in
nificant quan ced, and it bas but little nonprotein nitro-
gen. lis nigrogen content, 19.3 percent, corresponds to the factor

5.18. Tf the factor 5.18 be used, which corresponds to the actual
percentage of protein nilrogen in almonds, instead of the conven-
tional factor 6.24, the percentage of protein in almonds will be found
to be 17.4 pereent instead of 21 percent.

Even more striking is the discrepancy in the case of gelatin. Tts
protein content is given us 91.4 percent (N%X6.25). However,
when the factor 5.55 is used, which is the one corresponding tu the
actual nitrogen content of eclatin, the protein content will be found
io be 81.2 percent. Si viking, differences in the figures for other food
materials are shown in table 6.

The chief proteins in most. vegetable material contain more th
16 percent rogen. This is particularly true of seed proteins.
Many seeds, however, do contain, besides the one or two chief pro-
teins, one or more other proteins in smaller quantives, which almost
invariably contain less nitrogen. Tf only those proteins which make
up the greater part of the protcin contonl of vegetable material be
taken inte consideration, the average percentage of nitrogen in them
is about 17 percent. This corresponds, to the conversion [actor
5.84, When the percentages of protein in most foods of vegetable
origin, with the possible exception of green vegetables, fruits, and
tubers, are being calculated, this factor, therefore, would give the
truc proteim conient more closely than the factor 6.25, which is
now used. There are ecrtain foodstuffs concerning the proteins of
which sufficient is kuown to justify the use of special nitrogen con-
version factors. Although in most cases even these factors will
not give absolutely correct figures for protein content, they will
give them more ac tely than the arbitrary factor 6.25. Even
for substances Lhe nitrogen of which is known to be protein nitro-
gen, and [or whieh the relative proportions of the diferent proteins
have been fairly well established, available data are not sufficiently
complete for the caleulation of perfectly correet factors.

CHEMICAL COMPOSITION OF PLANT PROTEINS IN RELATION TO
THE PLANT ENVIRONMENT

The use of any factor for the conversion of the percentage of
nitrogen into terms of protein in a g ial presupposes that
the nitrogen content of the individual proteins contained in that



6 circ

HAR 183, Ue

have been prepared and analyzed many tines,

percentages w

Division of 4

he Bureau of Agri

ve determined in the Protein an

+ DEPAREMENT OF AGRICULTURE

Many of the nitrogen
nd Nutrition Research
cultural Chemistry and Engineering.

The others haye been compiled from the work of various investigators,

In nearly all

protein varies but little from the

rations of the
investigators,

Santee of protein

cases the perrentage listed in

same protein, many

the table for any given
percentages recorded Jor other prepa-
of which were determined by several

Tanse 1—Nitvagen content of various proteinat

—
! Protein j Nitto. i
yen

Souree of protein

{
1 Protein

Adauki beaus..

Almona_
Alfa'fa. leaves
Avocado

Mood.

Draail wut
Buckwheat...”
Butternut :
Canusloune seed.
Castor bean...

Chicken
Cohune nui
Gorn

Corn Jeaves.
Coconut,
Cottonseed

Cowpea.

Purum wheat

Filbert
Fish Chal
Flaxsved...
Gelatin ~
Mazelnast_
Hempseod
Jack bean.

Katir
Lentil

Lima bean

Locust tree bark.
Lupine
Marigold seed
Maple sews
Mills Coow'sy

Mung bean.

Navy bean.
| Sute-sotubie Siro
Aleghol alkali -sol-
uuble protein,
Aleohiobsolable pre
tein
Hordsin
Glonaltn
se Ghutcll
‘Sumin
unt globulin
rum afonmin

Os (dank).
Pea

Peach kernel.
Peanal

ato

Pot
Re

Ryo...

Seatlon.
Sesame seed.

Shrimp.
Si

Soybean...

Vignin.
Legumotin
Protarnin
Ovaibumin 2
; Conalbumin
Vitellin
Livetin.
Prolarnin
des
Corytin 712
‘Muscle Bicr
Globulin.
Gelatin .-

Sorghuus
Spelt

Veix
Vere

Tevumin..
Sia

amelin.
e-Globalin.
8-Giobulia
fstSumin .

‘Wheat germ.

Wheat bran.

Congluti
Globulin

Walnut, Engtish,
Walnut, black

j Kaelallinimin |
Aleobal-soluble pro- |
tein. i

Velvetboaa, Georgia,
etbean, Chinese.
i.

Wheat endosperm

an(obulir,.
B-lohulin.~
Phagcolin

Cony haseotin.
Prolamin...

-! Muscle fibe!
anit,

‘Puberin
Glubulin ”

do

Hohulin, eoautilable
at ite ©,

Globuil
Line ¢:

Orgeenit

toagulable

‘Radin

Glutelin”
Albumin
Musele fi
Globulin.
B-Glonnlin
Musee: ber
Seriein.
Fibro
_| Giyeinin
Legatmelin.
Prolamin.

Tporaoeir

Vrolamia

e-Globuiin.

Glomalin ~
e-Globrdin

| e-Gtibutin
Albussin

_ Stizelobin

+] Peunein -

Leaumelin

| Gas
Huton’

#2 Glutelin
Teacosin
Globulin.
Proteose.
Peokumin
Globulin.



PROTEIN CONVERSTON Facrors 19

(54) Jones, Th T., and Horn, M, J.
1930, THE PROPORTIONS OF ARACTIN AND CONARACHIN AND THI PROPOR-
TIONATE OCCURRENCE OF THKSE PROTBINS IN THE PEANUT, Jour,
Agr. Research 40: 673-68:
(55) ——— and Jonvs, C. 0.
1916. some pROTELIS

8 PROM THE JACK BNAN, CANAVALIA MABLPORMIS.

Tour, Biol, Chen, 28: 67-75.

(56) ——— and Jowss, ClO.
1918. THe HYDROLYSIS OF KAFTRIN. Jour, Biol. Chem, 36: 323-334.

(67) ——— and Jonna, C. 0.

1919, THE HYDROLYSIS OF ATIZOLORIN, ‘TE GLOBULIN OF THE CHINESE
VELVET KWAN, STIZOLOBIUM NIVEUM. Jour, Diol. Chem. 40:
135-448.
8} —~ and Jonns, C. 0,
1920, HYDROLYSIS OF THE GLOBULIN OF THE COCONTY, COCOg NUCIFERA.
Jour. Biol Chem, 44: 291 301,

(59) =» —-- and Jouns, C0,
1921. DETRRMINATION OF THE MONOAMINO-ACTDS IN THR HYDROLYTIC
CLEAVAGE PRODUCTS OF LACPALBUMEN, Jour. Biol. Chem,48: 347—
360,
(60) ——~ Moritrr, O., and Guxsnorrr, CE. F.

OF SoM AMINO.

1925. vHE NITROGEN DtsPKiBUTION 4
serierts (L.).

ACIDS IN Vuk MUSCLE OP Tu
Jour. Biol, Chem. 65: 59 66.

(61) ~~ and Moriun, 0,

1928, SOME RECENT DETERMINATIONS OF ASPARTIC AND GLUTAMIC ACIDS
TN vaniovs morsins. Jour. Biol. Chem, 79: 429-441.
(62) —— and Mvrrny, 3. C., with the assistauce of Mostizr, O.
1924. CYSTINH PEFICIENCY AND VITAMIN CONTEST OW THE LENTIL, LENS
ESCULENSTA MomNeu. Jour. Biol, Chem, 59: 243-253, illus.
(68) ——— and Warerman, B. ¢
1921, THE BASIC AMINO-ACTDS OF GLYCININ, THE CLORTLIN OF TIE soy
BEAN, SOI4 HISPIDA, AS DETERMINED BY YAN SLYKE’S METHOD.
Jour, Biol. Chem, 46: 459-462,

(61) ~——~ and Warenaan, I. C.

1922, STUDIES ON VEY MIGESTIBUATY OF PROTEINS IN VITRO.
CHEMICAL NATURE OF TIB_NicTRITIONAL DEFICLENC Le
cnx. Jour, Biol, Chens, 52; 357-266

- and Warurman, If. C.

1923, STUDINS ON THE DIGESTIBILITY OF PROTEINS IN yITHO, IV, ON THE
DIGESUIRILITY OF TI COTTONSEED GLONULIN AND THE BFFECT OF
HOSSYPOL UPON THE PEPTIC-TRYPITC DiGHSTION UF PROTEINS.
Jour, Biol, Chem, 56: 501-511, illus.

(66) ——~- and Wirson, R.

1928. THE DICARBOXYLIC AMINO ACID FRACTION IN GLIADIN. Cereal
Chem. 5: 473-47,

(67) Murry, J. C,, and Jonns, D. 1,

1926. PROTwiNS OF WHEAT BRAN, IT. TIE NUTHITIVE PROPERITES OF THE
PROTEINS OF Waka RRAN, Jour, Biol. Chem. 69: 85-99, illus

nuvan, M. X,, and Jonus, D. B.

1936. SrtDIES OF THD MOCHEMISTRY OF SULPHUR, VI. TRE CYSTINE CON-
VENT OF CONPHASEOLIN AMD PHASWOLIN, THE ALPIIA AND BETA
GLOBUIANS OV THE NAVY BEAN (PHASEOLUS vutGanis). Pub.

. Tieaith Rpts, [U. $.] Sup. $2, 9 p.

(69) Warmnwan, H.C.

1923, TR PREPARATION OF TRYPTOPHANE FROM THR PRODUCTS OF HYERO-
LYSIS OW LACTALBUMIN WITH DARYTA, Jour, Biol. Chem, 50:
T77.

(70) - --— and Jouns, C. 0.

1921. srupies ON FIR DIE
EWFECT OF COOKING ON TRE DIGESITHIITY OF PHASEOLE
Biol. Chem. 4 :

(71) ——— Jouns, C.O., and Jones, D. B., with the assistance of PurLutps, S.

1923. CONTHASEOLIN. 4 NEW GLOBULIN FROM THE NAVY BRAN, PHASEO-
hts vuLGAWs, Jour. Biol. Chem, -104,

\D FERCENTAGES
SHIRMP, PENET:

Ty, ON THE
OF AKAs

(65)

STIBILITY OF PROVHINS IN VITRO. 1. THE
Jour.



PROTEIN CONVERSION FACTORS 9

that “the evidence offered points to the chemical identity of the
gliadins from the different varictics of wheat.”

In a comparative study of the glutelins of cereal grains Larmour
(85, p. 1093-1094), concludes—
from the papers cited in the foregoing diseussion it seems evident that glutenins
from various types of flours are indistinguishable by present methods of ehem-
ieal analysis, and by means of the measurements of specific rotation, refractive
index, and absorption spectra.

Blish (75) could not differentiate chemically between a glutenin

repared from a high-grade patent flour and that prepared from a
iscuit flour.

Blish and Pinckney (76) found that samples of gliadin and
glutenin from widely different types and varietics of wheat were
identical, with the exception of leht differences noted in optical
rotation in the case of the proteins from onc unusual variety of
wheat.

What has been said regarding the constancy of the composition of
gliadin and glutenin, irrespective of the class or source of the wheat.
from which they wore obtained, can also be applied to the proteins of
other materials. Arachin, for example, a protein of the peanut,
has definite physical and chemical properties, irrespective of the
environmental conditions under which peanuts are grown. Edestin,
from hempseed, is a crystalline protcin having definite physical and
chemical properties. Preparations of edestin obtained by different
workers and from hempseed grown in different parts of the world
throughout a period of many years have been found to have the
same clementary composition.

The overwhelming evidence that the chemical composition of indi-
vidual proteins is not influenced by environmental conditions under
which the plants containing them were grown justifies the use of
nitrogen conversion factors based on the nitrogen content of indi-
vidual proteins.

On the basis of the properties and comp
most of our common, nitrogenous foodstuffs can be classed in five
general groups: Cereal grains, oilseeds and nuts, seeds of leguminous
plants, foods of animal origin, and fruits and vegetables. For ihe
sake of convenience in preseutation, the nitrogen conversion factors
for the proteins in a number of different food materials are con-
sidered under those classes.

Tt is realized {hat many food urticles are not included among those
for which special factors are given. Special factors are suggested
only for those articles concerning the proteins of which it is believed
there is sufficient information to justify the use of speeial factors.

ition of their proteins,

PROTEINS OF CEREAL GRAINS

WHEAT

The factor generally used for wheat flour and for the whole
heat kernel, 5.7, is based on the nitrogen content of the two prin-
cipal proteins in four, namely, gliadin and glutenin,
However, flour has a protein content about 1 percent lower than
that of the wheat from which it ig obtained, This las been assumed
2AISt—41—



Tes. “ '
Phd eae

CIRCULAR No. 183 ISSUED AUGUST, 1931
Slightly Revised February 1941

UNITED STATES DEPARTMENT OF AGRICULTURE
WASHINGTON, D. C.

FACTORS FOR CONVERTING
PERCENTAGES OF NITROGEN IN FOODS AND FEEDS
INTO PERCENTAGES OF PROTEINS

By D. Brnese Jonns, Principal Chemist, Protein and Nutrition Research Division,
Bureau of Agricultural Chemistry and Engineering

CONTENTS
Page

Introduction...
ratein
protel

ne conversion

INTRODUCTION

The importance of proteins in the dict is so generally recognized
as to necd here no detailed discussion. Both the quantity and the
quality of protein are matters of importance to everyone, particularly
to dietitians, physicians, housewives, school and institutional work-
ers, food and feed manufacturers, and animal husbandrymen.
Unless the diet contains enough protein of satisfactory quality proper
nutrition will not result. An excess of protein, on the other hand,
is at least uneconomical, because protcin is the most costly of the
food elements. The percentage of protein in a feed largely deter-
mines its price. Wheat is now commonly bought on the basis of its
protein content. It is, therefore, desirable to have as accurate
knowledge as possible of the quantity of protein in various materials.

Proteins have been defined as nitrogenous chemical compounds
of great molecular weight which give colloidal solutions and which
on analysis yield umino acids as cleavage products.

Without proteins there ean be no life. They play a predominating
role in the life process of every cell. Proteins entcr into the con-
struction of nearly all animal tissues and form the chief constituent
of many of the body fluids. Aside from their réle in nutrition, they
are connected with many physiological processes in health and

251818411 co

\S



12 CIRCULAR 183, U. $, DEPARTMENT OF AGRICULTURE

Total nitrogen in 4.4 grains of embryo proteins:
10X 4.4 0.168

0.411 gram albumin nitrogen.

18
2X AA XO1S3_ 0.221 gram globulin nitrogen.
BAEK ONT 9.195 gram proteose nitrogen.
rams
‘Total__..- en 7.)
Total nitrogen in 100 grams of total wheat proteins... ---—- 17. 161
+-17.161=5.83 conversion faetor.

The conversion factor 5.83 thus obtained is based partly on the
relative proportions in which the proteins have been isolated from
the different parts of the seed. The quantities actually isolated,
however, probably do not represent all the protein present, especially
in the embryo and the bran. It is scldom that the proteins in a
naturally occurring plant or animal product can be completely ex-
tracted, Furthermore, more or less unavoidable losses are always
involved in the separation and isolation of the proteins after they
have been extracted from the material which originally contained
them. Such losses are usually, however, rather uniformly distributed
among the different proteins of the seed and would therefore not
significantly affect the conversion factor. Although the now factor
here presented for the conversion of the percentage of nitrogen of
wheat into terms of protein does not differ groatly from the factor in
general use, it is believed that it will give the protein content of
wheat more accurately than the customary factor 5.7.

RYE, BARLEY, AND OATS

The proteins of these cereals, like those of wheat, have been found
to consist chiefly of an alcohol-soluble protein (prolamin) and.»
glutelin, Several investigators have shown that the nitrogen con-
tent of theso proteins agrees quite closely with that of gliadin and
glutenin, the corresponding proteins of wheat. Like wheat, these
cereals contain other proteins, but their quantity is proportionately
so small thaé they need not be considered in connection with the con-
versicn factor. The factor 5.88, which has been calculated for wheat,
enn therefore also be used for rye, barley, and oats.

RICE

The proteins of rice differ essentially from those of other cereals,
With respect both to the nature and properties of the proteins and
to their amino acid composition. Rice contains but an insignificant,
quantity of protein belonging to the class of prolamins. Its protein
content consists largely of oryzenin. Small quantities of two globu-
lins are also present. Tf the relative proportions in which the differ-
ent proteins are present in rice, and the percentages of nitrogen in
these proteins are taken into consideration the nitrogen content of
oryzenin, 16.8 percent, fairly represents the percentage of nitrogen
in the rice proteins as a whole. This pereentage corresponds to the
conversion factor 5.95.



PROLEIN CONVERSION FACTORS il

Tarun 4. Proteins af wheat bran and their nitrogen content

| oumtiey

| efieate
‘slated

from 100

Protein Nitrogen

Albumin.
Giobuli

Projamin

Total...

On the basis of these data, the bran proteins collectively conta
15.86 percent of nitrogen, corresponding to the conversion factor 6.31.

EMBRYO

Osborne and Campbell (94) obtained from wheat embryo about 10
pereent of albumin, 5 percent of globulin, and 3 percent of proteose,
which contain 16.8, 18.3, and 17.0 percent of nitrogen, respectively.
Calenlated on the basis of these figures, the embryo proteins as a
whole contain 17.24 percent of nitrogen, corresponding to the con-
version factor 5.80.

WHOLE WHEAT KERNEL

In their classic work on the nutritive value of the wheat kernel
and ils milling products, Osborne and Mendel (94) have estimated
the percentages of the total protcin of the wheat kernel which is eon-
tained in the endosperm, bran, and embryo as follows: Endosperm,
73.3 percent; br ercent; embryo, 4.4 percent.

‘These figures a s approxiniate proportions of the
named parts in the average moisture-free wheat kernel cow
2.2 percent of nitrogen. ‘The authors state: “The relative proportion
of these parts varies somewhet in different sumples of whee » but in
general we believe that these ”

By taking into account the foregoing proportions of the ‘pure in
the seed, the relative quantities of the dilferent proteins which have
been isolated from these paris, and the percentages of nitrogen in the
proteins, the conversion factor for Lhe nitrogen of the whole wheat
kernel way be calculated m the following manner:

Total nitrogen in 73.3 grauns of endoxperm proteins:

Grams
73.3 0.178: _. 12, 864
‘Total nitrogen in 22.3 grams of bran preted
2.87% 22.30. 425 eam albumin nitroso
BST XP3XO.} 325 gram alimmin nitrogen.
X22.3X0177 Ly 773 gram givbulin nitrogen.
0.153 1 7266 grams projamin nitrogen.
Total..-. 3, 537



8 DEPARTMENT OF AGRICULTURE

material is constant. For example, the adoption of the conve ion.
factor 5.7, which is generally used for wheat and wheat flour, is
based on the assumption that the nitvogen content of gliadin and
glutenin is independent of the environment under which the wheat
was grown, such as climatic and soil conditions; also 4hat the gliadin
and glutenin obtained from different varieties and classes of wheat
contain the same percentage of nitrogen. In other words, it assumes
that wheat gliadin and wheat glutenin are chemically definite
teins or sroups of proteins that have a constant chemical composition
i e of the conditions under which the wheat from, which
are obtained is grown. This must not be confused with the
well-known fact that different classes of wheat and wheat grown
in different environments vary with respect io the total quantity of
protein they contain and also with respect to their ratio of gliadin
to glutenin content. But these two proteins, in whatever quanti-
tics or in whatever ratio to each other they occur, have always been
found to contain practically the same percentage of nitrogen. This
has been demonstrated by the analyses of hundreds of preparations
of these proteins by different investigators covering a period of
more than 80 years. The remarkably close agreement of the nitrogen
content of gliadin and glutenin prepared by different workers not
only in different sections of the United States but also in other coun-
tries is shown in tables 2 and 3.

Tatu 2—-Nilrogen content and specific rotation of gliadin prepared from wheat
of diGerent varieties and ovigin us determined by various investigators

|
Specific
| year Ntgo| rotation
cen wy

Groh and Friedl (80. =a.
Tues (87) a 81.32
Ki a Woodman (708) ra
Osborneand Voorhees (od) , an. se

-- bo (9)
| Be me
92.089 | Pay

Dilland 4 m0

H
und Sandstedt
and Wilson (@4)._

ie lent indicated, the determined ions wers
‘his determination was made with sladin in 48 pereene 2

rues on elivelin ta 70 percent ethyt aleahal.
ahicl,

Taste 3.—.

trogen content of glutenin prepared from wheat of different varieties
and origin as determined by various investigators

Prreeut {|

Ritthausen 1372 TWA 1 ‘Tavue (9

oti fas | iPS) Gaeta

ese ie | ibd | sasssang
H

Cross and Swain (78) made a study of gliadin and glutenin pre-
pared from flour obtained from different varieties of wheat and from
wheat grown in different geographica) locations. They conclude



ORGANIZATION OF TIE UNITED STATES DEPARTMENT OF AGRICULTURE
WIEN THIS PUBLICATION WAS LAST PRINTED

woe. Cnaupe R. Wiewann.
Paut H. Avevesy.
Groviee BL Hron.
Morse Sanispvey.
Director of Extension Wor - - M. L. Wiuson.
Director of finance. anne W. A. Joa.
tor af Personnel ____ Roy BF. Henpiuexs
Director af Research James T, Jaxon
Director of Markeatite a -..-. Mino R. Peek
Soliciter— - -. .. Mastin G. Wire
Land Use - Conrdinator M.S. Eisennowne.
Ofice 0 of Plant and Opevations 00.0...
ce of & c sletivit

Seer
Unier Secretary .—
alssistant Secretary.

m A. Wrencrr, Li
Agricuitural Adjustinent Administration... By M. Evans, Administrator.
Bureau of Agricuitural Chemistry and 1 Beg

neering... ----
Rurenu of Agricnltural Ei
Agricultural Marketing '
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Huxny CG. Kasur, Chiey.
HR. ‘

joun Rt. Monier,
Credit Cor; Cant B. Ronprtns, President.
Commodity, Rrchange # a Josern M. Mehl, Chief.

Bureau of Dairy Lrdustry O. E. Rewp, Chief.

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Warm Credit Administration. _. ALG. Back, Gorernor.
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.. Bante A. Cuare, Acting Chief.
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Surplus Marketing Administration... Mio R. Pranins, Administrator.
22



4: CIRCULAR 185, U. S. DEPARTMENT OF AGRICULTURE

represent the actual protein. In many cases the percentages given
for protein are far from correct, and are likely to be misleading.
Much has been detracted from the practical and scientifie value of
many experiments in which the quantity of protein in the ration or
dict has been calculated only on the basis of nitrogen content.

In biological tests, unless pure, isolated protein is being used, or
the effect of nonprotein nitrogen is eliminated, would it not be more
recurate to speak of the protein value of corn, for instance, rather
than the biological value of its protein?

The quantity of protcin in materials can not be determined with
expediency by separating and weighing the pure protein as is done
with fats and other food constituents. The great difficulty and time-
consuming labor involved in. the quantitative isolation of proteins
in @ pure condition absolutely preclude this method. Instead, the
percentage of protein in a material is calculated from the quantity
of nitrogen it contains. The nitrogen is usually determined by the
Kjeldahl method or some modification of it.

All proteins contain nitrogen. Many years ago, when the com-
paratively few proteins known were chiefly of animal origin, such as
serum albumin and scrum globulin from blood and cascin from milk,
jt was found that these proteins contained about 16 percent nitrogen.
On the assumption, therefore, that proteins in general contained 16
percent nitrogen, the practice originated of estimating the per-
centage of protein in nitrogenous food materials by multiplying the
percentage of nitrogen by the factor 6.25 (100-16). With but few
exceptions, the pereentages of protein in foods as given in food-
composition tables have been calculated in this way; that is, by
multiplying the percentage of nitrogen by the factor 6.25.

This method of estimating protein resis on two assumptions,
neither of which is necessarily correct. In the first place it assumes
that all the nitrogen in food material is protein nitrogen. It is
known, however, that in many, if not in most cases, nitrogenous sub-
stances are present which are not proteins nor related to proteins.
In the second place it assumes that all proteins contain 16 percent
nitrogen.

NONPROTEIN NITROGEN IN FOODS

Tn connection with the first assumption referred to, it is obvious
that an estimation of the percentage of protcin based on the nitrogen
content of a material can not be accurate if that material contains
nitrogenous ct ituents other than protein. There are few, if
naturally occurring nitrogenous substances that do not have
ents other than protein which also contain nitrogen. An estimation
of protein based on total nitrogen content will, therefore, give values
that are too high. The following two cases may be taken as illustra-
tions. Only one-third to one-half the nitrogen of the common. potato
has been accounted for as protein. Concerning the nature of the
remaining nitrogen. in the potato but litue is known. | It has been
shown, however, that a considerable proportion of this nonprotecin
nitrogen belongs io asparagine. Potatoes, therefore, may contain not
1.5 to 2 percent: of protein (nitrogen X6.25), as given in tables show-
ing their composition, but only one-half, or Jess, of that amount
Again, alfalfa, an important fecdstuft, and one of the very few sub-
stances the nature of the total nitrogen of which has been system-



PROTRIN CONVERSION FACTORS 15

Tanun 6.—Percentages of protein in some food materials calculated with the factor
8.25 as compared with the values derived when using corrected factors

Protein hosed
a based on | Protein based on.
Food material factor | special factors
625"
Percent Percent

Almonds...
Barley meal and lou
Bread from bigh-erade patent flour
Buttermilk ~
Buttermuts, edibite portion
Trozil nuts, eulle portion
Cheese, Auirivan pale
Cheese, cotta,
Gueonitedibio portion
Geeonal, prenred, as purchased.
Filberts, edible. po
Flour, entive wheat
Tour, patent, bakers rade...
Gelatin
Hickors nnts,
Milk, whole.
) skienuied
Milk, condensed
Oatmeal.
Peunuts”~

a Bea) EAN XG 18)
stk

"tbe percentages in this column are based on date for erude protein given by Atwater and Bryant (74).

The lower figures for gelatin and nuts are believed to represent
elosely the actual protein content. Gelatin is known to contain
about 18 percent nitrogen, and most of the nitrogen in nuts occurs
in the globulin fraction, which contains approximately 19 percent
nitrogen. Although special conversion factors are sometimes used
for s few materials, notably wheat and wheat products, milk and
milk products, and gelatin, it should be emphasized that the factor
6.25 is the one that has been used in the preparation of nearly all
food-composition tables that are in general use.

SUMMARY

The method used for estimating protein in foods and feeds by
multiplying the percentage of nitrogen by the conversion factor 6.25
gives in many cases figures which are lar from, representing the cor-
rect values for real protein. This method rests on two assumptions,
neither of which is necessarily correct; namely, that all the nitrogen
in food material is protein nitrogen and that all proteins contain
16 percent nitrogen. Most naturally occurring nitrogenous sub-
stances contain nonprotein nitrogen. The nitrogen content of more
than 121 different proteins isolated from plant and animal sources is
given. In these- proteins the nitrogen content runges from 13 per-
cent to more than 19 percent.

The magnitude of errors arising from the use of wrong conver-
sion factors in estimating the protein content of foods is illustrated
by citing the figures for a number of foods. The chief proteins in
most of our common food materials have been isolated, and the per-
centage of nitrogen in these proteins is definitely known. On. the
basis of this knowledge, special nitrogen conversion factors have been
calculated for certain foodstuffs concerning the proteins of which
sufficient is known to justify the use of special factors. These foods



(17)
(18)

(19)

(20)

(21)

(22)

(23)

{30)
By
(32)

(33)

(34)

(35)

PROTEIN CONVERSION FACTORS 17

Jouns, C. 0. and Fixxs, A. J.
1918, ATIZOLOBIN, THE GLOBULIN OF THE CHINES® VELVET DBAS, STIZO-
ropum NivEuM. Jour. Biol, Chem, 34: 429-438,
~ and Finks, A. J.
1919. LYSINE AS A HYDROLYTIC PRODUCY ov HoRDEIN, Jour. Biol. Chem.
38: 63-66.
——— Finks, A. J., and Gersvorre, ©. TE. F.
1919. GLOBULIN OF THE COCONUT, COLOS NUCIFERA, I, PREPARATION OF
COCONUY GLOBULIN, DISTRIBUTION OF TIE BASIC NITROGEN IN
cocontr Gronuian, Jour, Biol, Chem. 37: 149-153,
Fuyxs, A.J, aud Paur, M.S
STUDIES IN NUTHIDION, I. TUB NUTRITIVE VALUR oF cOcoNUT
GLogUtIN AND cocoNDT press cakn. Jour, Biol, Chem, 37:
497-502, illus.
———- and Fivxs, A. J.
1920, STUDIES IN NUTRITION. IV, THE NUTRITIVE VALUE OF PEANUT
FLOUR AS A SUPPLEMENT TO WHEAT FLOUR, Jour, Biol. Chem.
412: 569-879, illus.
and Finks, A. J.
1920, STUDIES IN NUTRITION. 11. va¥ ROLE OF CYSTINE IN NUTRITION 48
EXEMPLIFIED BY NUPRICION UXPERIMENTS WITH TILE PROTEINS OF
THE NAVY BEAN, vHAskoL.tS yoLGaRis. Jour. Biol. Chem. 41:
379-389.
Fuss, A. J., and Pav, M.S.
1920, sTUDIES IN' NUTRITION. TI, THE NUTRITIVE VALUB OF COMMURCIAL
CORN GLUTEN MEAL. Jour. Biol. Chem. 41: 391-399.
and Frvxs, A. J.
1921, STUDIES IN NUTRITION. V, THE NUTRITIVE VALUE or SOYBEAN
FLOUR AS A SUPPLEMENT TO WHEAT FLCUR. Amer, Jour. Physiol.
35: 455-461, illus.
and Finks, A
1921. sruDIES IN NUTRITION. vit. TIME NuTRY
‘YNINS OF THE ADZUKI BEAN, PHASEOLUS ANGCTARIS
Physiol. 36: 208-211, illus.
———- and Grrsporrr, C. Hu. I.
1920, THE GLOBULIN oF THE COHLNW NUT, ATTALEA conUNE, Jour, Biol.
Chem, 45: 57-67, illus,
and Gersporrr, C. B. F.
1922. THY PROTEINS OF THE TOMATO SHED, SOLANTM ESCULENTUM, Jour.
Biol. Chem. 51: 439 452
and Jonzs, D, B.
1916, THE PROTEINS OF THN PRANUT, ARACHIS UYPOGATA. 1, THE GLOB-
UIINS ARACHIN AND coNAnAcHEN, Jour. Biol, Chem. 28: 77-87.
— and ronus, D.
1917, THE PROVEINS OF THY PEANUT, ARACHIS MYPOGEA. U. TE DIS-
TRIRUVION OF THE BASIC NITROGEN IN THE GLOPULINS ARACHIN
Nb cowaracnin, Jour. Biol. Chem. 30: 33-38.
——— and Jones, DB.
1917. rin PROTEINS OF THE PEANUT, ARACHIS WYPOG®.
Prac. 3: 365-369.
——— and Jonps, D. B.
1918, THE DEPERAINATION OF TYKOSINE IN PROTEINS. Jour, Biol, Chem,
36: 319-322.
—--— and JONES, .
1918, TI PROTEINS OW THA PHANUT, ARACHIS WYPOGmHA. UL THE We-
DROLYSIS OF ARACHIN, Jour. Biol. Chem. 36: 191-500.
--—— and Joxes, D. B.
1920, SOME AMINO-ACIDS PROM THH GLOBULIN OF THE COCONUT ag NETER-
MINED RY THH BRUTY! ALCOHOL EXTRACTION METHOD OF DAKIN.
Jour. Biol. Chem, 44: 283-200,
——— and Warvunay, TC.
1920, SOME PROTEINS FROM THE MUNG BEAN, PIASEOT:
wunag, Jour, Biol, Chem, 44: 303-317.
and Waterman, H. C.
1920. suMm PROTEINS FROM THD GEORGIA VELVET BEAN, S11ZoLOMIUM
DERRiNarandM, Jour. Biol. Chet. 42: 59-69.

191

VE VALUE OF THE PRO-
Amer. Jour,

Natl, Acad



PROTEIN CONVERSION FACTORS 3

which, at least, have been amply demonstrated to be essential for the
satisfa story wth and nutrition of anime ‘These are lysine, tryp-
tophane, ine, and cystine. These amino acids must ‘be supplicd
hy the food that is eaten, as the body ean not manufacture them from
other substances

QUANTITY OF PROTEINS IN FOODS AND FEEDS

Research on the proteins of food materials resolves itself, there-
fore, when viewed from a practical standpoint, into a study of the
quantity and quality of the different proteins in foods and fecd-
stuffs.

During the last 15 years the Protein and Nutrition Researeh Divi-
sion, of the Bureau of Agricultural Chemis teering has
carried on extensive investigations on the pr otcins of diferent mate-
rials and their constituent uni sa result, many
new data have been umulated relating to the proteins of many
foods, The proteins of some 51 different food 1 ite ais have been
lated in as pure condition as end al amino

Both in human nutrition and in the practical fecding of farm
animals it is obviously a matter of great importar to have as
accurate data as possible, not. only re: carding the qu: of the dif-
ferent feod proteins but also regarding the quantity in which they
are present in the various foods. Fer the feed manufacturer and
the practical feeder of farm animals it is an ever-present problem
to know how much of the different ingredients to se when com-
pounding a ration so that the mixture will contain, for example, 18
or 20 percent of protein of good quality. The same question con-
fronts the dietitian, A diet containing a definite percentage of
protein is prescribed for a patient. What proportions of the differ-
ent foods that arc to constitute this diel must be taken so that the
diet will contain the definite quantity of protein required?

Again, in scientific experimental work diets and rations are fre-
quently required that contain a definite, known perceniage of protein,

ESTIMATION OF PROTEIN IN FOODS AND FEEDS

In order to produce a mixture of several ingredients that will con-
tain a given quantity of protein from known sources, i. is obviously
necessary to know the percentage of in in each of th veral
ingredients. This is ascertained by referring to tables which give
the percentages of protein, fat, and carbohydrate in different food
materials. ‘The percentages of protein as there given are then used
in calculating the quantity of each constituent necessary in order that
the mixture shall contain the required quantity of protein.

Many users of food-composition tables will doubtless be somewhat,
surprised to learn that the percentages ¢ given for protein in different
foods represent the nitregen content, and itt only a few cases do they

articles which have been published in various
iS of Most Of these articles is extrausted, hus the
“din the biblingraphy at the end of this bulletin,

ations are described in som
jour The supply af rep
cansulted in the publications:

Phe results of these inv
scientific and technie:
original artic



CIRCULAR 183, U.S. DEPARTMENT OF AGRICULTORE

discal Mgrymg toxins, antitoxins, sensitization, immunization,
allergy Oe Mmitiar in the modern practice of medicine—are
largely interpreted in terms of protein.

Fat, carbohydrate, ash, and moisture in food materials can be
determined with a high degrce of accuracy, but the method used
for estimating protein is unsatisfactory. The percentages for protein
given in books and tables showing the composition of foods are
merely a form of cxpressing the total nitrogen, and in many cases are
far from representing the correct values for real protein.

Investigations on proteins carried on in the Bureau of Agricultural
Chemi: and Engineering and elsewhere during recent years have
yielded information regarding the nature and composition of the pro-
in a large number of different food materials. Lt is the purpose
of this circular, in the light of these results, to present some data which
will contribute to amore accurate estimation of the proiein content of
yarious foods and fecdstuffs. Also some of the shoricomings and
limitations of our present method of estimating p are pointed
out and discussed, so (hat those using the percentages given for protem
in various miterials can do so more intelligently by making certain
allowances.

The new protein conversion factors presented in this circular are
based upon the most reliable information available regarding the
nature and composition of the proteins in the materials concerned,
Although it is realized that their use will not give values which will
express the quantity of protein in the different food materials with
absolute accuracy, it is believed that they will give values represent-
ing the real protein content more closcly than those obtained by the
indiscriminate application of the factor 6.25, now in general use.
Tlow these factors arc to be applied must be left to the diseretion of
those who wish to use them in their own particular ficids.

QUALITY OF PROTEINS AS A FACTOR IN NUTRITION

One of the outstanding discoveries made within a comparatively
recent time in the field of nutrition is that proteins differ not only
with respect to their chemical make-up, but also in their food value.

Progress in the field of nutrition has necessitated a revision of the
ideas which formerly prevailed regarding the nutritional require-
ments of an animal.” Formerly all foods were classified according to
four basal factors—proteins, fats, carbohydrates, and minezal constitu-
ents. A diet was considered complete which contained these four
factors in sufficient quantities to meet the physiological needs of the
animal. In the older appraisement of food values, little considera-
tion, if any, was given to the quality of the protein, A protein was
a protein, and its food value was largely judged by the number of
calories it would furni: Today we know that a diet may supply an
abundance of calories, and may contain a sufficient quantity of pro-
tein, fat, carbohydraies, inorganic salts, and vitamins, and yet be
inadequate to provide for the normal growth and maintenance of an
animal if the protein is not of the right quality.

The quality of a protein depends on its content of ecrtain com-
pounds called amino acids, of which proteins are primarily com—
posed. Most proteins contain some 18 different ammo acids, 4 of



PROTEIN CONVERSION FACTORS 13

CORN (MAIZE)

The chief proteins of corn are zein and a glutclin. Tt also
globulins in relatively small quantities. The percentages of nitrogen
both in zein and in corn glutelin as recorded by numerous investiga-
tors are close to 16.1 percent, which practically gives the conve
factor ordinarily used, 6.25.

PROTEINS OF OILSEEDS AND NUTS

The predominating proteins in nearly all the oilseeds and nuts
which have been studied are globulins of relatively high nitrogen con-
tent, ranging from 18.5 to 19 percent, corresponding in general to
the factor 5.3. The proteins of the following food articles belonging
to this class have been studied: Almonds, hazelnut, walnut, Brazil
nut, butternut, coconut, castor bean, hempseed, cottonseed, sunflower
seed, flaxsced, squash sced, pumpkin seed, sesame seed, and cantaloupe
seed.

Although peanuts and soybeans are oilseeds, their predominating
proteins have a somewhat lower nitrogen content than those of the
oilseeds mentioned above. More than 87 percent of the total nitro-
gen of peanuts is protein nitrogen (44). The proteins consist almost
entirely of two globulins, arachin and. conarachin, both of which con-
tain 18.3 percent nitrogen. ‘This gives the conversion factor 5.46.
In view of the definite knowledge of the proteins of the peanut, it is
believed that the use of this factor will give quite accurately the real
protein content of peanuts.

The chief protein in soybeans is glycinin, a globulin that has 17.5
percent nitrogen. This corresponds to the factor 5,71.

PROTEINS OF LEGUMINOUS SEEDS

Studies conducted in this Bureau and elsewhere, on the proteins
of a number of different kinds of beans have shown that the percent-
ages of nitrogen in the proteins of these beans lie close to 16 percent.
The factor 6.25, therefore, can well be used for calculating the protein
content of the following varicties of beans: Common white navy
bean, Lima bean, adzuki bean, mung bean, velvetbean, und jack bean.
In view of the insufficiency of data regarding the proteins of other
leguminous seeds used for foods, it is recommended that the factor
6.25 be used tentatively for them also, with the exception of soybeans
and peanuts, which ave discussed under the heading oilsceds and nuts.

PROTEINS OF ANIMAL ORIGIN

MILK

From 75 to 80 percent of the nitrogenous maticr of cow’s milk
consists of casein. Most of the remainder is in the form of lactal-
bumin. Milk also contains small quantities of 2 globulin and of a
protein soluble in dilute aleohol. Casein and lactalbumin contain,
respectively, about 15.9 and 15.4 percent nitrogen. The factor 6.38,
ich has been in general use for some time in estimating protein in
, should be retained.



16 CIRCULAR 183, U. S, DEPARTMENT OF AGRICULTURE

include some of the more important cereal grains, oilseeds and nuts,
leguminous secds, and substances of unimal origin.

Although it is not claimed that these special factors will give
absolutely correct values, it is believed that thoy will give the true
protein content more accurately than will the indiscriminate use of
the conventional factor 6.25.

BIBLIOGRAPHY

PUBLICATIONS FROM THE PROTEIN AND NUTRITION RESEARCH DIVISION OF
THE BUREAU OF AGRICULTURAL CHEMISTRY AND ENGINEERING

(1) Aumy, T.. Tl.
1927, DECOMPOSITION OF FREE AND COMBINED CYSTINE WITH SPECIAL
REFERENCE TO CERTAIN EFFECTS PRODUCED BY HEATING FISH
FLESH. Jour, Amer. Chem, Soc. 49: 2540-2545,
(2) Bersvon, H. 8. Jones, D. B., and CsonKa, F. A.
1927. POLLEN #ROTEINS AND THEIR CLINICAL SIGNIFICANCE IN HAY FEVER,
South. Med. Jour. 20: 257-264.

II. TIDE GLUTELINS OF OAS (VENA SATIVA).
Jour. Biol. Chem. 75: 189-194.
(4) ——— Bernvon, I. 8., and Jonns, D, B.

1925. PROVEINS OF TIMOTHY AND ORCHARD GRASS POLLEN AND THEIR RELA-
pia VO VERNAL HAY FEVER. Soe, Expt. Biol. and Med. Proc.
23: 14-16,

(3) ——— Hown, M. J., and Jones, D. B.
1930. sruDIvS ON GLUTELINS. VIL THE OPTICAL ROTATION OF TAR GLUTE
LINS OF WHEAY, RYE, BARLEY, MAIZE, AND ice. Jour. Biol, Chem
89: 267-272.
(8) —and Jons, D. B.

1927. (sTUDIES ON UTELINS. (. THE @ AND # GLUTELINS OF WHEAY
(PRITICUM VULGARE). Jour, Biol. Chem. 73: 321-329,

(7) ——— and Jonws, 1). B.
1929. stroins oN GLUTELE ¥. THE GLUGELING OF RYE (SBCA CEREALK)
AND OF BARLEY (HORDEUM VULGARE), Jour. Tiel. Chem. 82:
17-21,
(8) ——— Muxpny, J. C., and Jonns, D. B.

1926. rue 1s0-KLECTRIC POINTS OF VATOUS PROTEINS. Jour. Amer.
Chem, Soe. 48: 763-768.
(9) Fixs, A. J., and Jouxs, C. 0.
1920. DISTRIBUTION OF ‘rHe BASIC NITROGEN IN PrAsRoLin. Jour. Biol.
Chem, 41: 375-377.
(10) - and Jomns, (. 0.
1921. SYUDIBS IN NUTRITION. VI. ‘(MH NUTRITIVE VALUE OF TOE PROTEINS
OF THE (IMA BEAN, PHASKOLUS LONATUS. Amer. Jour. Physiol.
36: 205-207, his,

(11) ——-— and Jouns, €. 0,
1921. SUUDIESIN NUTRITION. VIL. THE NUPRITIVE VALUE OP THe PROTEINS
OF TOMATO SEED PRESS CAKE. Amer, Jour. Physiol. 56; 104-407,
illus
(12) and Jouys, ©. O.

1921, STUDIES IN NUTRITION. IX, THR NUTRITIVE VALUE OF THE PROTEINS
PROM THE CHINESE AND OkORGIA VELVEY Beans. Amer. Jour.
Physiol. 47: 61-67, ily
(13) ~ Jonrs, D. B., and Jouns, C. 0
1922. THE ROLE OF CYSTINE IN THE DIRTARY PROPERTIES OF TUE PROTEINS:
OF THE COWPEA, FIGNA SI¥ , AND OF THE FIELD PEA, PISUM
saviyem. Jour. Biol. Chem. 52:'403 410, illus,
(14) Jonys, C. 0.
1919. THE VALLE oF PEANUY PROTEINS. Amer. Food Jonr, [4 (4): 21,39.
(Reprinted from Cotton Seed Press.)
(15) —-— and Benwarer, J. I’.
L916. KAFIRIN, AN ALCOHOL-SOLERLE PROTEIN FROM KAFIR, ANDROPOGON
sorantm. Jour, Biol, Chem. 28: 59-65.
(16) and Crmrxore. 1. JE
1918. oIE GLORDIAN OF BUCK WHEAT, rAcoDysE
Chom, 84: 439-445

MragorykuM, deur. Biol.