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SWEETPOTATO STORAGE

Agriculture Handbook No. 358

'0

Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE

in cooperation with

North Carolina Agricultural Experiment Station

CONTENTS

Page

Need for sweetpotato storage 1

Requirements for sweetpotato storage 1

A clean storage house and clean containers 1

Disinfection of storage and containers 1

Storage of sound, disease-free roots 2

Harvest to avoid injury to the roots 2

Injury from wet soil 2

Injury from cold soil 2

Injuries from harvesting and handling 2

Harvesting equipment 3

Storage containers 4

Handling equipment 6

Curing 7

Storage 9

Weight loss and development of pithiness 9

Sprouting 9

Chüling 9

Pathological losses 10

Internal cork ^ 10

Weevil control 11

Presprouting 11

Page

Physiological changes 11

Construction of storage houses 12

General requirements 12

Heating and cooling requirements 13

Heat of respiration 13

Vaporization of moisture and vapor barriers 13

Product load 14

Loss of heat through walls, ceilings, and

floors 15

Infiltration 20

Heat distribution and source 20

Removal of heat 23

Ventilation 24

Refrigeration 26

Humidiñcation 26

Dehumidification 28

Floor plans or blueprints 29

Storage with several rooms for curing and

storing 30

Storage with separate rooms for curing. __ 30

Literature cited 31

This handbook contains information formerly published in Farmers' Bulletin 1442, ''Storage of

Sweetpotatoes," by J. M. Lutz and J. W. Simmons.

Washington, P.C. Issued February 1969

For sale by the Superintendent of Documents, U.S. Government Printing Office,

Washington, D.C. 20402 - Price 35 cents.
List of Figures

1. Harvester with filled crates on racks at the side of the machine. Rotary mower at left is followed by tractor (not in

view) barring off the sides of the hill.

2. Orates for harvesting and storing sweetpotatoes ; left to right : Storage Durabox ; 3-way Durabox ; Hybrid crate.

3. Method of stacking Hybrid crates of sweetpotatoes on a slatted floor raised above the earth floor.

4. Placing topmost pallet of Duraboxes in position, five layers of boxes per pallet.

5. Pithy condition of sweetpotato root.

6. Internal cork : A virus disease that develops corklike areas in susceptible varieties at storage temperatures above

7. Wall and roof details sihowing relation of construction to thermal resistance.

8. Theoretical condensation conditions during curing at 85° F. or storage at 55°.

9. Average annual minimum temperature (°F.) expected in sweetpotato-producing areas of the Southeastern United

States'.

10. Suggested construction details for heating system : A, Location of ducts in concrete floor ; B, trench detail ; C, location

of thermostat.

11. Schematic floor plan for a storage composed of several rooms. Room size may be changed to meet needs, but four

or more rooms are recommended for this type of storage. (Courtesy R. Ritchie, Dept. Biol. and Agr. Engin., N.O.

State Univ.)

12. Schematic floor plan for a storage composed of a curing room (or rooms) and a separate storage area. This system

is recommended only for storages that are equipped to handle palletized field crates (or bulk bins). The sweetpotatoes are moved carefully with a forklift to the storage room after they are cured. (Courtesy R. Ritchie, Dept.

Biol. and Agr. Engin., N.O. State Univ.)

13. Suggested construction details for ventilating systems : A, Air-intake vent at or below floor level for convection-type

ventilation; B, ridge-exhaust vent for convection-type ventilator; C, hood and bafile for intake blower

mounted in roof of storage room ; 2>, exhaust louver for overhead ventilation.

14. Humidity measuring devices: Left, sling psychrometer (that is, portable wet and dry bulb) ; center, wall-mounted

wet and dry bulb with circular conversion scale below ; right, indicator actuated by hair element.

15. Water vapor content of air at 50,90, and 100 percent relative humidity.
SWEETPOTATO STORAGE

By L. J. KusHMAN, research plant physiologist, Market Quality Research Division, and F. S. WRIGHT, agricultural

engineer, Agricultural Engineering Research Division, Agricultural Research Service

NEED FOR SWEETPOTATO STORAGE

Sweetpotatoes have an important place in the

human diet. Storage of them benefits both the producer and the consumer by making them available

throughout the year. Durmg 1961-65, sweetpotato

production in the United States averaged about

900,000 tons a year with a farm value near $75 million. Not all of this crop was stored since approximately 25 percent was marketed soon after harvest

and another 25 percent processed (primarily

canned). A small part of the processed p^ortion of

the crop was cured and stored before being processed. The remaining 50 percent of the crop, or

approximately 450,000 tons, was stored for marketing during the winter and spring or was used as

"seed" to produce the next crop. The primary purposes of curing and storing sweetpotatoes are to

permit orderly marketing during the several

months that follow harvesting and to promote

high plant production from seed roots.

Proper curing and storage aid in assuring good

market quality, as exemplified by good appearance,

desirable culinary quality, and freedom from defects. If improper conditions for curing and storage are used, losses from decay, shrinkage, and

poor appearance can be tremendous and culinary

quality reduced. It is obvious, therefore, that a

well-designed, well-managed storage facility

serves a necessary function by helping to maintain

the quality of the sweetpotatoes after harvest with

a minimum of loss {97, 98,109,119)}

To get the most satisfactory results in storing

sweetpotatoes, the following six points must be

observed.

• The storage house and containers must be

clean.

• Boots showing disease, especially black rot,

should be kept out of storage.

• The crop must be harvested in time to avoid

low-temperature or wet-soil injury.

• The sweetpotatoes should not be cut, bruised,

or skinned during harvesting, and during filling,

stacking, hauling, and handling storage containers.

• The sweetpotatoes should be placed under

proper curing conditions promptly after harvest

without exposure to conditions that cause sunscalding, chilling, or desiccation.

• The proper storage conditions must be maintained after curing.

REQUIREMENTS FOR SWEETPOTATO STORAGE

A Clean Storage House and Clean

Containers

Before sweetpotatoes are placed into the storage

house, all old or decayed sweetpotatoes and other

debris should be removed. All storage containers

should be emptied, if they already have not been,

and the floor of the storage house flushed or swept.

False floors should be removed and cleaned underneath. Any repairs that are needed should be made.

Disinfection of Storage and Containers

Fumigating the storage is not necessary if it is

cleaned as indicated. The most common storage

losses result from infections of the soft rot organisms {Rhizopus spp.) and the end or surface rot

organisms (Fmariv/m spp.). These organisms are

almost universally present. No evidence is available to indicate that fumigating the storage and

storage containers will control these diseases.

If black rot is present, or suspected as being

present, the storage house and storage containers

should be treated to eliminate the disease before

they are used again. The black rot organism is

sensitive to heat. It is killed by exposing diseased

roots or contaminated containers to temperatures

of 110° to 115° F. for 24 hours (19,26, 66) or for

6 hours at 122° {92). Scurf, another sweetpotato

disease, is also killed by exposing the roots for 6

hours at 122° {92). To be sure the temperature is

^ Italic numbers in parentheses refer to Literature Cited,

p. 31.
AGRICULTURE HANDBOOK 358, U.S. DEPARTMENT OF AGRICULTURE

high enough in all parts of the storage house, it

should be warmed to 120° for 24 hours or longer.

Only a small amount of heat would be needed to

do this at low cost if it were done on a warm summer day after all the previous crop had been removed. Any objects, such as paint or aerosol containers, that could be damaged by the heat should

be removed from the storage before it is heated.

Storage of Sound, Disease-Free Roots

To successfully store sweetpotatoes, roots must

be free of disease. Many diseases attacking sweetpotatoes occur first in the field. By following approved cultural practices, these diseases can be

controlled. No field, however, is completely disease

free. At harvest, then, the roots should be inspected

before they are placed into storage containers and

those diseased or unmarketable kept out.

Eoots affected with growth cracks do not keep

well (86) and should not be stored. Some roots

with scurf should not be stored, although small

amounts are allowed in the U.S. No. 1 grade (up

to 15 percent of the surface) and larger amounts.

but not over 15 percent of the surface of any root,

in No. 2 commercial grades. Grade standards (i^^,

1^5^ 126) and marketing outlets should be considered m handling these roots. Since the scurf

fungus does not penetrate into the roots, the eating quality is not affected. But when stored,

scurfy roots shrink more rapidly than others and

may become unmarketable. Although this disease

does not spread sufficiently from diseased to sound

roots to cause a problem, the infected areas may

enlarge and new infections may appear during

storage when the humidity is high, making these

roots unmarketable. This does not occur, however,

when the humidity is low {24).

Infection by the organism causing black rot occurs in the field. However, the disease can spread

during harvesting from diseased to healthy sweetpotatoes so the presence of black rot lesions in a

crop at harvest, even in small amounts, may lead

to large losses in storage. Usually it is desirable

to market the sound roots from such a crop as

quickly as possible. In this way any infections

occurring at harvest are not given an opportunity

to grow and spoil the roots.

HARVEST TO AVOID INJURY TO THE ROOTS

Sweetpotatoes may be harvested whenever they

reach a salable size. Under good growing conditions, harvesting may begin 90 to 100 days after

planting of some varieties and continue until well

after frost has killed the vines and leaves {28^ 33).

Injuries to the roots detract from their value and

cause losses. One usually thinks of injuring sweetpotatoes physically or mechanically ; but they may

also be injured in several other ways that impair

their keeping quality, and, at times, their culinary

quality, too.

Injury From Wet Soil

Rainfall that i)roduces saturated or nearly saturated soil conditions causes carbon dioxide to

accumulate in the roots {69). Sometimes this is

accompanied by a depletion of oxygen in the roots

also. When considerable amounts of carbon dioxide are accumulated in the roots, wounds heal

slowly, if at all, even when given favorable conditions after harvest. Such sweetpotatoes are especially subject to soft rot.

Wet soil may injure the roots at any time, either

before or after frost. Because heavy rains often

accompany frosts that kill the vines and leaves,

there is, at times, an association of frost with the

damage. Since the damage caused by wet soils is

associated with an accumulation of carbon dioxide

in the roots, damage is produced more quickly in

warm, wet soil than in cool, wet soil. Loss or removal of the vines increases the damage.

Not all roots damaged by wet soil decay, but

those that do not often have an off-flavor or noxious odor when cooked.

Injury From Cold Soil

Sweetpotatoes freeze at about 30° F. and are

ruined. They are injured at temperatures below

55°, especially those below 45°. Many growers delay digging until the first light frost occurs. Some,

because of large acreage, cannot harvest at least

part of the crop until after frost has damaged or

killed the leaves and vines. In general, no damage

will result unless frost injures the top of exposed

roots or the air temperature drops low enough to

freeze the top inch or two of the soil and the root

tissue in this layer. If the roots are dug promptly

after a light frost, they will keep as well as before

the frost. The soil temperature at the time of the

first light frosts usually will be near 60° in the

vicinity of most of the roots {69).

Harvesting should be concluded as soon as possible after frost kills the vines and leaves. Destruction of the leaves prevents an increase in yield, and

delaying harvest provides an opportunity for wet

or cold weather to damage the undug portion of

the crop.

Injuries From Harvesting and Handling

The quality of sweetpotatoes coming out of

storage is influenced by the kind and amount of
SWEETPOTATO STORAGE

injuries inflicted during harvesting and handling.

Bruises and abrasions must be kept at a minimum.

The sweetpotato root is covered by a thin, delicate

skin that is easily removed or broken. The sweetpotatoes may be cut or bruised so as to produce

crushed tissue. High levels of fertilization that

cause late vine growth has been associated with an

increase in decay, apparently because the roots

were more easily damaged than when grown with

lower fertility. {25). Potash levels influence keeping quality slightly (29),

Although curing, described below, usually heals

injuries quickly, the injured roots are not as attractive as those uninjured. Severely damaged areas,

especially those involving crushed tissue, are slow

to heal even with ideal curing conditions {120^

128). Therefore, the amount of decay that develops

often reflects the severity and extent of injuries

received at harvest and the degree to which good

curing conditions are provided.

Exposure of sweetpotatoesto bright sun, chilling

temperatures, or drying winds for periods sufficient to damage tissue makes healing more difficult

and may result in considerable loss from decay

(5^, 9Jf^ IOS). On a warm day an hour or more of

exposure to í)right sunlight will cause temperatures

at exposed surfacesto rise over 100° F. and increase

the amount of surface rot in storage. Leaving roots

overnight in the field when air temperatures drop

near freezing or letting them lie in the field for

several hours when dry winds are blowing also

will increase surface rot on most varieties.

Harvesting Equipment

Various methods are used for digging sweetpotatoes (U, 87,105,107, 110, 111, /^i). Usually

the first step is to remove as much of the vines and

leaves of the plant as necessary to allow digging

and picking up the sweetpotatoes with minimum

interference. Frequently the vines are removed by

barring off the hill on each side with disks and

passing a rotary mower over the hill (fig. 1).

Several other practices are also used to cut the

vines and leaves. One is using a blade mounted in

a drag and flails of rubber or chain. Another, and

probably the best, is to cut the vines with an 8-inch

shielded rolling colter set shallow and offset 5 to

6 inches from the center of the hill. The shield

prevents the colter from going too deep and forces

the vines down against the soil where they will be

cut cleanly. On a turning plow the colter is lined

up with the plow landside. A 16- to 18-inch tractordrawn turning plow with the shin of the plow

lined up with the off edge of the hill provides a

good complement with the shielded rolling colter.

In any event, the cutting must be at the side of

the hill or across the top without cutting into the

hill proper where the marketable roots are located.

Because turning the hill with a plow usually

exposes only part of the sweetpotatoes, considerable hand labor is needed to scratch the potatoes

out, sort them, and place them in storage containers. In areas where soils are light, part of the

moldboard section of the plow is cut away and a

rotary spiral installed to help sift soil from the

potatoes. The sweetpotatoes should not be thrown

into heap rows or handled any more than necessary. Soft cotton gloves are usually worn to protect

the sweetpotatoes from being damaged by

fingernails.

White-potato diggers are often used to harvest

sweetpotatoes because they usually expose most of

the roots. In dry or sandy soil, however, these diggers often damage the roots so much that the roots

become unacceptable for marketing. In an effort

to reduce such injury, several modifications have

been made of these diggers. Short-bed diggers,

mounted on the 3-point hitch of a tractor are used

in many areas. The hydraulic system of the tractor

can be used to carefully control the depth of digging so that soil can be kept on the shaker chain to

cushion the potatoes until just before the roots fall

back onto the ground. If the vines are removed

before digging by barring off the hill on each side

and the operator is careful to keep soil on the

shaker chain, bruising can be kept as low as or

lower than when a moldboard plow or middlebuster is used to dig. Under normal field conditions as much as 30 to 40 percent less time may

be required to sort and pick up sweetpotatoes when

a short-chain tractor-mounted digger is used than

when a moldboard plow is used. In addition, up

to 25 percent more marketable roots may be obtained following the short-chain tractor-mounted

digger {107).

To further reduce the labor required for digging, sorting, and crating sweetpotatoes, several

types of harvesters have been developed. On many

of these harvesters the sweetpotatoes are placed in

crates by hand. Placing them in crates, bulk (l^

ton) boxes, or bulk trucks directly from conveyors

usually causes too much injury for sale on the

fresh market. Considerable handwork is necessary

to sort the roots out of the soil and vines and to

place them into crates. The filled crates are stacked

temporarily on platforms located either at the

side or at the rear of the harvester (fig. 1). Some

platforms have been designed to hold one or two

pallets on which the crates are stacked. The palletized crates are removed with a tractor-mounted

fork and placed onto a truck that hauls them to

the storage house.

Most mechanical harvesters used to harvest

sweetpotatoes for processing plants cause more injury than can be tolerated for roots going to the

fresh market or into storage.

Depending upon field conditions, yield, type of

harvester, and factors related to management, a
AGRICTJLTURE HANDBOOK 358, tJ.S. DEPARTMENT OF AGRICULTURE

BN-32770

FiGDEE 1.—Harvester with filled crates on racks at the side of the machine. Rotary mower at left is followed by tractor

(not in view) barring off the sides of the hills.

harvester with 6 to 12 laborers usually can harvest

about 1 or 2 acres of sweetpotatoes a day.

Tractor-drawn sulky-type harvesters require

several feet of turning area at the ends of the rows

and are not suited for use in fields with short rows.

Terraces, constructed to prevent soil erosion, usually cause problems where harvesters are used.

Harvesters are usually equipped with a standard

type of white-potato-digger shovel followed by an

elevator chain that is gently agitated to remove

most of the soil. On some models the elevator chain

becomes horizontal after attaining a specified elevation and extends on back through the harvester.

On others the elevator drops the roots, soil, and

vines carried up the elevator onto a second linkchain conveyor which runs back to the rear of the

harvester. To reduce injury to the roots, the links

may be covered with rubber or made of sections of

wood that enables more soil to be carried. On some

harvesters a rubberized belt is used as a second or

cross conveyor after most of the soil has been

allowed to fall back to the field to prevent injuries

to the roots.

Storage Containers

Inherent in the harvesting and handling operations is the container used for storing the sweetpotatoes. Three types commonly used are (1) the

tub-bushel basket, (2) the James or Hybrid crate

or a modification of it, and (3) the Durabox, which

is available in two styles.^ These containers are also

used for shipping sweetpotatoes to market. Occasionally, about a 14-ton pallet or bulk box is

used. Burlap bags are not recommended for storing sweetpotatoes as they remove too much skin

from the roots.

The tub-bottom bushel basket has been used for

many years, especially in Eastern United States.

Crates, usually Duraboxes, are rapidly replacing

baskets for two reasons : The sweetpotatoes receive

much less injury in the crates than in the baskets

and the crates may be palletized and handled with

forklift equipment to save labor.

The James or Hybrid crate (fig. 2) has been

used for several years in Louisiana and in neighboring States as a storage and shipping container.

These containers were designed as shipping containers. As such, they can be packed with a bulge

on the top and then stacked for shipment on their

sides, at which time the box has its greatest structural strength for supporting weight. When

" Trade names are used solely In this publication for the

purpose of providing specific information. Mention of a

trade name does not constitute a guarantee or warranty of

the product by the U.S. Department of Agriculture or an

endorsement by the Department over other products not

mentioned.
SWEETPOTATO STORAGE

BN-32772

FiGUEE 2.—Crates for harvesting and storing sweetpotatoes, left to right : Storage Durabox, 3-way Durabox, Hybrid crate.

stacked upright as a storage container, these crates

may be stacked 10 high (fig. 3).

Ifthe Hybrid crates are stacked 12 or more high,

the bottom crates usually are crushed (108). The

one-bushel Hybrid crate has a volume of about

2,460 cubic inches and outside dimensions of

14=1/4 by I61/2 by 13% inches high without a bulge.

They cannot be nested 1 in 2 when empty (3 crates

in the space occupied by 2; that is one crate is

placed inside the other two crates that are placed

with the openings toward each other).

Two kinds of Duraboxes are used. One is designed for field and storage use only; the other

for field, storage, and shipping use. They are commonly referred to as storage and 3-way boxes,

respectively. Both are designed with the supporting cleats m the ends (fig. 2\. Consequently, they

will support considerable weight when assembled

properly, and are suited to palletizing and high

stacking.

The 3-way Durabox measures 203^ by 13%6 by

11% inches high with a capacity of about 2,593

cubic inches. The bottom and lower side shooks

are %-inch thick and the top side shooks are %-

inch thick. A stapled wire near the center of the

box helps support the bottom and sides, which

tend to give with the weight of the sweetpotatoes.

With the addition of a lid, labelj and paper strips,

the container can be used for shipping.

The storage Durabox is slightly larger than the

3-way Durabox and made with heavier shooks. The

outside dimensions are 203^ by Ií^Q by 111/2

inches high with a capacity of about 2,729 cubic

inches. The bottom and lower side shooks are %-

inch thick and the top side shooks are y^^-^nch

thick. A wire stapled around the box helps support

the sides and bottom, which usually do not give

noticeably when the container is full of sweetpotatoes.

321-902 O—69 2

N-8023

riGUBE 3.—Method of stacliing Hybrid crates of sweetpotatoes on a slatted floor raised above the earth floor.
AGRICULTURE HANDBOOK 358, U.S. DEPARTMENT OF AGRICULTURE

The approximate weight of sweetpotatoes that

may be stored in the containers at harvest can be

estimated by dividing the calculated cubic-inch

capacity by 50, since one pound of root will occupy

about 50 cubic inches. Eoots of the variety Nugget

tend to be considerably heavier per unit volume

than other varieties, so the containers will hold

more weight of Nugget roots than of other

varieties.

Bulk handling of sweetpotatoes in palletized

bins of about %- to 1-ton capacity is relatively

common for sweetpotatoes going to canneries, but

not for those intended for the fresh market. However, some growers do pick up sweetpotatoes in a

field container and pour them carefully into pallet

boxes in the field. Better methods of filling and

emptying the pallet bins are needed.

Containers should not be filled to the poinf

where roots will be damaged by stacking one box

on another. The box, rather than the sweetpotatoes,

should support the weight of additional boxes.

Storage containers, such as bushel baskets, James

or Hybrid crates, and 3-way Duraboxes, can generally be used once for storage and once for marketing, if care is taken to prevent them from breaking

and discoloring. The effects of rain, soil, decaying

roots, or mold growth during harvesting and storage often make wooden containers unattractive.

Provisions must be made for storing empty containers from one season to the next. The number of

containers held over from one season to the next

depends primarily upon the kind of container, the

number shipped to market, and the number damaged too much for reuse. The space occupied by the

empty containers is, at times, a factor which influences overall management of the storage.

Handling Equipment

Because of the trend toward palletization of

sweetpotato containers, pallet design and use must

be considered in connection with the storage containers and also in storage house design and management. Obviously the pallet must be designed

for the storage container and, then, the storage

designed to permit efficient handling and storing

of the palletized containers as a unit. If a storage

is already available and is suitable for a forklift

operation, the planning may of necessity be in the

reverse order to avoid loss of considerable storage

area.

There are many factors to consider in selecting

a pallet size. The dimension that coincides with

the direction in which the forks of the forklift

penetrate the pallet (that is, the dimension across

the field-truck bed) should be 48 inches or less.

Thus, two pallets across the truck bed will conform to the 8-foot allowable maximum width of

the load on the highway and the center of the pallet load will not exceed the 24-inch load center

designed into most forklift equipment. The other

dimension is usually about 48 inches and preferably a little less so that the load on the pallet will

not cause it to bend appreciably beyond where

the forks supportthe pallet.

Pallets for the storage Durabox are usually 44

by 44 inches (112). The boxes are stacked with

their ends toward the sides of a field truck to prevent the boxes from rocking off when the truck

turns corners. Both types of Duraboxes are 21

inches long and six of them will stack in a single

layer on a pallet with approximately 1 inch

left on each side of the pallet at the ends of the

boxes. Since the strength of the Durabox is in the

ends, there is a little room left for some margin

of error in stacking to be sure the ends of the boxes

are on the pallet. The 44- by 44-inch dimensions

permit stacking the pallets in the storage in any

direction without special consideration of space

utilization necessary with rectangular pallets. Although the same size of pallet may be used with

the 3-way Durabox, which is slightly narrower

than the storage Durabox, a 2- by 4-inch board is

needed between pallets to properly aline the boxes

on the pallets. Therefore, a 40- by 44-inch pallet

is used in some storages with the 3-way Durabox.

Another consideration is the bulk of the palletized unit. Duraboxes stacked six in a layer and five

layers high produce a gross weight approaching 1

ton (30 times approximately 60 pounds plus a

pallet weighing about 100 pounds).

The storage Durabox will stack and handle

satisfactorily five high on a pallet; the 3-way

Durabox, four high, although stacking five high

is preferred if the storage house will permit it.

Stacking five high conserves space, pallets, and

forklift operation. If the 3-way Duraboxes are

stacked five high, the top layer should be tied with

a wire or string to keep the boxes in place. This is

needed because the 3-way Durabox is narrower

than the storage type and because the boxes cannot be interlocked.

Allowing 6 inches for the thickness of a pallet,

three pallets of storage Duraboxes stacked on one

another are nearly 16 feet tall; and if 3-way Duraboxes are used, the total height is slightly more

than 16 feet. Some clearance must be allowed for

placing the top pallet in position and for ventilation, so the ceiling should be 171/2 or more feet

from the floor (fig. 4 ).

For handling a 1-ton gross pallet load, a 2,000-

pound forklift may be used, but this size of truck

is often too unsteady and taxes the unit when placing a pallet in the third layer of a stack. A 3,000-

pound forklift is safer, quicker, and nearly as

maneuverable as a 2,000-pound one. Some operators prefer a 4,000-pound forklift truck if it can

be turned and handled easily in the storage area.

In any event, the lift must be capable of elevating

the forks to a height about 6 inches above the
SWEETPOTATO STORAGE

BN-32768

FIGURE 4.—Placing topmost pallet of Duraboxes in i)ositlon, five layers of boxes per pallet.

boxes upon which the highest pallet will be placed.

In the illustration of the Duraboxes stacked five

layers high (fig. 4), the height of two pallets is

slightly over 10 feet. Fortunately, the standard extension of many types of forklifts is IQi^ feet or

more.

For rapidity and accuracy in stacking pallets,

the lift should be equipped with a "sideshifter."

The fork should be long enough to reach all supporting boards of the pallet. In the Durabox illustration (fig. 4), the fork would be 42 inches,

but 36-inch forks may suffice if wide boards are

used at the edge of the pallets. The shorter forks

permit a shorter turning radius for the truck,

which is important in some storage areas. A sideshifter will reduce the turning radius necessary

also. A rack or set of crash bars on the forklift

truck will help to prevent injury to the operator

if boxes fall oiï the pallets.

Since a forklift truck will have many uses

around a storage house, especially where sweetpotatoes are graded and shipped to market, one

should be selected with care.

Handling sweetpotatoes in properly palletized

field boxes with a forklift truck usually produces

very little injury to the roots. The field trucks may

be unloaded quickly atthe storage house, and large

quantities of sweetpotatoes may be placed in storage with relatively little labor other than the forklift operator. Handling sweetpotatoes with a forklift truck is so gentle that the sweetpotatoes may

be moved after curing practically injury free. This

in turn has permitted the development of a system

of storage-house management and design utilizing

separate curing and storage facilities (p. 30).

CURING

Sweetpotatoes should be cured immediately after harvest, preferably at 85° F. and a relative

humidity of 85 to 90 percent for 4 to 7 days. Sufficient exchange of air in the curing room should

be allowed to prevent the accumulation of carbon

dioxide produced by the roots or the depletion of

oxygen consumed by them. If condensation is excessive, it is removed by ventilation.

The primary purpose of curing is to heal injuries so that the sweetpotatoes remain in good

condition for marketing during the winter and to

preserve "seed" roots for the next crop. Healing

takes place rapidly at 85° F. and 85 to 90 percent

relative humidity {3, 6, 21, 77, 78, 83, 101, 128).

Curing should start as soon after harvest as possible to heal injuries before disease-producing organisms gain entrance (70). Healing involves production of cells that are very much like the skin

in their ability to prevent infection. These new

cells form in a layer just below the surface of the

injuries. Because this layer is corky, it is commonly

called wound cork. Healing is more rapid under

clean cuts and skinned areas than in deep wounds

where tissue is crushed {120, 128). The rate of

healing differs a little among varieties.

At 85° to 90° F. wound cork begins to form in

2 days and is well developed in 5 or 6 days. At

lower or higher temperatures than this, wound

cork forms less rapidly. Above 95° very little, if

any, wound cork develops and such temperatures

should be avoided. Satisfactory wound-cork formation takes about 4 to 7 days at 85°, 8 to 10 days

at 80°, 15 to 20 days at 75°, and 25 to 30 days at

70°. At 55° or below, wound cork does not form.

Slow formation of wound-cork increases the opportunity for decay-producing organisms to gain

entrance to the root before the layer is sufficiently

well formed.

Because the layer of wound cork forms near the

surface of wounds, desiccation of the wounds re-
8 AGRICULTURE HANDBOOK 358, U.S. DEPARTMENT OF AGRICULTURE

tards such formation by making the cork form

several cells deeper, or, if the wound dries rapidly,

prevents the formation of wound cork by desiccating the cells that form the cork layer. Usually,

a relative humidity below 70 percent retards healing, causes high weight losses, and makes injuries

dark, sunken, and unsightly. Kelative humidity

near saturation (95 to 100 percent) often allows

considerable condensation on storage walls and

ceiling, and may cause discoloration of the surface

of the roots (^i). The relative humidity around

the roots in a storage container is usually slightly

higher than that in the air around the containers

or in the rest of the room (82).

A sticky, milklike juice often exudes from injuries (57), This juice dries in a few hours and

may appear to close the wound but actually offers

little or no protection against rotting. Also, the

presence of a dried and hardened surface over a

wound is no indication that the wound has been

healed, although this surface may retard the entrance of decay-producing organisms for a time.

Phenolic substances have been associated with

wound-cork formation but their role in producing

resistance to infection is not established {22, 89),

The conditions to which the roots are subjected

for the first 3 or 4 days after harvest determine

the degree of success in controlling decay by curing (55, 90), Long curing periods at temperatures

below 85° F. do not compensate for the difference

in temperature. Curing may be extended to set the

skin of the roots if they are to be graded and

shipped soon after harvest. During the last few

days of the extended cure, the relative humid'ty

may be reduced to dry the roots some in an effo t

to help set the skin.

At harvesttime the skin of the sweetpotato is

thin and delicate and easy to slip or remove. Often

roots that are run over grading equipment lose

noticeable amounts of their skin. The skin usually

slips more easily 2 or 3 days after harvest than at

harvest; but as curing progresses, the skin slips

less and less easily and eventually tightens {123).

The ease with which the skin slips at harvest

and the rate at which it develops adherence to

the root vary considerably. Factors inñuencing

this are not well understood, but roots should be

protected from being skinned during preparation

for market. It is important that conditions of curing and storage that speed setting the skin be

maintained. Experienced storage-packinghouse

operators usually allow 4 to 6 weeks after harvest

to set the skin through proper curing and storage.

Some operators indicate that drying roots after

curing aids in setting the skin when grading must

be done soon after curing.

By the time roots have been cured (healed) they

usually have a few short sprouts and a somewhat

velvety feel. If curing conditions are maintained,

these sprouts will grow and need to be removed

during grading. In addition, prolonging curing

will reduce the storage life of the roots. Reducing

the temperature in the storage house to 55° to

60° F. will prevent further sprouting. Some varieties (Nugget, Julian) sprout more slowly than

others (Jersey Orange, Goldrush).

The organism causing black rot, if present in the

field at harvest, may infect roots, and these roots

even though they are cured properly develop

typical black-rot spots {76), While it is possible to

prevent black-rot infection by curing infected

roots at 95° F. {ßJi) or to kill the organism by

holding the roots at temperatures over 100° {19,

26, 66, 92), neither of these treatments has proved

practical for commercial use because the temperature must be closely controlled. If black rot is

present at harvest, even in very small amounts, it

is usually advisable to market the sound sweetpotatoes as soon as possible without curing. Fortunately, this disease is not as prevalent as formerly because control measures are follow^ed during production. If the disease is present, it may be

controlled reasonably well on roots going to market by sanitation practices {100) and by treatment

with SOPP (sodium o-phenylphenate {65)),

Only a small amount of ventilation is required

during curing to prevent an accumulation of carbon dioxide or a depletion of oxygen. Usually sufficient exchange of air takes place in the normal

opening and closing of doors or through infiltration. However, as better or tighter storage houses

are constructed or rooms are designed specifically

for curing, presprouting, or refrigerated storage,

requirements of air exchange must be considered.

During curing at 85° F., sweetpotatoes that are

sound—that is, not excessively damaged by handling, wet soil, or chilling—consume oxygen at a

rate close to 2 cubic feet per ton per day or the

amount of oxygen in 10 cubic feet of air. Equivalent amounts of carbon dioxide are given off. This

amounts to about 1,250 cubic feet of air per 24

hours for a room holding 5,000 55-pound boxes of

sweetpotatoes. But four or five times this amount

may be needed because each exchange of air will

not completely replace the oxygen consumed or

remove the carbon dioxide produced.

If the roots are severely damaged by handling,

wet soil, or chilling, the respiration rate may be

three to four times faster than that for essentially

nondamaged roots {69), Decaying roots respire

even faster ; the rate depends upon how fast the

decay develops. If a curing room is filled with

roots damaged by rough handling, wet soil, or

chilling, it is possible to accumulate carbon dioxide

and to deplete the oxygen in the relatively small

amount of air left in the room. The storage-house

manager should increase ventilation to prevent

this from occurring and to remove any excess heat

produced by respiration (p. 24).
SWEETPOTATO STORAGE

STORAGE

After curing, sweetpotatoes should be held at

55° to 60° F. with a relative humidity of 85 to 90

percent (^0, 26). A small amount of ventilation is

needed to replace oxygen consumed and to remove

carbon dioxide produced. For refrigerated storage,

fresh air should be introduced at a rate equal to

the cubic-foot capacity of the storage every 2 hours

to prevent the accumulation of carbon dioxide

(108).

The primary purpose of these storage conditions

is to minimize physiological and pathological

losses. Some of the factors that should be considered have been discussed in connection with curing, but the primary storage problems are development of pithiness, sprouting, chilling, internal

cork, and decay.

Weight Loss and Development of

Pithiness

Most weight loss is due to evaporation of moisture through the skin. In addition, sweetpotatoes

lose weight as a result of respiration that consumes

some of the stored food in the root, primarily sugar

and starch. During respiration, carbon dioxide and

water are given on, thus decreasing the weight of

the root {60, 127). For some reason, not fully

understood, sweetpotatoes lose water and carbon

dioxide in such a manner that the ratio of water

to dry matter (such as starch and sugar) in the root

changes very little even when the roots are held

under widely different conditions of temperature

and relative humidity {I^l ). Loss of weight is encouraged by high storage temperature and low

relative humidity. At 55° to 60° F. and 85 to 90

percent relative humidity, properly cured sweetpotatoes usually lose about 2 percent of their

weight per month although differences exists

among varieties.

One important aspect of the loss of weight from

sweetpotatoes is its association with the development of pithiness in the roots {6, J¿7). Although

sweetpotatoes lose both weight and volume during

curing, there is little change in volume during

storage (72). Consequently, with loss of weight

during storage the roots become pithy (fig. 5).

Even at harvesttime 5 to 10 percent of the tissue of

sweetpotato roots is made up of intercellular

spaces {72). As weight losses exceed volume losses

during storage these spaces increase and eventually

become visible—that is, the root becomes pithy.

When more than 12 percent of the volume of a root

becomes intercellular space, pithiness can usually

be seen by cutting the root in half. Obviously, then,

weight losses should be keptto a minimum to retain

good quality during storage.

Varieties of sweetpotatoes differ in the amount

of intercellular space present at harvest as well as

in the rate at which this space increases in storage.

Varieties that become pithy rapidly, such as Jersey

Orange and Gem, should be marketed early in the

storage season. Other varieties, such as Porto

Kico, Goldrush, and Nugget, do not become pithy

rapidly and may be stored from 5 to 7 months.

Sprouting

At any time that temperature in storage rises

above 60° F. sprout growth will occur. A high

relative humidity encourages growth if the temperature is high enough (see under Curing). Since

heat and moisture often collect in the top of a

storage room, excessive sprouting is most often

noticed in the top layers of sweetpotatoes. Sprout

growth contributes to the development of pithmess.

A chemical sprout inhibitor, isopropyl-N(3

chlorophenyl) carbamate, is being tested on a

commercial scale and may become available for inhibiting sprout growth during storage. Maleic

hydrazide treatment before harvest has proved

detrimental (^5), although it has some effect on

apical dominance {106,115).

Chilling

Although reducing physiological activity and

decay of the roots by lowering the storage temperature below 55° F. may seem desirable, sweetpotatoes cannot be stored very long at temperatures

below 55°. At temperatures from 32° to 50°, they

BN-32773

FIGURE 5.—Pitliy condition of sweetpotato root.
10 AGRICULTURE HANDBOOK 358, U.S. DEPARTMENT OF AGRICULTURE

develop chilling injury and, like many other crops,

freeze and are ruined if held below 30°. A few days

at 50° is often not noticeably detrimental, since the

extent of the injury is proportional to the decrease

in temperature below 55° and the length of time at

a chilhng temperature (69). A day at 32°, however, produces a noticeable effect. Often the effect

of chilling is not noticeable until the roots have

been returned to the recommended storage

temperature.

Chilling injury is expressed in many ways. The

most common injuries are internal breakdown

{6ß), increased susceptibility to decay (76), failure to sprout {18, 31), and im;paired culinary

quality {W, 74) • In addition, chilling produces

such physiological responses as loss of ascorbic

acid (Vitamin C) (81) ; increase in chlorogenic

acid (81), which is associated with discoloration

upon exposure to air; increase in sugar content,

wnich sometimes is detectable when the roots are

cooked {4, 69) ; inability to synthesize carotene,

which usually increases slightly during storage

{38) ; and accumulation of carbon dioxide in the

root during chilling {68) ; accelerated respiratory

activity {69,79) ; a change in acidity of the tissue

{71); and other changes {9,10,80, 99,113).

Varieties differ slightly in their ability to withstand chilling injury. Puerto Rico sweetpotatoes

are less susceptible than most other varieties, but

all varieties are sufficiently susceptible to make it

desirable to avoid chilling conditions.

Pathological Losses

Infections of decay-producing organisms not

prevented by curing will progress into various

stages of decay during storage. Soft rot is caused

by a species of RMzopus that produces a soft decay

that consumes the roots quickly, even at 60° F.

Surface rot and end rot are caused by species of

FusañuTn that grow slowly; it may take several

weeks for an entire root to be destroyed. Several

other diseases also are often encountered {34, 48,

52). Usually the development of decay is more

rapid at high temperatures than at 55° to 60°.

With few exceptions, the decay-producing organisms do not move from diseased roots to roots

healed by curing. A new disease, blister, of unknown origin has been associated with storage of

sweetpotatoes {104). Blister has been detected

more frequently on roots of the Nugget variety

than on roots of other varieties.

Internal Cork

Storage temperatures above 60° F. encourage

the development of a virus disease of sweetpotatoes that causes the development of corky areas in

the roots of susceptible varieties (fig. 6) (7, 63, 67,

91,102,129).

Although some internal cork may be present at

harvest, especially if warm weather precedes harvest, during storage the corky areas develop and

enlarge at storage temperatures above 60°. The

varieties Porto Rico, Goldrush, Georgia Red, and

Centennial develop cork, if the virus is present in

the roots, at a warm storage temperature. Roots of

BN-32769

FIGURE 6.—Internal cork : A virus disease that develops

coriililie areas in susceptible varieties at storage temperatures above 60° F.
SWEETPOTATO STORAGE 11

most other varieties may carry the virus but they

do not develop the internal cork symptoms even

when stored at temperatures above 60° (5,61).

Weevil Control

In the few weevil-infested areas where sweetpatatoes are grown, the roots are often dusted or the

storage area is sprayed with DDT {17^ 100),

Sometimes the temperature of the storage is allowed to rise sufficiently to permit the weevil larvae present in the roots to grow and to emerge. The

infested roots can then be identified and eliminated

during grading. In addition, when the insect

CAVTION

Do not feed DDT-treated sweetpotatoes to

dairy animals or animals being finished for

slaughter. Sweetpotatoes must be washed thoroughly before they are eaten or offered for sale.

emerges, it comes in contact with any DDT on the

roots. The DDT-containing dust is washed off during preparation for market. A residue of 7 parts

per million is allowed by the Food and Drug

Administration.

PRESPROUTING

Sweetpotatoes saved for seed stock must be in

good condition and free of disease. Special production practices are ofte-n used to eliminate

diseases from seed roots. In addition, these roots

should be cured and stored using recommended

practices. In the storage house, presprouting—a

practice not recommended for roots going to the

fresh market—is often applied to seed roots to enable them to produce plants faster when bedded.

Presprouting involves the application of curing

conditions to cause seed roots to produce sprouts

1 to 2 inches long before bedding {27),

Usually j>resprouting is started about 4 weeks

before bedding by raising the temperature of the

seed roots to about 85° F. with a high relative

humidity and maintaining such conditions until

the roots sprout sufficiently. Some ventilation is

desirable because the roots respire rapidly when

the temperature is raised. Although varieties that

are slow to sprout usually require a longer presprouting treatment than others, the treatment is

also more beneficial for them. Presprouted roots

produce more plants, especially early plants, than

roots not presprouted m a given period of time

after bedding.

Before bedding, seed roots are often sorted to

remove those decayed or off-type. Those remaining

are then placed in fairly uniform size categories to

aid in uniform plant emergence. Presprouting is

often used to heal injuries inflicted during this

sorting and handling, since presprouting conditions are the same as those used in curing to heal

injuries inflicted during harvesting (55, 122),

Bulk-tobacco-curing barns, after some modification, have been used for presprouting {Jß)^ but

most storage houses with adequate curing facilities serve very well.

PHYSIOLOGICAL CHANGES

Some physiological changes take place in roots

after they are harvested. Sweetpotatoes are more

palatable when held for a few days after harvest

before being cooked than when cooked immediately after harvest {23, Jfi, 58,5^, 65,66,116, ISO),

The major change involves the increase in conversion of starch to sugars and dextrins during

cooking. Although the sugar content of the raw

roots usually increases some during curing and

may increase still more during storage {Í¡9, 90,

llJf), these changes are small compared with the

change of starch to sugars and dextrins that occurs during cooking. Freshly dug roots convert

less of their starch to sugar "and dextrins during

cooking than comparable roots after being cured.

Storage for a month or two often increases the

conversion of starch to sugars and dextrins during cooking.

Varieties of sweetpotatoes differ in the degree to

which they convert starch to sugars and dextrins

during cooking. Varietiesthattend to convert most

of their starch to sugar and dextrins become very

sweet and moist; they are referred to as "wet-"

or "yaon-" type sweetpotatoes. The varieties Porto

Rico, Goldrush, and Georgia Red fall into this

class. Other varieties, such as Big Stem Jersey

and Jersey Orange, convert less of their starch to

sugar and dextrins during cooking and are usually somewhat firm and "dry'' tasting.^ Several

varieties, such as Nemaigold, are intermediate. The

classification of varieties into wet or dry types is

arbitrary since the classes are not easy to define,

and all varieties tend to be more dry at harvest

than after curing.

Ascorbic acid content usually decreases some

during curing and storage {38, Jfi, Jß, 117, 118),
12 AGRICULTURE HANDBOOK 3 58, U.S. DEPARTMENT OF AGRICULTURE

Some changes in pectin content occur that are associated with the softening of the flesh of cooked

roots (i, 11^ 12^ 58), Changes in amide- and aminonitrogen have been recorded (SO). Carotene content usually increases during storage (ß, 36^ 37^ 39,96) since the site for synthesis is in the root (59.

95).

CONSTRUCTION OF STORAGE HOUSES

General Requirements

The storage house should be designed to provide the temperature, humidity, and ventilation

recommended for proper curing and storing of

sweetpotatoes throughout the season in the location selected.

Heat is usually necessary in the fall during curing, in the late winter or early spring for presprouting, and often in the winter during the

coldest weather. The storage should be designed

to provide the amount of heat needed as well as

its proper distribution to maintain reasonably uniform temperatures. To do this, heat is usually

added beneath the stored sweetpotatoes, since the

lower levels of the storage are usually cooler and

harder to heatthan the top.

Excess heat is removed from the storage house

in two ways. Warm air may be exhausted from the

top of the storage and cool air introduced through

doors and special vents near floor level. Or cool

air is introduced into the top of the storage with

ventilation equipment or mechanical refrigeration.

High humidities are usually rather easily maintained in most of the sweetpotato-producing areas

because the roots give off some moisture and the

relative humidity is often fairly high in the outside air. But when large amounts of heat are added

to the storage, or in dry climates, some provision

for humidifying the air in the storage is needed.

Conversely, because outside air temperatures often

are considerably lower than the temperature inside

the storage room, some provision for removing

condensation is desirable. This is usually provided

in the system of ventilating that is used both for

cooling and for exchange of air.

Locating the storage near the point of production will minimize hauling costs. Size of the storage influences the relative cost per unit stored.

Usually large storages are more advantageous than

small ones. Large storages often beneñt by complementing storage with marketing and processing

facilities.

Since all circumstances of weather, construction,

or management cannot be anticipated, the following sections will provide some guidelines to consider in constructing and managing storage houses

to obtain the objectives. The storage house should

be located on a well-drained site with a foundation

or footing appropriate to the site and construction

of the building. Often the building is designed to

include more than storage of sweetpotatoes.

For large storages, or small ones which may be

expanded, it is desirable to plan for the following

five steps which usually make up the complete

operation.

1. Unloading of field trucks directly into the

curing room or to the grading equipment (if this

is present or to be added in the future).

2. Curing with forklift handling of palletized

boxes or provision for it in the future.

3. Storage, either in the curing room or in separate rooms provided for it, also with provision

for handling by forklift.

4. Grading, preferably with equipment that includes washing and treating to prevent decay during marketing.

5. Loading trucks (or railcars) for shipment to

market.

In large storage houses, office space and restrooms are included, too.

The storage should be located where an adequate supply of electricity and water are available

or can be made available. Depending upon the facilities included in the plans, electricity will he

needed for lights, conveyors, humidifiers, grading,

heating, ventilating, or refrigeration. Special considerations are necessary if the demand for electricity is great ; that is, when electricity is used for

heating the storage or nearby plant beds. Water i^

used for humidifying and cleaning the storage,

and, usually in large amounts, for washing the

roots during preparation for market when this is

performed at the storage house.

The exact requirements for operating a storage

house will be determined by the size and facilities

included. The plans should include provisions for

expansion or addition of facilities. For example,

a storage house containing 250 tons or more (10,000

bushels) of sweetpotatoes probably will justify

pallet and forklift handling, especially if labor

costs are high, and the forklift will find use in

other activities such as loading and unloading fertilizer. If the forklift is included, the floor, doors,

ceiling height, and room dimensions should be

suitable for such equipment. In storages of 750

tons or more, the addition of grading equipment is

probably justified. Some refrigerated storage space

is usually added, especially in the southernmost

producing areas in storages above 1,000 tons

capacity.
SWEETPOTATO STORAGE 13

Although earth floors are popular in small buildings because of their low cost and the ease of maintaining high humidity, they are not practical in

the larger storages where forklift equipment is

used. False floors with underfloor heating work

well, but this type of flooring will not support the

great weight of forklift equipment.

Since walls and ceiling must withstand moisture,

they should be constructed with moisture-resistant

materials.

Selection of materials and type of construction

will depend upon local availability of materials,

cost of materials and labor, and the need for a

temporary or a permanent building. Features of

wall, ceiling, and floor construction will be considered from the standpoint of heating, cooling,

aerating, humidifying, and dehumidifying.

Heating and Cooling Requirements

Heat is needed in a sweetpotato storage house to

warm the sweetpotatoes and storage containers to

the proper temperature ; replace heat lost through

walls, ceiling, and floor by direct transmission and

radiation; replace heat lost through inñltration

of air at cracks around doors, windows, and ventilators; and replace heat absorbed by evaporation

of moisture from the roots or storage containers

and adhering soil {13).

Heat of Respiration

Although some heat is produced by the sweetpotatoes, it is usually insufficient for curing and

presprouting, and except in large storages in the

southernmost sweetpotato producing areas, for

maintaining the storage temperature during the

winter. These same factors must also be considered

when cooling, since then heat must be removed

instead of added.

If essentially sound sweetpotatoes are harvested

and placed at 85° F. for curing, respiration will

produce about 275 British thermal units (B.t.u.)

per 55 pounds of roots (10,000 B.t.u. per ton) for

the first day or two. If the roots are placed at a.

lower temperature after harvest, respiration will

be proportionately less by about 22 B.t.u. per day

per 55-pound box for each 5° reduction, or about

165 B.t.u. at 60° (6,000 B.t.u. per ton). The rate

of respiration, and therefore heat production, varies considerably from one lot of sweetpotatoes to

adiother; consequently these figures must be considered as reasonable approximations only {69),

Mechanical and physical injuries increase the rate

of respiration. Cured roots respire less than freshly

dug roots when held at the same temperature and

at 60° produce about 100 B.t.u. per 55-pound box,

or 3,600 B.t.u. per ton {68,69,108).

The production of heat by sweetpotatoes damaged by wet or cold soil, chilling during storage,

321-902 0—69 3.

and decay deserves special attention. The respiration rate and heat production of roots damaged

by wet or cold soil and chilling is often two to three

times greater than for roots not damaged in these

ways {69). If the damage is great enough, some

roots decay, increasing the production of heat still

more. The rate of increase depends upon the rapidity and extent of the decay ; if decay is extensive, it may be great. Moisture is given off rapidly,

too, because of the decomposition of the roots.

Extra precautions are usually necessary to provide

sufficient ventilation to remove the excess heat and

moisture produced.

Vaporization of Moisture and Vapor Barriers

Some evaporation of water from the sweetpotatoes is to be expected. The roots themselves may

be carrying moisture or the moisture may be in the

soil clinging to the roots or be in the wood of the

boxes or pallets. This moisture, along with some

produced by the roots, usually evaporates slowly.

In calculating heat requirements, the amount of

heat absorbed by evaporation or released by condensation is usually disregarded. But during curing, sweetpotatoes usually lose 3 percent or more

of their weight, primarily through evaporation of

moisture. This evaporation absorbs approximately

60,000 B.t.u. per ton of sweetpotatoes and is approximately the amount of heat produced by respiration during curing. Therefore, in calculating

the heating requirements during curing, the heat

produced by respiration of healthy roots is not

considered nor is heat absorbed by evaporation of

moisture, even though respiration may provide

slightly more heat than that absorbed by evaporation. During storage moisture evaporates very

slowly and loss of heat from this is negligible.

From the standpoint of storage-house construction, vaporization of moisture must be controlled,

A good moisture barrier is desirable in the storage

to maintain the high humidity recommended, to

prevent excessive evaporation, and to prevent excessive accumulation of moisture in the walls.

Evaporation of moisture will aid in maintaining

the high humidity desired. But at times, especially

when large quantities of moisture are carried into

the storage house during harvesting from wet soil,

moisture levels in the house go above those recommended; that is, above 90 percent, and approach

saturation.

When the relative humidity approaches saturation, moisture will begin to condense on any surface that is below the temperature of the air containing the moisture. Usually such condensation

occurs in the top of the storage. Warm air, which

is capable of holding more moisture than cool air,

rises to the top of the room and is cooled slightly

by the loss of heat through the walls and ceiling.

With the loss of heat, the air deposits moisture on

any surface that is cool enough to be at the dew-
14 AGRICULTURE HANDBOOK 358, U.S. DEPARTMENT OF AGRICULTURE

point temperature for the air near it. The amount

of condensation is kept to a practical minimum by

providing the proper amount of insulation and by

removing excess moisture by ventilation.

To avoid undue deterioration in the walls or

ceiling, condensation should be prevented within

them. This condensation is not as obvious as the

condensation on the inside wall surfaces or ceiling;

consequentl}^, the deterioration often occurs before

the trouble is detected. If a relative humidity of

90 percent is maintained at 85° F. in the storage,

the vapor pressure is 0.535 pound per square inch.

If the relative humidity outside is 50 percent at

35°, then the vapor pressure there is 0.048 pound

per square inch, less than one-tenth that on the

inside. Under the influence of this pressure, water

vapor will tend to move out through the wall.

Often the temperature within the wall is below

the dewpoint temperature of the air in the room

and moisture will condense within the wall (or

ceiling).

A moisture barrier retards movement of water

vapor. As long as the vapor can move out of the

"cold" side of the wall faster than it moves in from

the "warm" side, little trouble is experienced. InsuflSicient data are available from sweetpotato storage studies to recommend the degree of resistance

to moisture movement to provide in the vapor barrier, but recommendations for similar structures

have proved useful {16). Sheet-type vapor barriers, such as polyethylene sheets, 2 to 6 mil thick,

are often used for this purpose in the coldest parts

of the sweetpotato-producing areas. When these

sheets are used, they should oe applied carefully

to prevent damaging them. Asphalt or oil varnish

paints are useful vapor barriers if two or more

coats are applied. Other paints such as aluminum

and alkyd gloss are also effective.

Shiny-surface roll roofing, heavily asphalted

building felt with a shiny surface, and duplexkraft paper with aluminum foil on one side or

with asphalt between laminations are effective

moisture barriers and are necessary in a frame wall

filled with insulation.

Board-form insulation having an asphalt coating is preferable to the uncoated type for vaporproofing. The joints between the insulation panels

should be filled with a waterproof compound and

the entire surface mopped heavily with two coats

of hot asphalt or painted with two coats of asphaltbase aluminum paint. Concrete- or cinder-block

walls may be made vapor-resistant by painting the

interior surface with a cement-water wash. When

this is thoroughly dry, two coats of asphalt-base

aluminium paint should be applied. Specific details

for installing commercial insulation and vapor

barriers are usually available in the directions

given by manufacturers.

The exterior walls of the storage house should

be constructed or treated in some way to prevent

entrance of moisture into the insulation from driving rain or snow. An overhanging roof is very

helpful. Two coats of cement-water paint should

be thoroughly scrubbed into the pores of concreteand cinder-block walls.

Ventilating a wall or ceiling cavity is effective in

oontrolling condensation. Unvented insulation in

the ceiling is particularly troublesome since the

insulating value of any material is reduced by the

accumulation of moisture. This trapped moisture

in turn causes portions of wooden joists and the

ceiling to decay rapidly. No vapor barrier completely prevents moisture from moving through

the walls or ceiling. Construction then should

provide some ventilation to remove this moisture.

In frame walls, small openings may be made toward the outside at the top and bottom of the stud

spaces to permit moisture to escape, or holes may

be bored through the plate, sill, or cornice.

On unvented insulated walls under extreme

conditions, as found in farm structures used for

sweetpotato storage, an 8-1 perm ratio of outside

to inside wall covering is necessary. This limits

sheathing moisture to 20 percent (dry basis),

which is the maximum desirable to prevent decay

in wood members. A perm ratio of 3-1 or greater

is needed to prevent the formation of free moisture

within insulated walls. One perm equals the transmission of 1 grain of water per hour per square

foot per inch of mercury vapor pressure.

In view of the problems often encountered with

moisture in sweetpotato storage structures, recent

trends have been toward masonry construction,

even in the ceiling. Fire insurance rates are usually

considerably lower also where wood and other

flammable materials are eliminated or nearly

eliminated.

Product Load

Sweetpotatoes delivered to the storage house for

curing and storing usually arrive at a temperature

near that of outside air. Early in the harvest season this may be near the 85° F. temperature

recommended for curing, whereas late in the season on cold days the temperature occasionally may

be as low as 40° or 45°. The amount of heat required to warm the sweetpotatoes to the recommended 85° will vary according to their temperature on arrival and the quantity delivered. Generally, enough heat should be provided in the

storage house to raise the temperature of the sweetpotatoes to 85° within 24 hours so that the roots

brought in each day will reach curing temperature

before more are placed in the curing room.

Certain assumptions are needed to design a

storage that will meet most situations arising during the season. The first assumption is that the

sweetpotatoes usually will not be chilled in the

field or by delayed delivery to the storage house.

Therefore, they will arrive at temperatures no
SWEETPOTATO STORAGE 15

lower than 55° F. most of the time; that is, no

more than 30° below the recommended 85°. A second assumption is that the sweetpotatoes will be

handled in field boxes containing from 50 to 55

pounds of sweetpotatoes, so we may accept 55

pounds as a working figure. Furthermore, the

empty boxes will weigh approximately 4 pounds

each, soil and moisture adhering to the roots and

in the box is negligible (that is, 0.1 to 0.2 pound)

and the roots will not be excessively damaged by

harvesting or by wet or cold soil.

With these assumptions, it is possible to calculate the amount of heat needed to bring the sweetpotatoes to 85° F. for proper curing, if heat produced by respiration and lost by evaporation of

moisture is disregarded (p. 13).

To warm 55 pounds of sweetpotatoes 30° (from

55° ;to 85° F.) requires about 1,320 B.t.u.; that is,

30 times 55 times 0.8, where 0.8 is the specific heat

of the sweetpotatoes at 75 percent moisture content. To warm a 4-pound wooden box 30° requires

about 48 B.t.u. ; that is, 30 times 4 times 0.4, where

0.4 is the specific heat of the wood. Similar calculations may be made if it is assumed that sweetpotatoes will arrive at the storage at some temperature other than 55°. For the practical purpose of

storage-house design, it is probably safe to assume

that not over 1,300 B.t.u. will be required to raise

the temperature of one box of sweetpotatoes (55

pounds) to 85° throughout most of the harvest

period. During warm weather no additional heat

may be required {61), If the rate of filling the

house is known and losses of heat through walls,

ceilings, and floors and by infiltration are estimated, the total heat requirement may be

calculated.

Approximately the same calculations may be

used to determine the heat required to raise the

temperature of stored roots to presprouting conditions. Because presprouting is usually done during February or March, a room with the smallest

outside wall area is often used.

Loss of Heat Through Walls^ Ceilings^ and Floors

Loss of heat through walls and ceilings depends

primarily upon the amount of resistance provided

to the movement of heat through the materials

used in the walls or ceiling. The insulating value of

construction materials is rated by a factor k^ defined as the rate of heat transfer in B.t.u. per hour

through a 1-square-foot area of 1-inch thickness

per unit of temperature difference between the

outside and inside surface. This factor is often

difficult to use for combinations of construction

materials and a simpler method rates insulation

in thermal resistance. Thermal resistance (Ä) is

related to k, according to the equation Ä=^. If

the resistance values of different materials in a

wall or roof are known, they may be added together to give the total resistance to movement

of heat. Eepresentative B values for some commonly used construction materials are given in

table 1, along with commonly accepted values for

air films that provide some resistance to heat loss,

too.

TABLE 1.—Thermal resistance of building materials

frequently used in construction of sweetpotato

storage houses

Material and description Thick- Resistness anee ^

Inches R

Air space:

Dead air space in wall J^-4 0.97

Dead air space in ceiling %-4 ! 85

Air surface:

Outside air film (15}^ m.p.h.)._ . 17

Inside air film (still).. _ . 68

Board:

Asbestos-cement board _. y^ . 03

Plywood }i * 31

Do % .47

Do }i .63

Wood sheathing 2^2 .98

Asphalt impregnated sheathing J^ 1. 32

Do 2^2 2.06

Building paper:

Felt, 15-pound grade .06

Felt, 30-pound grade . 12

Masonry :

Brick, common 4 .80

Brick, face 4 . 44

Concrete block, 3 core 4 . 71

Do 8 1.11

Do___- 12 1.28

Lightweight aggregate block 4 1. 50

Do 8 2.00

Do 12 2.27

Lightweight aggregate block,

core filled with granulated

insulation 8 2558

Do 12 2 6.67

Siding:

Asphalt roll . 15

Asphalt insulating siding >^ 1. 45

Wood . 80

Roofing:

Asbestos-cement shingles .21

Asphalt-roll . 15

Asphalt shingles .44

Wood shingles .94

Insulation:

Mineral or rock wool 1 3. 70

Plastic (foamed) polystyrene 1 3.45

Corkboard 1 3.70

Cement and granular insulation (1:4) 2 * 4. 07

Do 4 »6.45

^ Abstracted from Heating Ventilating and Air Conditioning Guide (1958) Ch. 9, except as indicated.

2 Concrete Masonry Manual, N.C. Concrete Masonry

Association, Raleigh, N.C, April 1964.

3 Specifications and Data Perlite Insulating Concrete

Over Metal Form Units, Perlite Institute, Inc., International Association of Perlite Miners and Processors,

May 1962.
16 AGRICULTURE HANDBOOK 358, U.S. DEPARTMENT OF AGRICULTURE

Representative wall and ceiling construction is

given in figure 7.

Masonry walls can be insulated in several ways.

Lighweight aggregate or cinder block or structural-clay tile cores can be filled with a pellet or

granular type of insulation (expanded mica). This

type of insulation is easily added as the wall is

built and is generally less expensive than other

types. In this connection, 12-inch blocks may be

used to build walls in most areas to a height of

20 feet without pilasters or other support for the

walls except where girders or trusses are to be

OUTSIDE AIR FILM 0.17

12" BLOCK, GRANULAR FILL 6.67

DEAD AIR SPACE, 2" 0.97

Vi" ASPHALT IMPREGNATED

SHEATHINGWOOD STRIPS, r'x4"-

INSIDE AIR FILM

-1.32

-0.20

-0.68

B

OUTSIDE AIR FILM——

WOOD SIDING

ROCK WOOL BATT, 3 "-

ASPHALT ROLL SIDINGWOOD STRIPS, l"x4"-

INSIDE AIR FILM

-0.17

-0.80

-11.10

-0.15

-0.20

-0.68

TOTAL THERMAL RESISTANCE--10.01 TOTAL THERMAL RESISTANCE--13.10

VENTOUTSIDE AIR FILM———0.17

4-PLY BUILT-UP ROOF 108

I'/i" RIGID INSULATION BOARD—4.19

4" PERLITE-CEMENT MIX 6.45

CORRUGATED SHEET IRON 0.05

INSIDE AIR FILM-^ 0.68

TOTAL THERMAL RESISTANCE--12.62

OUTSIDE AIR FILM 0.17

4-PLY BUILT UP ROOF 1.08

ROOFING BOARDS, 25/32" 0.98

4" ROCK WOOL BETWEEN

JOISTS 14.80

4 mil POLYETHYLENE 0.10

%" EXTERIOR PLYWOOD 0.31

INSIDE AIR FILM 0.68

TOTAL THERMAL RESISTANCE--18.12

FIGURE 7.—Wall and roof details showing relation of construction to thermal resistance.
SWEETPOTATO STORAGE 17

used to support the roof. Local or other applicable

codes should be followed in determining the need

for pilasters or other supports. An 18- to 20-foot

height is high enough for most palletized operations if proper clearance is maintained.

If the granular-fill type of insulation is not desired, insulation board attached to 2- by 2-inch

or 2- by 4-inch vertical furring strips is often used.

Strips 1 by 3 inches or 1 by 4 inches spaced about

a foot apart should be nailed over the insulation

board so that holes will not be punched in it. A

moisture-resistance finish may be applied to seal

nailhead and other holes.

Masonry walls, usually cinder or concrete blocks,

are relatively windtight and watertight, permanent, and low in upkeep. Asbestos-cement boards

are not affected by moisture or decay, are fire- and

vermin-resistant, do not require painting, and are

reasonably strong when properly supported. Galvanized-sheet steel is relatively low in first cost,

is easy to apply, and has advantages similar to

those of asbestos-cement boards. Aluminum sheets

are also satisfactory as interior lining or exterior

finish, but should not be used in contact with earth,

concrete, mortar, or other metals unless protected

with asphalt paint. Aluminum nails should be used

for applying sheets. Moisture-proof plywood,

laminated asphalt-felt board, and emulsifiedasphalt gypsum board are suitable materials when

properly installed. All commercial materials

should be installed according to the recommendations of the manufacturers.

A wood interior lining has a reasonably long life

if the walls and ceilings are properly insulated so

that little condensation occurs. The wood can be

protected by covering it with a vaporproof membrane and fastening the lining down tightly with

closely spaced lath or other wood strips.

Fortunately, the amount of thermal resistance

needed to prevent excessive consumption of fuel

corresponds fairly well with the amount of thermal

resistance needed to maintain the high relative

humidity recommended. The inner surface of a

wall or ceiling cools as heat is transmitted through

it. Kapid loss of heat will mean that the inside

surface will cool more than if heat is lost slowly.

Therefore, the insulation properties determine the

temperature on the inside surface which, in turn,

determines whether moisture condenses on this surface because the dewpoint temperature is reached.

Figure 8 shows the theoretical outside temperature at which condensation will begin in a storage house with walls of a given conductance when

the inside relative humidity is held at a given point

for both the curing temperature of 85° F. and the

storing temperature of 55° (16) .^

3 Presumably from equation T,-Ti-^{Ti-To) (44)-

oc

3

O.

80

CURING TEMPERATURE, 85°F.

60

Relative humidity: ^^ "

90^.x""^^

^

40 / /'^y^ ^

20

y^

= 60

Û

(/)

O

40

20 -

STORAGE TEMPERATURE, 55°F.

Relative humidity:

\

90%

0 0.1 0.2 0.3 0.4 0.5

WALL CONDUCTANCE, B.t.u./hr./sq. ft./deg. F.

I 1 \ \ 1

10.0 5.0 3.3 2.5 2.0

THERMAL RESISTANCE OF WALL

FIGURE 8.—Theoretical condensation conditions during

curing at 85° F. or storage at 55°.

For example, a wall with a thermal resistance of

5.0 should conduct 0.2 of a B.t.u. through each

square foot of wall surface in 1 hour for each degree Fahrenheit difference in temperature between

the room and outside air; that is, 6 B.t.u. per hour

per square foot for a 30-degree difference. Also,

this same wall should permit condensation on its

inner surface if the temperature in the room is 55°

F. and if (a) the outside air temperature is 30° and

the relative humidity in the room exceeds 90 percent, or (b) the relative humidity in the room is

90 percent and the outside air temperature drops

below 30°.

From figure 8, then, it can be determined how

much insulation is needed to permit the maintenance of the high relative humidity recommended if the outside air temperature is known.

Or, to summarize, the following combinations of

outside air temperature and insulation permit the

maintenance of 90-percent relative humidity at

85° or 55° F.
18 AGRICULTURE HANDBOOK 3 58, U.S. DEPARTMENT OF AGRICULTURE

Outside temperature {°F.)

70

60

50

40

30

20

10

0.

Thermal resistance value midity during the coldest weather expected, the

of wallfor— maximum loss of heat will be within reasonable

55° 55° limits, and an estimate of the rate of maximum

3 1 heat loss through walls and ceiling obtained :

5. 2 Wall area (in square feet) times 5 plus ceiling

J-^ Í"R'" " ^^®^ (^^ square feet) times 314 =B.t.u. per hr. loss

jj ^ ^ g at the outside air temperatures that will cause con13] 5 8. 3 densation to begin to form if the relative humidity

15. 7 10. 6 inside is 90 percent. This calculation assumes that

^'^- ^ ^^' ^ all walls are outside walls, but this is seldom true.

Since harvesting continues in all producing areas

Therefore, a wall with a resistance value of 11.4 ^P *^ f¿ ^^^^,^^f^, *T^ ^^1}^ ^^^ ^^^^^^ ,f^

would permit the outside air temperature to drop ^^^^^^9* ^^^ TöÄ ""A ' '^ '' fPPf f.^^ ^^^^

as low as 30° during curing at 85°, or to 10° dur coMitions of 85° F and 90-percent relative huing storage at 55°, befort condensation would ^\^!^y 'f'??w^ ^^^i?^ ^^ ' ^^ 'T?^ ^^! ^^^'^^

ocSur if the relative humidity was 90 percent or f^^^ P^^/^ Pj *^^ .^ ^- However, such low temperalower in the storage house. ^^^^^ ^^^^^^^ ^^f *^^ ^^^^ ^ ^^^^ ^ ^' ^^^ au

Each of the combinations of outside air temper- ^^^^^^ temperature over several hours, or even a

ature, wall insulation (thermal resistance), and ^f^^'. ^^ ^^^^ ^^f.^^^ ^^ estimatmg requirements.

90-percent relative humidity will allow about 5 to ^^^^i^^ ^^^^^^^^ J^ maximum insulating require51/2 B.t.u. heat loss per square foot per hour. If the ^?^.^s ^^^ ^^ estimated from the average annual

thermal resistance of the ceiling is II/2 times minimum temperature m the area (fig. 9).

greater than in the walls, about 31/2 B.t.u. heat loss This temperature will vary m different parts of

will occur per square foot per hour for these com- the country. To reach a compromise between rebinations. If, then, thermal resistance is provided quirements at harvest for curing and during the

as indicated to maintain the proper relative hu- winter for storage, the following thermal resistPERIOD 1899-1938

FiGUEE 9.—^Average annual minimum temperature ( " F. ) expected in sweetpotato-producing areas of the Southeastern

United States.
SWEETPOTATO STORAGE 19

anees are suggested for areas where the average

annual minimum temperature is as indicated :

Average annual minimum

temperature ° F.

0

10

20

30

Thermal resistance (R)

Wall

10-13

8-10

6-8

4-6

Ceiling

15-20

12-15

9-12

6-9

The roof or ceiling should be better insulated

than the walls so that any condensation will occur

first on the walls where it cannot drop on the sweetpotatoes. In general, a factor of at least 1.5 times

as much insulation should be provided in the ceiling as in the walls of the same room. If fuel is

relatively expensive, the higher thermal resistance

values should be considered for the appropriate

average annual minimum temperatures. Interior

walls may require less insulation than exterior

walls since the temperature in adjoining rooms

is usually no lower than 55° F. Eooms used only

for storage after curing in another room require

less insulation than curing or presprouting rooms.

Loss of heat through the ñoor is usually of little

consequence since part of it is radiated back to the

room. Loss around the periphery of the room may

be estimated by multiplying the linear feet of outside wall by 14 to obtain the B.t.u. loss per hour

(13).

Heat requirements should be calculated for a

proposed storage house based upon the construction details and the way the storage house is to be

operated, as well as the expected weather in the

area of construction (13). An allowance for the inefficiency of the heating system usually is included

also. This inefficiency may amount to 25 to 50 percent of the calculated requirements. An estimation can be obtained as follows :

Given a room 22 by 50 by 18 feet high with a

thermal resistance of 5 in the walls and 7.5 in the

ceiling, a capacity of 5,000 55-pound boxes to be

put in at a rate of 1,000 per day, and an outside

temperatures that will average no less than 55 ° F.

during a 24-hour period at harvest and develop an

outside annual mean minimum of 20° ; then estimated maximum heat requirements per hour are :

A. For curing

B.t.u.lhr.

No. 55-lb. boxes/dayX551 (product load) or

(1,000X55) 55, 000

Walls (sq. ft.)Xdegree2XB.t.u./sq. ft./hr./degree ^

or (2,592X30X0.2) 15, 552

Ceiling (sq. ft.) X degree ^ X B.t.u./sq. ft./hr./degree ^

or (1,000X30X0.13) 3, 900

Perimeter (linear ft.) X14 or (144 X14) 2, 016

Total 76,468

Plus infiltration (10 pet.) ^ 7, 647

Grand total 84, 115

B. Storage

B.t.u./hr.

Walls (sq. ft.) X degree 5 X B.t.u./sq. ft./hr./degree 3

or (2,592X35X0.2) 18, 144

Ceiling (sq. ft.) X degree ^X B.t.u./sq. ft./hr./degree 3

or (1,100X35X0.13) 5, 005

Perimeter (linear ft.) X14 or (144X14) 2, 016

Total 25, 165

Plus infiltration (10 pet.) * 2, 517

Grand total 27, 682

Less No. 55-lb. boxesX4 6 (5,000X4) 20,000

Net total 7, 682

1 1,320 divided by 24, assuming heat produced by

respiration equals heat lost by evaporation.

2 85° F. inside less 55° outside.

3 From figure 8, or J^

R,

* See discussion on p. 20.

^ 55 less average annual minimum temperature from

figure 9.

Ö 100 B.t.u./24 hours divided by 24 (heat of respiration

at 60° F.).

The heat produced by respiration enters the

problem of storage-house design and management

in another way. As the capacity of storage rooms

increases, the wall and ceiling areas increase but

not as rapidly as the room capacity. This influences

the rate at which heat produced by respiration is

lost. For example, a room measuring 24 by 40

and 18 feet high is capable of holding about 5,400

boxes of sweetpotatoes (30 boxes per pallet and

pallets 6 by 10 and 3 layers high). If the walls

have a thermal resistance of 5 and the ceiling 7.5,

a loss of about 17,500 B.t.u. will occur each day for

each degree the outside temperature is below that

inside the room (table 2). The sweetpotatoes, at

60° F., will produce about 540,000 B.t.u. per day

or enough to compensate for heat lost through the

walls and ceiling at an outside temperature of

about 29°. A smaller room provides less and a

larger room more protection as long as the rooms

are well filled.

Obviously, this is an ideal illustration and under

commercial conditions the situation may be considerably different. All walls were assumed to be

outside walls, which is frequently not trup. Furthermore, infiltration may be greater than the 10

percent allowed or decay may add more heat than

calculated as heat of respiration. The room may

not be full. The temperature of the sweetpotatoes

will probably not be exactly 60° F. and, of course,

more heat will be produced above 60° than below.

Evaporation of moisture will probably consume

some heat.

Despite all the variables involved, the example

illustrates that the mass of sweetpotatoes in storage may be large enough so that little or no additional heat will be required during the winter.

This would depend, of course, on adequate insulation in the storage house and proper distribution
20 AGRICULTURE HANDBOOK 358, U.S. DEPARTMENT OF AGRICULTURE

TABLE 2.—Relation of size of storage rooms to calculated heat requirements ^

Heat losses (per °F. per day)

Room dimensions (feet) 2

12 X 24 24 X 40 40 X 48

B.t.u.

Walls (ß=5) 5,216

Ceiling (Ä=7.5) 888

Perimeter (ft.X14) 1,008

Infiltration (10 percent) 711

A. Total heat loss/°F./day 7, 823

B. Heat of respiration at 60 °F., boxes X 100= B.t.u./

day - 162,000

C. Protection from heat of respiration 21

D. Outside air temperature to which protected __, 39

B.t.u. B.t.u.

11,059 15,216

2, 995 5, 976

1, 792 2, 464

1, 680 2, 366

17, 526 26, 022

540, 000 1, 440, 000

^F. "F.

31

29

55

5

1 Assuming all walls are exterior walls and rooms are filled with palletized boxes containing 55 pounds each, 30 boxes per pallet, pallets 3 layers high; that is, 90 boxes per 4-

X 4-foot ñoor area.

2 All rooms 18 feet high.

of the heat produced by the sweetpotatoes. Unfortunately, this last requirement is not easily obtained unless air circulation is provided, since heat

tends to collect at the top of the storage rooms.

Infiltration

The amount of infiltration of air through openings, such as cracks, open doorways, around ventilators, is difficult to estimate. Usually, about 10 percent of the heat required to bring the sweetpotatoes

to the proper temperature or replace heat lost

through the walls, ceiling, and ñoor is assumed to

be lost by infiltration. If it is assumed that infiltration will be equivalent to one air change per hour

(IS), an estimate can be obtained from the

equation

B.t.u./hr. =0.018 Ç (Ti-To),

where Q is the capacity of the room in cubic feet,

and Ti and To are the inside (room) and outside

air temperatures, respectively. This will provide

an estimate closer to 15 percent of the heat requirement to raise the product temperature and to replace heat lost through walls and ceiling. If the

storage is not well constructed or managed, this

value of infiltration may be more realistic than 10

percent. These rates of infiltration provide sufficient air exchange to prevent accumulation of

carbon dioxide or depletion of oxygen under most

circumstances (p. 13).

Heat Distribution and Source

After certain assumptions have been established

for the construction of the storage house regarding

its size and general management and expected

weather conditions, the heat requirements may be

estimated. In general, selecting a heating system

with sufficient capacity for the job is no problem;

but often one or two factors are overlooked in the

design of the storage that affect proper distribution of the heat.

When possible, the heat should be placed under

the sweetpotatoes; even then heat will rise either

through or around the stacks. After the proper

temperature is reached for curing or presprouting,

some circulation of air is often desirable to prevent heat from accumulating at the top of the

room, especially if the sweetpotatoes are stacked 10

feet or more high. Second, it is not uncommon for

some of the sweetpotatoes near the heating element

to become overheated, resulting in the temperature

of a few boxes of roots to rise over 95° F. As a

result, these roots do not heal properly, or even

worse, the temperature near them may soar over

110° for several hours, causing the roots to

decompose.

Heat is easily distributed with electric-strip

heaters placed under a false floor in a pattern that

provides slightly more heat near outside walls

than near the center of the room {SO, 32,121), The

cost of this equipment is not high and thermostatic

control reduces labor cost. Baffles are placed over

the heaters to prevent overheating potatoes directly over them and to reduce fire hazard. A 1,000-

watt (1 kw.) heater will produce about 3,400 B.t.u.

per hour.

Where earth or false-floor construction is used,

the storage containers should be at least one foot

above the ground to permit free circulation of air.

Sills or other wooden parts near the ground should

be treated with a preservative to prevent them

from decaying. A slatted floor made of l-by-é's
SWEETPOTATO STORAGE 21

spaced %-inch apart is necessary to permit air circulation. A false floor should be made in movable

sections to permit cleaning and repairs underneath

and with joists capable of supporting the stack of

sweetpotatoes.

In areas where power rates are relatively high,

heating with electricity may not be economical. In

addition, because of seasonal use of electricity and

the necessity of some companies to provide heavy

duty service at times, power companies make

demand charges. These charges may be reduced by

use of a seasonal rider offered by some companies

or by operating selected circuits rather than all

circuits at one time. Records for small, well-built

houses in the Southeastern States show that sweetpotatoes can be cured and stored for several months

with an electric-energy consumption of not over

4 kilowatt-hours per 55-pound box. All electric

installations should be in accord with the rules of

the National Board of Fire Underwriters.

Gas- and oil-fired furnaces or burners, vented to

the outside and with proper safety controls and

fans are widely used for heating sweetpotato

houses. In small storage houses the heat may be

released under floor racks or false floors. Ducts

placed in or, if necessary, on concrete floors are

used to release air through registers at appropriate

locations about the room. The number and size of

ducts should be determined by the heating contractor to supply not over 5,000 B.t.u. per hour to each

register (fig. 10,^).

Too often, though, to keep costs down, too few

registers are provided and the roots near the registers have been damaged. Once registers are

installed, it is difficult to change their position.

When sufficient registers are properly distributed

floor ducts work well. Circumstances and register

requirements vary, but needs can be estimated as

follows :

Allow apfjroximately 1.5 square feet of register

and 450 cubic feet per minute (c.f.m.) air movement for each 1,000 55-pound boxes in storage. For

each 250 to 300 boxes, Ys to % square foot of register is desirable. With proper distribution of

registers, heated air leaving the furnace may be

120° to 130° F. and 95° to 100° at the registers to

deliver about 5,000 B.t.u. per hour per register.

When the heated air is allowed to spread under a

false floor or palletized boxes, roots near the register seldom become overheated.

Registers in the floor or ducts on the floor create

problems with forklift operations. Even if the

registers are only 4 inches across, they become

uneven and cause the forklift to shake noticeably

when going over them. A system of trench distribution of heat was developed for use with forklift

handling. Trenches should be made to support no

less than 2% times the weight of the rated capacity

of the forklift, since the weight of the fork and the

fork load will be borne primarily by the front

wheels of the fork. A span of 12 inches, with loadbearing trench shoulders of 4 inches on each side

of the span, works well with forks rated at 1 to 2

tons when 2- by 6-, 2- by 8-, or 2- by 10-inch hardwood free of knots and with 8 percent or less slope

in the grain is used. Depth of the trench is optional

as long as it will handle the air from the furnace

(fig. 10, 5).

A trench level with the concrete floor is highly

desirable and is usually attained if finished lumber

is used for forms and for covering the finished

trench. A trench 12 inches wide and 12 inches deep

can handle large quantities of heated air, or electric-strip heaters.

If electric-strip heaters are used, the boards over

the trench, which are usually fastened together in

sections with 2- by 4-inch stringers, may be spaced

about 1 inch apart on all trenches. If warm air is

forced through the trenches, the boards over the

trenches should be placed alongside one another

over approximately one-fourth of the total trench

area next to the source of heat. The boards should

be spaced approximately i/4 inch apart over the

next section of trench, then 1/2 inch, and finally

1 inch apart at the farthest distance from the

source of heat. Because the heated air is warmest

mear the furnace and because the boards do not

fit perfectly (usually after some shrinkage), sufficient heat will reach the roots in the area where the

boards have been placed alongside one another.

Adjustments, however, are easily made if needed.

After the proper temperature of the storage

house has been reached, during curing and

presprouting, the furnace fan should be operated intermittently or continuously to prevent

stratification.

Trenches are spaced every 8 to 10 feet apart

(figs. 11 and 12). Where pallets are used in the

storage, a trench should be provided for each two

rows of pallets—that is, for 44- by 44-inch pallets,

trenches should be 8 feet apart (on centers) to

allow room for two pallets and a few inches of

clearance in stacking and unstacking as well as

sufficient air movement. This will also avoid stacking crates against walls.

A thermostat should be used to control the heating equipment. The thermostat, or its sensing element, should be mounted near the top of the

stacked sweetpotatoes but not less than 1 foot

below the ceiling (fig. 10, O), The controls should

be accessible from the doorway into the room.

Sensing el^^^^^onts on long capillary tul)es may be

placed wlier rhe thermosutts are shown and the

controls placed in a more accessible position. However, the sensing elements should not be placed

next to a wall or where infiltration will affect

them.

When heat is introduced and properly distributed at the bottom of the room under the stored

sweetpotatoes, it will rise slowly and warm the
4"

4"^

CONCRETE SLAB

VAPOR BARRIER

GRAVELREGISTER

W^W0ißi//^MmWM y-^^

Is3

l"x24" PLASTIC FOAM

INSULATION

-CRATE

44"x44" PALLET

4"

12"

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EXHAUST

LOUVER

CORRIDOR

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GRADING

AREA

^ TIMECLOCK

'-(¡n series with

cooling thermostat)

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d

FIGURE 10.—Suggested construction details for heating system : A, Location of ducts in concrete floor ; B, trench detail ; 0, location of thermostat.
SWEETPOTATO STORAGE 23

-COULD BUILD PACKING SHED ACROSS END-

-ÎFURN

DAMPERFANS AT PLATE LEVEL.X

^SOLID COVER

AT CORNERS HEATING TRENCHESn

STORAGE ROOMS

16'x28'xl2'.APPROX. 2,000 BU. PER ROOM 8'xiO' SLIDING DOORS

COVER-2X8 BOARDS W/'A" CRACKS

INCREASE TO ^1^' AT FAR END- MAK

6' TO 8' SECTIONS TO PERMIT REMOVAL

CLEANING. (SEE NOTES ON ELECTRIC HEAT)

AT FURNACE ENDCE COVER IN

OVAL FOR yi

]

PACKING SPACE OR [!

- HALLWAY (MAKE l]

A'S WIDE AS NEEDED) I]

I!

II

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FLOOR PLANADDITIONAL ROOMS»

II

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II

II

II

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PROVIDE SCREENED VENT STRIP 4" WIDE

AT EACH GABLE END TO VENT SPACE

ABOVE INSULATIONy

>NOTE: IF DOUBLE WIDTH BUILDING

y^W/CENTER PACKING SPACE IS BUILT,

i r~ CARRY EXHAUST THRU ROOF. ^^'

2,000 CFM INTAKE

FAN W/SHUTTERS

SIDING—

(3/8" EXT. PLYWOOD,

METAL SIDING,

WOOD SIDING, OR

COMPOSITION SIDING

OVER WOOD SHEATING)

^-

BLOCK

6" CONC. FTG.

CROSS SECTIONFIGURE 11.—Schematic floor plan for a storage composed of several rooms. Room size may be changed to meet needs,

hut four or more rooms are recommended for this type of storage. (Courtesy R. Ritchie, Dept. Biol. and Agr. Engin.,

N.C. State Univ.)

entire stack. Thermostatic control of the temperature near the top of the room provides for reasonably uniform heating, prevents overheating at the

top, and permits coordinating the heating system

with a ventilating system.

The heated air may be blown into the top of the

storage room from heaters mounted or ducted near

ceiling level, but this is not recommended. Roots

in the top of such rooms are often 10° to 20°

warmer than those near the floor, and excessive

sprouting of roots near the ceiling is common.

Removal of Heat

After curing, the temperature should be lowered

to 55° or 60° F. as soon as possible. A period of 7 to

10 days usually is allowed for this but it should be

done in 2 or 3 days if possible. Cooling is usually
24 AGRICULTURE HANDBOOK 358, U.S. DEPARTMENT OF AGRICULTURE

accomplished with ventilation, but it may be done

with mechanical refrigeration. Ventilation is the

more popular method, since outside air temperatures at night are usually low enough during the

last part of the harvest period to permit efficient

cooling.

Ventilation

Convection and overhead ventilating systems are

used. In convection-type ventilation, cool air enters the storage through doors or specially constructed vents located on outside walls near the

f

PIT OR

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PACKING SHED

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FLOOR PLAN-

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SCALE

t I I I 1

0 10 20 30 40 FT.

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CROSS SECTIONSCALE

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0 5 10 15 20 FT.

FIGURE 12.—Schematic floor plan for a storage composed of a curing room (or rooms) and a separate storage area.

This system is recommended only for storages which are equipped to handle palletized field crates (or bulk bins).

The sweetpotatoes are moved carefully with a forklift to the storage room after they are cured. (Courtesy R.

Ritchie, Dept. Biol. and Agr. Engin., N.C. State Univ.)
SWEETPOTATO STORAGE 25

ground (fig. 13, A) and warm air is exhausted at

the top of the building (fig. 13,5).

The removal of warm air from the top of the

storage may be increased with an electric fan or

blower, controlled by a refrigeration-type (cooling) thermostat with contact closure on temperature rise. This system brings cool air into the bottom of the storage where the sweetpotatoes are

usually a few degrees cooler than those in the top

of the room. It is not uncommon for the sweetpotatoes at the bottom to become too cool while

those at the top are still too warm. During the

winter, chilling often occurs around intake vents

near the bottom of the storage, even when these

vents are tightly closed.

In general, for convection-tj^pe ventilation the

area of intake and outtake ventilators should each

equal approximately 1 percent of the ñoor area.

More than this may be needed in the southernmost

areas and less in the northernmost areas of production. Some insulation is usually needed to prevent

condensation on the dampers and vent covers during cold weather. Screens should be used on intake

vents or doorways to prevent rodents from

entering.

The overhead ventilation system uses a blower

to move cool air into the top of the storage (figs.

10, C and 13, 0) so as to form a layer of cool air

across the top of the room {73). In figure 13, G^

details are given for hood and baffle for an intake

blower mounted in the roof of a storage room.

Areas of air movement {A and B) should equal

or exceed IV^ times the area of the fan orifice, and

the baffle-board dimensions (¿7) should equal or

exceed li/^ times the fan diameter. The shutter is

spring-loaded and opens with the pressure of the

fan. In some instances, two sides of the opening

(B) may be enclosed and the other two closed with

a lightweight plastic damper to reduce condensation on the fan. The hardware cloth shown prevents entry of birds.

The air passing over the sweetpotatoes is allowed

to pass through pressure-operated louvers to the

outside at the top of outside walls (fig. 13, i>). In

mild climates the door shown in figure 13, D can be

eliminated and the louver mounted on the exterior

of the wall. The door, however, if one is used, must

be propped or fastened open when the blower system is operating.

With this system the cool air is placed in the

top, where cooling is most needed, and in the

process will carry moisture condensed in the top

of the room out of the building. Since warm air

rises and cool air falls, cooling is fairly uniform.

LINTEL

2"x6

FRAME

ROOFING

DOOR-2 LAYERS

T& G LUMBER

WITH BUILDING

PAPER BETWEEN

LAYERS TURN ROOFING UP

ALONG EDGE "

DROP STICK OF OPENING

SHEATHING

DOOR BUILT

IN 10' SECTIONS

B

2"x2"

BLOCK

EYE BOLT/

/CONTROL ROPE

¿-RUNS INTO

SHED ROOM

MESH HARDWARE CLOTH

-BUILT-UP ROOF WITH INSULATION

METAL HOOD-^

FLASHING

2"xl0" FRAME

¡ZZZZTZZZZZIk

STEEL-WEB JOIST

5/8" SQUARE

PLYWOOD BAFFLE

2"xl2" PLATE

12" LINTEL BLOCK

2"xl2" FRAME

EXHAUST LOUVER

MESH HARDWARE

CLOTH

12" MASONARY

BLOCK

SPRING DOOR

HINGE

Vi" PLYWOOD

DOOR

l"x2" STRIP

ROPE

mounted in roof of storage room; D, exhaust louver for overhead ventilation.
26 AGRICULTURE HANDBOOK 358, U.S. DEPARTMENT OF AGRICULTURE

Early in the morning it is not uncommon to find

the top of the storage slightly cooler than the bottom. This system works well when outside air

temperatures at night average below 55° F.

Overhead ventilation can be made automatic

with a timeclock to select the hours of blower

operation (usually 8 p.m. to 6 a.m.) and a refrigeration-type thermostat (or sensing element)

located near the top of the room. At any time that

the temperature in the top of the room is reduced

to the desired temperature, about 60° F., the

thermostat turns the blower off and chilling is

prevented regardless of outside air temperature.

Furthermore, if any part of the top layer of sweetpotatoes is cooled below the desired temperature,

it is soon warmed by warmer air rising from below.

Since no openings are provided at or near the

ground, there is little opportunity for chilling to

occur and rodents to enter as when air is introduced into the bottom of the storage room. A differential thermostat may be used instead of a

timeclock to provide power to the blower anytime

the cooling thermostat is not satisfied and the air

outside the storage is cooler than that inside {73).

Since air is blown into the storage, any air leakage

is usually from instead of into the storage.

The blowers should have a capacity of 1 to 2

c.f.m. per 55-pound box of sweetpotatoes at a static

pressure of at least y^o inch. At least I1/2 feet or

more clearance should be allowed over the stacked

sweetpotatoes to allow for movement of ventilating air. Intake hoods, baffles, and exhaust louvers

should provide sufficient cross-sectional area to

prevent restriction of the air movement created by

the fan. Usually at least li/^ times the area of the

fan orifice will suffice for both the intake and the

exhaust.

Mechanical ventilation and automatic controls

require much less attention than hand operation

of ventilation equipment.

Refrigeration

Nearly all operators of large storage houses

have some refrigerated storage. Eefrigerated storage permits storing roots into warm weather when

ventilation is inadequate to maintain low-enough

temperatures {93), Often the large operator has

customers he supplies regularly, customers who

want sweetpotatoes throughout the year if they

can be obtained.

Cooling sweetpotatoes after curing or during

storage with refrigeration involves essentially the

same problems and amount of heat as for warming

the roots, except that heat of respiration must be

removed too. The amount of refrigeration

required can be calculated in the same way, except

that heat will be moving into instead of out of the

storage. In some installations, ventilation has been

supplemented with refrigeration, while in others.

refrigeration provides all the cooling needed and

vents are used only to introduce fresh air.

One approximation of the amount of refrigeration needed for a storage house insulated to produce a thermal resistance of about 10 in the walls

and 15 in the ceiling is 4.6 B.t.u. per hour per cubic

foot of storage space {108), This storage room

was designed to hold 8,000 to 10,000 boxes of

sweetpotatoes and to maintain 60° F. when temperature outside reached 95° or to cool the sweetpotatoes from 85° to 60° within 10 days.

Another means of estimating refrigeration

requirements indicates that approximately 1 ton

of refrigeration (theoretically 288,000 B.t.u. per

24 hours) is needed for each 1,000 55-pound boxes

stored in rooms insulated sufficiently to maintain

the recommended curing temperature with an outside air temperature of 30° F. or lower.

The refrigerated air should be distributed across

the top of the storage room and air returning to

the unit picked up in the lower part of the room.

The refrigeration thermostat may be located in

the return air close to the return-air duct if air

circulation is continuous, or near the top of the

room and away from refrigerated air discharged

into the room.

A refrigeration engineer should be consulted

since the rated capacity of refrigeration units is

often 20 percent less under storage-house conditions than under the customary conditions of testing for home or office use, and thermal resistance

of walls and ceilings are slightly different than

when heat is lost from the room. Often a blower

larger than usually supplied for home use is

needed, and the cooling coil should maintain a

wet-bulb temperature of 56° F. and a dry-bulb

temperature of 60°.

Humidification

At times the moisture given off by the sweetpotatoes, crates, or soil is insufficient to maintain

the relative humidity at 85 to 90 percent. This is

especially true when large amounts of heat are

added to w^arm cold roots late in the harvest, and

during presprouting when the roots and storage

containers have come to equilibrium at storage

conditions of 60° F. and 85 to 90 percent relative

humidity.

Dirt floors, under false floors or racks, can provide moisture. Wetting the soil, if necessary, before the storage season usually provides adequate

supplementary moisture. On some occasions water

has been run onto concrete floors when the storage

containers were up on racks. After the storage

house is filled, it is difficult to determine whether

the floor is wet and distribution of moisture is

often not even. With the development of larger

operations, more efficient systems should be considered {45),
SWEETPOTATO STORAGE 27

A mechanical humidifier, which consists of an

electrically driven, high-speed fan upon which

water is dripped, will produce a fine mist that can

be picked up by circulating air and distributed

throughout the house. Such a unit is usually

mounted where heavy particles of moisture falling

from it will not produce undesirable wet areas.

A good humidistat may be used to control one or

more humidifiers and may be mounted in the return air stream of refrigeration equipment or in

an accesible location near the furnace return-air

pickup. The humidifier should be mounted at the

opposite end of the room from a mechanical

refrigeration or heating unit.

A mechanical humidifier should have a capacity

of 2 gallons or more per hour for a room containing 5,000 boxes of sweetpotatoes. For presprouting, it should have a capacity of 1 gallon per hour

per 1,000 boxes. Unfortunately, the rated capacity

of mechanical humidifiers is indicative of the

amount of water vaporized or atomized under

favorable conditions, but this is only a general

indication of its performance under the customary

conditions encountered in a storage house.

Furthermore, the storage-house manager should be

able to check the room's relative humidity and the

humidistat for accuracy since the humidistat can

easily get out of adjustment. A sling _psychrometer

is useful for measuring relative humidity (fig. 14)

and is more accurate than a stationary wet- and

dry-bulb hygrometer because of low rates of air

movement in the storage.

Although humidity-indicating devices are easy

to use, they are delicate instruments and easily get

out of adjustment after a while.

Another method of maintaining relative humidity is discussed here partly because of its simplicity and partly because it puts the moisture

in a desirable place. When hot air in trenches is

used for heating, water may be introduced into the

trench so that the heated air, which is low in relative humidity, may pick up moisture (and lose

some of its heat) before it reaches the sweetpotatoes. A drain in the trenches to the outside

of the building prevents excessive addition of

water and allows for ñushing and cleaning the

room and trench after storage. The drain should

be capped or sealed when not in use. With a little

experience, the amount of water to be added can

be judged fairly well. Although this system is not

automatic, it permits a rapid pickup of moisture

when heat is being added to the room. This is also

the most desirable time to add moisture. It is better for the warm air to pick up moisture from the

trench than from the sweetpotatoes.

An atomizing nozzle placed in the furnace

plenum and operated with a solenoid valve

actuated by the furnace burner circuit can also be

used as a humidifier. Clogging of the nozzle and

failure of the solenoid to operate because of variable water pressures however, have produced problems with this system. In addition, use of such a

system may shorten the life of the furnace.

The approximate moisture (and heat) requirement for vaporizing water may be calculated as

follows :

Vaporizing 1 pound of water will provide all the

moisture in approximately 500 cubic feet of air at

85° F. and 100 percent relative humidity or in

about 1,250 cubic feet of air at 60° and 100 percent

FIGURE 14.—Humidity measuring devices: Riglit, sUng psyehrometer (tliat is, portable wet and dry bulb) ; center, wallmounted wet and dry bulb with circular conversion scale below ; left, indicator actuated by hair element.
28 AGRICULTURE HANDBOOK 358, U.S. DEPARTMENT OF AGRICULTURE

relative humidity (fig. 15). Vaporizing 1 pound

of water will consume approximately 1,000 B.t.u.

of heat.

Insulation and a vapor barrier are needed to

permit the maintenance of the high relative humidity recommended (p. 13).

Dehumidification

It is impossible to avoid all conditions that produce relative humidity in excess of that recommended or to prevent condensation at all times. A

relative humidity as high as 95 percent is satisfactory for the sweetpotatoes. But at relative humidities of 95 to 100 percent, it is very difficult

to prevent condensation. Often the relative humidity risesto 95 percent and above in the storage when

the outside air temperature is approximately the

same as that inside. Under such conditions a decrease in outside air temperature will produce condensation on any inside surface that is cool enough

to be slightly below the storage-room temperature.

Whenever the outside air temperature is considerably below inside air temperature, the possibility

of condensation exists according to the insulating

properties of the walls and ceilings as indicated in

figure 8.

Usually excess moisture is removed by ventilation. Air containing less moisture than the air in

the storage is introduced to replace some of the air

3 -

D

w

§2

o

<

>

cc 1

<

0

._ /

—

AT SATURATION //

(100% humidity) //

—

^^^^90% relative^

^^^r humidity ^r —

1 1

^^^^^^ 50% relative humidity

1 1 1 1 1

-

30 40 50 60 70 80 90 100 110

FIGURE 15.—Water vapor content of air at 50, 90, and 100 percent relative humidity.
SWEETPOTATO STORAGE 29

containing a high level of moisture. Air that is at

the same temperature but with a lower relative

humidity or cooler air, which cannot hold as much

vapor, will reduce the moisture level.

For example, 1,000 cubic feet of air at 85° F.

will hold about 2 pounds of water at saturation, but

at 63° will hold only about 1 pound at saturation

(fig. 15). If 500 cubic feet of saturated 85° air

can be removed and 500 cubic feet of saturated

air at 63° can be added, the amount of moisture in

the air would be reduced from 2 to 1.5 pounds;

and if the temperature was maintained by addition

of heat, the relative humidity would be reduced

from 100 to 75 percent. At 75 percent relative

humidity, condensed moisture could be picked up

by evaporation into the drier air, if the insulating

properties of the wall permit the interior surface

temperature to remain above the dewpoint under

these conditions.

In effect, then, the moisture content of the air

can be lowered by introducing either dry air at

the same temperature, or by introducing cold air,

and warming it so that, in effect, it becomes dry

air at the same temperature. On some occasions a

relatively large amount of air is needed to remove

moisture and the ventilating system designed for

cooling the room may be used. This is additional

justification for an efficient ventilating system.

Floor Plans or Blueprints

The floor plans and construction details given

here are designed to aid in the development or

adaptation of floor plans or blueprints suitable

to individual needs, especially in the construction

or design of a storage for palletized handling.

Two plans are discussed. The first deals with the

procedure for curing and storing roots in a storage composed of several rooms without moving the

roots until they are to be sent to market (fig. 11).

This has been the recommended manner of curing

and storing for many years because it was not possible to move the roots after curing without causing

injuries that resulted in considerable decay. The

second plan deals with a procedure for curing

roots in palletized field boxes (or bulk bins) in one

room and moving them after curing to a second

room for storage (fig. 12). The last sweetpotatoes

cured would not be moved from the curing room

unless there was sufficient space for them in the

storage room and the storage-house manager

wished to move them.

Some features are common to both systems.

Posts and other obstructions in the rooms should

be as few as practical to permit easy and accidentfree operation of the forklift. The turning radius

(including the pallet) of most 1%- and 2-toncapacity forks is nearly 12 feet. Therefore, posts

should be at least 12, and preferably 16 or more,

feet apart. In planning, 4-foot units or multiples

of 4 feet are used since a 44- by 44-inch pallet

works well in a storage designed to allow 48 by 48

inches per pallet. The extra space is needed for

posts, air passages, allowances for irregular stacking, and leaning of stacks. If another size of pallet

is used, similar provision for these features should

be made.

Because the turning radius of most forks is 12

feet or less, aisles and doorways are usually 12 feet

wide. Doorways should also be high enough to

exceed the height of two loaded pallets (that is,

about 11 feet for two pallets of Duraboxes stacked

five layers high on each pallet). The 12-foot width

of the doorway permits stacking all but three rows

at right angles to the doorway. These three rows

may be filled by driving straight into the doorway.

Only space for three pallet loads are lost just inside the door, because a third layer is not possible

at the doorway unless the doorway goes all the

waj^ to the ceiling. In some instances, doubleswinging doors lined with sheet metal have proved

useful since the doors remain closed when not in

use. The doors shown in figure 11 are too small

for best forklift operation.

Because it is practical to stack and unstack two

rows of pallets very close together with a sideshifter, and because it is convenient to stack two

rows of pallets over a trench for heating, a narrow

air channel between each double row of pallets

aids in cooling the stacks with ventilation or refrigeration. To illustrate this with 44- by 44-inch

pallets, place one pallet close to another over a

heating trench so that the pallets touch over the

center of the trench. With trenches on 8-foot centers and with reasonably straight stacks, up to 8

inches will be left between this pair of pallets and

those centered over another trench 8 feet away. In

actual practice the 8-inch space usually becomes

2 to 4 inches. Whatever size of pallets is used,

trenches or ducts should be placed to provide even

distribution of heat and permit stacking to allow

some air channels for ventilation and prevent

stacking against outside walls.

A smooth and level floor, including any trenches

or registers in the floor, is necessary. A slight unevenness, which may result in as little as % inch

deviation from the level between the two front

wheels of the forklift, may result in 2 inches or

more of sideward movement of a pallet 12 feet off

the floor. If the change is abrupt, boxes may shake

off the pallet, especially if they have not been

properly stacked on the pallet. Ceiling height and

clearance over the stacks are determined by container and pallet stacking (p. 6). Usually three

pallets with either four or five layers of boxes are

used, with five layers per pallet worth considering

for field containers less than 12 inches high. The

5-layer-per-pallet load saves pallets, forklift time,

and cost of storage space.

The floor should be capable of supporting about
30 AGRICULTURE HANDBOOK 358, U.S. DEPARl'MENT OF AGRICULTURE

21/^ to 3 timos the rated capacity of the folklift. It

is possible with a 2-ton fork to pick up and move

two pallets, one on the other, at one time. In that

event, the fork, which is handling about 2 tons, has

a counterweight of more than this and the gross

weight on the two front wheels may exceed 4 tons.

A 4-inch concrete floor poured on firm soil is

usually adequate, but this should be confirmed for

each site.

Free, unobstructed stacking height should exceed the height of the stacks by at least 6 inches to

allow the forklift to raise the top pallet high

enough to clear those below and maneuver the

pallet into position before lowering it into place.

At least 18 inches should be allowed for ventilation over the stacks and to prevent holding the

topmost sweetpotatoes too near the ceiling where

heat and moisture often accumulate.

Storage With Several Rooms for

Curing and Storing

The basic concept in working with a storage

composed of several rooms is that enough sweetpotatoes will be harvested in 5 days or less to fill

a room. This will permit the room to be kept at 85°

F. and 85 to 90 percent relative humidity throughout the period of filling and for an additional 4 or

5 days without excessively curing the first roots

placed in the room or inadequately curing the last

ones. Each room must be equipped to provide heat

for curing and to prevent chilling during winter,

ventilation for cooling, aeration, and removal of

condensation, and moisture for humidifying dry

air. This type of operation requires a sizable investment in equipment for each room if the facilities are to be automatic and thereby ensure good

control of curing and storage conditions without

considerable time and effort on the part of the

storage-house manager. Furthermore, if the investment in such equipment is to be justified and

the rooms are to be large enough for efficient use of

a forklift, the rooms should be able to hold 2,000

55-pound boxes or more, preferably considerably

more.

A final qualification concerning this type of

operation is indicative of the number of rooms

needed to make the system function properly. If a

room is to be filled in 5 days or less, there must be

enough rooms to provide at least one room for

each week of the harvest season, even if the size of

the rooms differs in proportion to the quantity to

be harvested each week. Usually no less than four

roomsshould be used, unless the farming enterprise

or enterprises are too small to warrant use of a

forklift or the construction of a storage holding

8,000 to 10,000 55-pound boxes of sweetpdtatoes.

The schematic floor plan illustrated in figure 11

shows operation of two rooms off one furnace and

the storage made up of two or more such two-room

units.

Although the system of using several separate

rooms within the storage house provides a reasonable degree of success in curing and storing sweetpotatoes, it has some disadvantages. The primary

disadvantage is the inability to fill a room adequately in 5 days or less on many occasions. The

important factors contributing to this are rainfall,

which prevents harvesting; breakdown of equipment; inadequate labor supply; and, in cooperatively operated storages, the desire of each producer to put sw^eetpotatoes in a separate room.

The most successful storage-house managers are

able to overcome most of these problems exceptthat

of rainfall. Lack of success usually means that the

curing temperature is reduced to permit slow curing to accompany slow filling of rooms and, eventually, even the elimination of the separate rooms

which are not needed with slow curing, although

the results are far from those desired.

Storage With Separate Rooms for Curing

With palletization, sweetpotatoes can be cured in

palletized field boxes in a room designed to provide recommended conditions for curing, and, after

curing, the sweetpotatoes can be carefully moved

with a forklift to a room in which storage conditions are maintained continuously. This system

provides the recommended curing and storage conditions, but also has flexibility enough to cope with

most of the problems that cause irregularities in

harvesting. Furthermore, less investment in construction and equipment is necessary than in the

system using several storage rooms. More than one

curing room or storage room may be used, but

available information indicates that curing space

should make up one-fourth to one-sixth of the

total storage capacity.

This estimate is based on the observation that

the turnover through the curing room allows the

curing room to handle four to six times its capacity

during the harvest period. If the harvest period

lasts about 6 weeks and the roots are handled so

that they remain in the room about 2 weeks, it

would be possible to fill the curing room about four

times. If the roots are placed in the curing room

and removed on a 7-day schedule, it would permit

filling the curing room the equivalent of six times

in 6 weeks. The 7-day schedule would fit recommendations regarding the length of the curing

period and allow some flexbility by decreasing the

time to a minimum of 4 days if necessary.

With this system the palletized boxes may be

stacked in rows, two pallets wide over a trench,

in almost any order they are received regardless

of variety, grower, or quality as long as some identifying mark or tag is affixed to each pallet in a

prominent place. If more than one curing room is

available, seed roots may be cured in one room and

marketable roots in another.
SWEETPOTATO STORAtJE 31

Because of the need to move the palletized units

into and out of the curing room several times during a season, slightly more clearance between pairs

of rows may be desirable ; that is, instead of 8 feet

on center for 44- by 44-inch pallets (as shown in

fig. 12), heating trenches could be placed on 81/^-

foot centers or some other comparable arrangement

for the size of palletto be used.

Often three pallets from one grower (or truck)

are placed on top of one another to prevent contamination, however slight, from soil sifting down

from the pallets on top. A convenient truckload is

six pallets. When the palletized boxes are moved to

the storage area, the topmost pallets in the curing

room should be placed on the ñoor in the storage

area in an effort to keep sprouting at a minimum

on all roots. At this time the palletized containers

may be sorted into rows according to desire : that

is, by grower, variety, quality, intended use (seed,

market, processing), or other subdivisions.

The curing room (or rooms) should be equipped

to heat, humidify, and ventilate the sweetpotatoes

just as recommended for storages with several curing rooms. In the plan shown (fig. 12), furnaces

also provide heat in the storage area near outside

walls. With proper damper arrangement, a furnace

in the grading area may be used to heat this area

instead of the storage area. With proper management, the curing facilities may be used for presprouting seed roots in February or March.

The storage area should be equipped primarily

for ventilating to cool the roots, although in some

areas of production, especially in small storages,

some provision for heating to prevent chilling will

be needed. The addition of heat to such storage

areas need not be as elaborate as in the curing

rooms. The storage area should be easily reached

from the curing area because forklift time is particularly valuable during the harvest period. Preliminary data indicate one forklift can handle up

to 50,000 palletized field boxes in this system with

a peak load during the harvest period of about 60

hours of work per week, including some time servicing a grading line.

The forklift is the key unit in this system. A

maintenance program for all equipment should be

provided, but especially for the forklift. Facilities

should be available for prompt repairs or emergency rental of other forklift equipment.

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1952. EFFECT OF CURING AND STORAGE TEMPERATURES ON DEVELOPMENT OF INTERNAL CORK

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1965. OVERHEAD VENTILATION OF SWEET POTATO

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1956. SWEET POTATO DEHYDRATION : TIME AND

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1959. EFFECT OF CHILLING ON THE CHLOROGENIC

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34 AGRICULTURE HANDBOOK 358, U.S. DEPARTMENT OF AGRICULTURE

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1958. A STUDY OF THE SYNTHESIS OF CAROTENE

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U.S. GOVERNMENT PRINTING OFFICE: 1969 O—321-902