Historic, archived document
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Circular No. 13
October 1944 e Washington, D. C.
UNITED STATES DEPARTMENT OF AGRICULTURE ‘=
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Freezing Injury of Fruits and Vegetables,
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By D. H. Ross, senior physiologist, R. C. WRIGHT, physiologist, and eé “OQ,
BRATLEY, pathologist, Division of Fruit and Vegetable Crops and Diseases, Re
Bureau of Plant Industry, Soils, and Agricultural Engineering, Agricul-
tural Research Administration
CONTENTS
Page Page
Severity of injury dependent on tem- Length of time required for freezing
perature and its duration................... 2 to occur—Continued.
Methods of protecting fresh fruits and IGEMON'S es ee en eae 15
vegetables against freezing.................. 2 Wettuces fee. rice peace oP Pee) keeled ek. Ry. 16
Length of time required for freezing Onions and garlic 2.0.0.0... cee 16
TOMPOCCUT Ree Re I PL A he 4 Oranges and grapefruit .........00....... 17
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Carrots)... Mew Okie aekiie Beri mwect notators Sota ee eee aa
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WS ey ee in See ae ier oe 14] Rates of cooling of fruits and vege-
(CEH EEI. (OOERDE ocx ecccticentorecce 805: oce ear OeSCEaa ie tables Atti SA ee eid te ee
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TEE EH ON AUE Wie chocspcoeae BERS E RCEECER epee ere 15 DVOGUCCR er een een Series eet 28
N THE HANDLING, STORAGE, AND TRANSPORTATION
of fresh fruits and vegetables, it frequently becomes neces-
sary to protect them against freezing or to care for them and
determine whether they are usable after they have frozen. How to
deal with both of these problems is of importance to a great many
people, including officials of the Army and Navy responsible for
perishable produce destined for the armed forces, market inspec-
tors, employees of railroad and truck companies, market receivers
and distributors, and cold-storage operators. It seems desirable,
therefore, to summarize the information now available on the sub-
ject, in order to keep preventable loss and damage to a minimum
and so help to conserve food materials that are so essential to the
war effort. This circular presents the results of observations on
the market and of experimental investigations at controlled
temperatures 7° to 18° F. below the freezing point of water.
1
2 CIRCULAR 7138, U. S. DEPARTMENT OF AGRICULTURE
SEVERELY OF INJURY DEPENDENT ON TEMPERATURE
AND ITS DURATION
Problems that arise in connection with the protection of fruits
and vegetables against freezing and with the care of these com-
modities when they have frozen can be solved more intelligently if
a few fundamental principles are understood and used as guides in
handling produce.
Fruits and vegetables do not freeze immediately when exposed
to a freezing temperature, and many of them are not immediately
injured even if they do freeze. Much depends on what fruit or
vegetable is concerned, on how low the temperature goes, and on
how long the product is exposed to the dangerous condition. Pota-
toes and tomatoes are injured if any ice forms in their tissues even
though the exposure is very brief and the temperature barely
reaches their freezing points. In this respect they resemble sweet-
potatoes and other products listed on page 30 in the group most
susceptible to cold or freezing injury.
On the other hand, apples, cabbage, carrots, and other produce
in the groups moderately susceptible or least susceptible to freez-
ing injury (p. 30) are injured very little by exposure to tempera-
tures only a few degrees below their freezing points, whereas at
temperatures below about 20° F. they are in much greater danger.
However, if these two different sets of conditions are assumed, the
time element becomes of the utmost importance. Apples exposed
to 25°, say for only a few hours, will be damaged very little if at
all, whereas if they remain at that temperature for 3 or 4 days
they may be so softened and otherwise injured that their keeping
quality is seriously impaired. At 15° to 18°, however, they may be
damaged as much in 12 to 15 hours as they would be at 25° to 27°
in 3 or 4 days. If held at the lower temperature (15° to 18°) for
several days, they become “frozen to death” and worthless. Simi-
lar statements are true for numerous other products in the mod-
erately and least susceptible groups.
The important fact in the preceding discussion is that for any
given product the combination of time and temperature determines
the severity of injury. Enough time must elapse for the forma-
tion of ice in the tissues; and, for reasons that are not well under-
stood, enough more time must elapse so that the tissues suffer in-
jury from which they cannot recover. The lower the temperature
the shorter the dangerous, critical period. The length of this
period varies with different products, but its significance and
importance should always be kept in mind.
METHODS OF PROTECTING FRESH FRUITS AND
VEGETABLES AGAINST FREEZING
Methods of protecting fresh fruits and vegetables while in cold
storage or on the way to market are fairly well known, although
they are not always used intelligently or with full SS of
the risks involved if they are not used.
Sometimes these perishable products are held in storage = too
low temperatures or in rooms where the temperature occasionally
FREEZING INJURY OF FRUITS AND VEGETABLES 3
falls below their freezing points; sometimes they are loaded into
ears or truck without adequate protection from low outside tem-
peratures during the loading process; in still other instances they
are transported in cars where heater service is not furnished at the
proper time or in the necessary amount; or they are hauled to
market in trucks with little or no protection except that furnished
by the containers. In all such instances the use of foresight and
care will do much to prevent damage from freezing. In cold
weather the transportation of fruits and vegetables by truck can
be made safer by placing straw, hay, or paper around the inside of
the truck body and tarpaulins over the top of the load. Under ex-
treme conditions it may be advisable not to attempt long-distance
transportation of these commodities by truck.
If a carload of produce showing signs of freezing arrives on the
market there are several possible ways in which it may be handled,
although the receiver may not always be free to choose the one he
will use. In many instances he can do nothing but unload the car
and put the produce into trade channels. On the other hand, he
may be able to leave it in the car to thaw out either because of mod-
erate outside temperatures in the local area or by the use of heaters
placed in the car. As a third choice, he may unload the produce
and take it to a pier, warehouse, or store and leave it to thaw
slowly.
A temperature of about 40° F. has been found most satisfactory
for the thawing of apples, onions, and potatoes, and probably it
would be desirable for other fruits and vegetables. Too high tem-
perature—60° to 70°, for example—will favor decay and may
bring about yellowing or withering of vegetables and too rapid
ripening of fruits if the produce is left at such temperatures for
several days. If packages have to be handled in order to put them
in a place where the produce will thaw, they should be stacked in
such a way as to permit free circulation of air around them. In
the experimental work discussed on pages 5 to 25 and partially
summarized in table 1, thawing temperatures of 45° and 60°
were frequently used in order to determine within a relatively
short time how much and what kind of deterioration occurs in
products that have frozen. There is no intent to imply that the
thawing temperatures shown in column 9 of table 1 are the
optimum for the different commodities.
Whatever method is used in caring for a shipment, it should
always be remembered that produce should not be handled while
frozen if such handling can possibly be avoided; the reason for this
is that when fruits or vegetables are in a frozen condition the ef-
fect of even slight bruising extends deep into the flesh and much
more damage results than from similar bruising of unfrozen prod-
ucts. Leafy vegetables when frozen are easily broken and mashed
by being handled.
If fruits or vegetables have to be hauled to a pier, warehouse, or
store in freezing weather, they should be protected by means of
paper, hay, or straw around the inside of the truck body and tar-
paulins over the top. Individual packages in small lots, if hauled
to stores in unheated trucks in freezing weather, can be protected
by wrapping them in heavy paper.
4 CIRCULAR 713, U. S. DEPARTMENT OF AGRICULTURE
If the shipments arrive at market in good. condition but, after
being unloaded, have to be held on a pier or in a receiver’s ware-
house or storeroom during severely cold weather, the danger that
they may freeze depends on (1) the quantity of the fruits or vege-
tables to be stored and their temperature before exposure to the
conditions where they must be held, (2) the amount of artificial
heat provided, (8) the tightness and insulating quality of the pier
or warehouse construction, (4) the temperature of the storage
space during the previous day, and (5) the quantity of other
commodities held in the storage space and their capacity for
retaining heat.
Additional facts that should be kept in mind in attempting to
prevent freezing are (1) produce cools more slowly if packed
tightly than if packed loosely and more slowly in tight boxes, bas-
kets, or barrels than in slatted crates or hampers; (2) certain
produce (pears, apples, and citrus fruits) cools more slowly if
wrapped than if not wrapped; (8) close stacking of packages
gives some protection from freezing; (4) the bottoms of outside
stacks need the most protection because freezing occurs there first;
(5) sawdust along the bottoms of outside doors helps to keep cold
air out; (6) a covering, such as a tarpaulin, over the stacks and
tucked carefully around them, especially at the bottom, helps to
retain both the heat already present in the produce and that which
it produces by virtue of being alive (heat of respiration).
Even if there is no permanent equipment for heating the room
or pier, substantial help in keeping up the air temperature can be
obtained by setting barrels, oil drums, or buckets of hot water
under the tarpaulins where heat is most likely to be needed. Since
water will freeze before any of the fruits or vegetables do, the
heat it gives up on freezing is available for further protection. The
heat that could be furnished by a lighted lantern, an oil heater, or
a container full of hot water set under a tarpaulin covering stacks
of produce might seem small, but it might be just enough to keep
the produce from freezing. The beginning of ice formation on
water in a container can be taken as a sign that the surrounding
temperature is becoming dangerously low. It should be remem-
bered, however, that the heat given up by the water as it freezes
can help to protect the produce until all the water is turned to ice.
Care should be taken, of course, that overheating does not occur.
LENGTH OF TIME REQUIRED FOR FREEZING TO OCCUR
Because of qualifying factors already mentioned (p. 4) it is
not safe to make general statements concerning the time required
for fruits and vegetables to freeze in a place where the air tem-
perature is below 32° F. Even if surrounding conditions were
identical there might still be differences in freezing time, because
of differences in varieties or kinds, in maturity of the commodity,
and in its freshness. Freshness would be important chiefly in
connection with leafy crops such as spinach, lettuce, and kale.
In order to obtain information on freezing under specific condi-
tions, single commercial packages of 4 kinds of fruit and 23 kinds
of vegetables were held at 14° to 25° F. until freezing occurred in
FREEZING INJURY OF FRUITS AND VEGETABLES 5
at least the most exposed parts of the pack. The length of expo-
sure varied from 8 or 4 hours to several days. Details of the tests
are given in table 1. Brief descriptions of the symptoms of freez-
ing injury in the various commodities appear in the text. In many
instances statements about freezing injury in the particular com-
modity as observed on the market are also given. Information as
to the temperature of the produce at different positions in the
package was obtained by means of thermocouples and a potenti-
ometer. The thermocouples were thrust into the produce to a depth
of 1 or 2 inches; hence they did not show the temperature at the
surface. Statements concerning the time of the first occurrence of
freezing of each commodity are usually based on evidence obtained
by visual and manual examinations.
The wraps used for apples, grapefruit, lemons, and oranges
were of the kind commonly used for these fruits in commercial
operations.
In these tests it was possible to determine within rather narrow
limits the time when freezing occurred in a given commodity, espe-
cially in the more exposed parts of the package. It should be
clearly understood, however, that the periods given in table 1,
columns 6 and 7, are to be accepted as only approximate if it is de-
sired to apply them in the handling of other packs and kinds of
packages of the same commodities under various commercial
conditions.
APPLES
EXPERIMENTAL OBSERVATIONS
Apples in two eastern boxes were held in an 18° F. room for 51
hours; those in one box were individually wrapped and those in the
other were not wrapped. The boxes were not papered over the
outside, but both were lidded. (For rate of cooling of these two
lots see figure 1.) When the fruits were examined immediately
after removal from the freezing room, freezing was found to
extend to the center of both boxes.
After a week at 31° F.. it was found that the severest injury was
in the unwrapped lot. The apples in that lot showed numerous
water-soaked bruises, with browning underneath, extending deep
into the flesh, and the flesh was generally dry and mealy. The
bruises occurred where the apples had touched each other or the
box, although they were not under pressure from the lid. Some of
these bruises were similar to those that can be produced by jolting
apples under pressure; others extended so deep and the flesh was
so much browned that the injury could have been caused only by
freezing. A few fruits in this box were brown throughout, mushy,
and clearly frozen to death. The same symptoms, to a much
smaller degree, occurred in the wrapped apples, except that none
were brown throughout, watery, or mushy.
Apples in two unpapered eastern boxes were held in a 21° to 24°
F. room for 54 hours. Those in one box were individually wrapped
and those in the other were not wrapped. On removal from the
freezing room, fruits were found frozen in all parts of both boxes
but not all fruits were frozen. When the fruits were cut, bruising’
CIRCULAR 713, U. S. DEPARTMENT OF AGRICULTURE
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Sa. CIRCULAR 713, U. S. DEPARTMENT OF AGRICULTURE
was more noticeable in those that were frozen than in those that
were not but it was only slight even in the frozen fruits. After a
week at 31° neither the wrapped nor the unwrapped lot showed
any change from the condition found when the fruit was removed
from the freezing room.
MARKET OBSERVATIONS 1
Many persons think that the presence of ice in an apple is prima
facie evidence of freezing injury. Theoretically they are right;
practically they may not be. The confusion is due to a failure to
distinguish between slight freezing and freezing to death in which
enough ice is formed to cause permanent and visible injury.
Doubtless the least incipient ice formation injures the apple flesh
to some degree, but so far as present knowledge goes there is no
visible evidence of injury by such slight freezing and no effect upon
the market value of the fruit. If, however, the freezing process is
carried somewhat further, a slight noticeable injury results, even
though the cells may appear practically normal; if freezing is car-
ried still further, the cells may be killed, in which event they turn
brown. Regardless of how much ice appears in the fruit it is inac-
curate to say that the apple shows freezing injury unless a signifi-
cant proportion of the cells show this browning.
EXTERNAL AND INTERNAL APPEARANCE OF FROZEN APPLES
If freezing of an apple has been slight, there may be no marked
external symptoms of any sort; if it has been severe, the general
outside appearance is strikingly affected. The surface is discol-
ored in irregularly shaped areas—becomes so, in fact, very soon
after the apple thaws—and appears considerably darkened. It
often assumes a water-soaked, brown color closely resembling that
of apple scald, or the color may become much darker, in some cases
almost black: When apples are in a frozen condition the skin be-
comes slightly shriveled, but the shriveling usually occurs in the
form of a network of wrinkles rather than as parallel lines of
shrinkage such as are produced by normal evaporation. Careful
measurements have shown also that the fruit actually becomes
smaller, sometimes by as much as 10 percent of its original volume.
On thawing it regains practically its original volume unless the
freezing was very severe.
When apples thaw after having been badly frozen the skin
becomes shriveled, particularly if the air in the storage place is
very dry. This form of shriveling seems to be due to rapid evapo-
ration, after thawing, of the water withdrawn from the cells and
changed into ice in the spaces between the cells during the freezing
process. Shriveling when slight is accompanied by a reduction
chiefly in size and when severe by a marked reduction in both size
and weight.
Apples that have been severely frozen frequently show notice-
ably sunken spots, which may be a quarter of an inch or more deep
1 Adapted from Ross, D. H., Brooks, C., FISHER, D. F., and BRATLEY, C. O
MARKET DISEASES OF FRUITS AND VEGETABLES; APPLES, PEARS, QUINCES. U. S. Dept. Agr.
Misc. Pub, 168, 71 pp., illus. 1933. (See pp. 16, 17, 26-29.)
FREEZING INJURY OF FRUITS. AND VEGETABLES 9
and have a superficial diameter about equal to their depth. In vir-
tually all cases these sunken spots develop at places that were
bruised while the apples were still frozen.
When freezing occurs the cells of the vascular, or water-conduct-
ing, system are usually the first to be affected, especially if the
freezing takes place rapidly; and they may be the only ones so af-
fected. In cross section this injury is shown by a brown discolora-
tion of the 10 large main water-conducting bundles, the color being
visible evidence that the cells have been frozen to death. Similar
browning may occur in the smaller strands through the flesh and
in the core tissue; it is frequently restricted to one side of the apple
because of lower temperature on that side. In more extreme cases
all the tissues may be affected; the flesh then shows a solid color
throughout, which varies from bright golden brown to darker
brown or almost black, depending on the variety of apple and the
severity and freshness of the freezing injury. These browned
areas in whatever tissue found usually have a water-soaked
appearance and in milder injury are translucent.
It snould be remembered, however, that during a short exposure
to an air temperature several degrees below freezing considerable
ice may form within the tissues and yet produce little or no effect
that could be diagnosed as freezing injury.
FREEZING INJURY AND TRANSIT BRUISING
Apples that have been both bruised and frozen while in transit
by rail frequently show flattened areas 114 to 2 inches in diameter
that are somewhat sunken and soft toward the center and have a
dull-brown or slate color over most or all of the surface.
Another kind of bruising occurs in boxed apples in transit. This
injury is usually found only in apples at the lower side of the bot-
tom layer of boxes in the car and for this reason is frequently at-
tributed to freezing. It is more common during winter than dur-
ing fall and spring, but nevertheless it has been found in boxed
apples shipped in the fall before freezing weather has occurred in
the producing districts or anywhere along the route taken by the
shipment. The injury has also been found so late in the spring
that there was no possibility of the fruit having been exposed to
freezing weather in transit.
This transit bruising is characterized by flat, bruised areas on
the sides of the apples that were in contact with the lower side of
the box as the latter lay in the car. These areas have a water-
soaked, darkened appearance, are generally rather firm, and may
be an inch or more in diameter. Occasionally the skin covering
them is discolored in spots or streaks. In cross section there is
usually a water-soaked, glassy, wedge-shaped area extending from
the skin toward the center of the apple. It may be shallow or may
extend quite to the core. In some instances the inner edge of this
area appears as a fairly smooth curve, convex toward the core; in
others it is broken by strands or rays having the water-soaked ap-
pearance just mentioned and extending radially for as much as
three-quarters of an inch beyond the main affected area. Small,
water-soaked patches or streaks are sometimes seen also under-
10 CIRCULAR 713, U. S. DEPARTMENT OF AGRICULTURE
neath bruises produced by the pressure of one apple against
another.
Bruises like those just described can be produced by subjecting
apples to a jolting similar to that received while they are in transit
by rail. Glassy, water-soaked bruises are not necessarily a sign of
freezing injury; neither are wedge-shaped injured areas that ex-
tend to the core nor brown bruised spots under the skin, in which
browning of the water-conducting bundles has oecurred. All of
these can be produced by the jolting and pressure that apples are
subjected to while in transit in railroad cars.
EFFECT OF FREEZING AND THAWING
It is generally believed that frozen apples are injured less by
gradual than by rapid thawing. Investigation has shown, how-
ever, (1) that apples frozen rapidly show an equal amount of in-
jury when thawed at 32° and 72° F. and (2) that slowly frozen .
apples show more discoloration when thawed slowly (at 32°) than
when thawed rapidly (at 50° or higher).
Frozen apples are often dry and mealy, probably because of loss
of water through evaporation from the injured tissues. The de-
gree of mealiness increases with the amount of freezing but is not
entirely absent even when freezing is only slight. The flesh some-
times appears flaky or corky and always lacks the normal crisp-
ness; in severely frozen specimens it collapses and becomes viscid,
soft, and mushy.
Apples frozen but not frozen to death may thaw out with no
apparent aftereffect except a slight softening of the flesh. This
softening, however, means that their prospective storage life has
been shortened. The amount of the reduction will depend on the
variety, the degree of maturity of the fruit when frozen, and the
severity of the freezing. There is no doubt that apples that have
been frozen solid throughout, even though for only a short time,
will not hold up so well in storage, or for so long a time, as similar
apples from the same orchard or the same storage lot or shipment
that have not been frozen.
Apples should not be handled while frozen, because of the
danger of serious damage from bruising. Bruises produced in this
way frequently extend deep into the fruit, and the affected flesh is
usually brown, soft, and somewhat watery.
FREEZING INJURY AND INTERNAL BREAK-DOWN
During January or even earlier and through the remainder of
the storage season, it may sometimes be difficult to distinguish be-
tween freezing injury and internal break-down due to overripe-
ness. The difficulty will be greatest when there is no evidence of
freezing in transit. Internal break-down may be followed by
browning, but the color change, unlike that which often follows
freezing injury, does not begin in the main water-conducting
bundles. Instead, it may begin at any place in the flesh and usually
does begin at many places. In cross section, fruits affected with
internal break-down often show the following symptoms: An
outer shell of healthy flesh about a quarter of an inch thick sur-
FREEZING INJURY OF FRUITS AND VEGETABLES 1l
rounding a brown zone which extends inward in roughly triangu-
lar patches as far as the bundles of conducting vessels or a little
beyond; next to this another zone of healthy flesh; and in the flesh
at the core a second area of brown.
Internal break-down is usually worse in large apples and more
marked at the blossom ends than at the stem ends. Freezing in-
jury may affect apples of any size and is not necessarily or uni-
formly worse at one end than at the other. Yet when one side of
an apple or even the whole apple shows a uniform brown color in
cross section, it will be hard to determine whether the color is due
to freezing injury or internal break-down. Reliance should then
be placed on all the symptoms that can be found in as many apples
as can be examined conveniently rather than on any one symptom
or the examination of one apple.
ASPARAGUS
Asparagus was held in a standard asparagus crate in a 20° F.
room for 24 hours. The crate was papered over the sides and
bottom. 2 After 48 hours’ thawing at 60° many stalks were limp
and water-soaked. Counts made on 2 bunches showed that 64
stalks, or 55 percent, had been injured by freezing. The other
stalks showed no signs of freezing injury. When only part of the
stalk had been injured, the affected region—the upper, tender end
of the stalk — was easily detected by the limp, water-soaked,
slightly shriveled condition.
LIMA BEANS
Lima beans in a bushel hamper, papered around the sides and
covered with a regular hamper lid, were held in an 18° F.. room for
48 hours. Freezing, recognizable by a water-soaked appearance of
part or all of the pod, began in 1 hour at the top of the hamper. At
the:end of 6 hours pods around the sides and about 3 inches down
from the top had frozen, but those in the center of the mass were
not frozen. On removal from the freezing room, all pods were
found frozen except those in a roughly cylindrical region about 3
inches in diameter, extending from about 4 inches below the top of
the mass of beans to within about 3 inches of the bottom. After 48
hours at 45° the pods and seeds that had been frozen were limp and
water-soaked.
Lima beans in a 54-bushel, papered hamper were held in a 24° F.
room for 7 hours. At the end of that time pods at the top and next
to the sides of the hamper were found slightly frozen and showed a
few water-soaked spots. After 24 hours at 45° the only symptom
of injury that could be detected was a slight limpness of the pods
that had frozen. The seeds showed no signs of injury.
SNAP BEANS
Snap beans in a 54-bushel hamper, papered over the sides and
bottom, were in a 24° F. room for 7 hours. After thawing for 24
2 Paper was used on this crate and on packages of certain other commodities in
other tests, in order to restrict air circulation and so simulate to some extent the
coud tous to which packages in a refrigerator car or in a stack on a pier might be
subjecte
12 CIRCULAR 713, U. S. DEPARTMENT OF AGRICULTURE
hours at 45° scattered specimens in the top 2 inches and along the
sides of the hamper showed water-soaked mottling in small
patches on the surface of the pods. When the injured pods were
bent toward one of these patches, moisture oozed out on the sur-
face. None of the pods appeared to have been completely frozen,
and none of the beans in the center of the package showed injury
of any kind. After a week longer at 60° the patches that had
previously been water-soaked and discolored were dried out and
turning brown.
Beans on the market that have been frozen are water-soaked and
limp, and they quickly dry down or are invaded by bacteria or
fungi.
BUNCH BEETS
Bunch beets in a standard crate, papered over the sides, ends,
and bottom, were held in a 24° F. room for 28 hours. On removal
from the freezing room the tops and roots were found frozen at the
top, along the sides and ends, and at the bottom of the crate. Im-
mediately after thawing the leaves had a water-soaked appearance.
There was no freezing at the center of the crate. After 17 hours
at 60° there was no sign of injury in any leaves or roots; after 12
days the small rootlets were becoming moldy, but the main roots
(the beets) were still sound and firm.
On the market, freezing injury is shown by flabbiness of the
roots and small radial cracks in their centers.
SPROUTING BROCCOLI
One dozen bunches, or “heads,” of broccoli in a crate, papered
over the sides, ends, and bottom, were held in a 24° F. room for 72
hours. On removal from the freezing room the stems were found
frozen at the base, but the flower heads were not frozen. After the
crate had been held at 55° for 24 hours, no injury could be detected
in any of the bunches.
On the market, freezing injury shows first as water soaking of
the stem core.
CABBAGE
An unpapered crate containing two layers of heads of new cab-
bage was held ina 24° F. room for 78 hours. On removal from the
freezing room some heads were found frozen solid and others only
on the outside to a depth of one-quarter to one-half inch. After 6
days at 60° none of the heads showed any signs of injury.
Several heads of old cabbage, unprotected (not in a container),
were placed in a 24° F. room and left for 6 days. On removal and
while still frozen, they had a glassy, water-soaked appearance in
the heart and some of the larger midribs. After 24 hours at 60°
they were completely thawed. When cut lengthwise they looked
entirely normal except that the lowermost leaves of the head
tended to separate from the stem or heart. Such separation also
FREEZING INJURY OF FRUITS AND VEGETABLES 13
occurs in old, unfrozen cabbage, but in the heads discussed here
there was no yellowing as in old cabbage.
After 2 days at 18° F. unprotected heads of new cabbage were
found frozen solid. On thawing they were found to be worthless—
soft, water-soaked, and very leaky.
For a discussion of the rate of cooling of three other lots of
cabbage see page 26. (For rate of cooling of one of these see
figure 3.)
On the market, cabbage frozen severely enough to show injury
usually is worst affected in the stem and the heart leaves. These
parts are water-soaked and light brown. Severe or frequent
freezing causes the outer leaves to become paper thin. Slight
injury is often revealed by brown streaks in the stem (or heart).
CARROTS
EXPERIMENTAL OBSERVATIONS
Carrots were stored at 18° F. for 51 hours and then at 14° for
48 hours more. The container was a standard crate lined with
paper and with a pad under the lid. No extra paper was placed
over the outside. The carrots had been topped, leaving 2-inch
leafstalk stubs. (For data on the rate of cooling of carrots, see
also page 25 and figure 1.) On removal of the crate from the
freezing room all the easily accessible roots were found frozen and
numerous roots at the top were seen to be cracked lengthwise.
After 72 hours at 45° all the roots had thawed. Cracking was
fairly uniform throughout the crate, 48 percent being cracked and
52 percent not cracked. In many of the cracked roots the outer
layer, to a depth of about one-eighth inch, had separated from the
central cylinder and could easily be removed. There were no in-
ternal cracks extending outward from the center (radial crack-
ing) except in a few specimens that were spongy and leaky. Some
of the leafstalk stubs were plainly frozen to death (limp and
water-soaked) ; others appeared to be uninjured. After 72 hours
at 45° there was no increase in radial cracking and no softening.
Another standard crate of carrots with the tops attached was
held in a 20° F. room for 48 hours. The crate was not lined but
was papered over the sides, ends, and bottom. On removal from
the freezing room the roots next to the sides, ends, and top of the
crate were found frozen. After 12 hours at 60° the only injury
that could be detected was in the limp, water-soaked tops that
were exposed along the edges of the lid. After 4 more days at 60°
the tops and roots at the top of the crate were wilted and the roots
had a poor flavor. Roots away from the outside of the crate
showed no injury and had a good flavor. No lengthwise cracking
was observed in any roots used in this test.
MARKET OBSERVATIONS
Freezing injury of carrots seen on the market consists of flabbi-
ness of roots, radial cracking of the central cylinder, and tangen-
tial cracking of the outer cortex. Water soaking is rarely seen.
14 CIRCULAR 713, U. 8. DEPARTMENT OF AGRICULTURE
CAULIFLOWER
Cauliflower was held in a crate containing 1 dozen heads in a
20° F. room for 31 hours. The sides, ends, and bottom of the crate
were papered. On removal from the freezing room the leaves and
curd were found frozen solid. After 17 hours at 60° there was no
evidence of injury to either leaves or curd. After repeated freez-
ing and thawing, the edges of the outside leaves became dry and
papery and had a bluish color. The curd was not affected, and
when cooked it had a normal flavor.
Cauliflower repeatedly or severely frozen in transit or on the
market has a glassy, water-soaked appearance of the pith of the
stem and of part or all of the curd. A pronounced odor of spoiled
cabbage is given off. At room temperature the water-soaked
areas are quickly invaded by soft rot bacteria.
CELERY
EXPERIMENTAL OBSERVATIONS
Blanched celery was held in a standard crate in a 20° F. room
for 7 hours. The crate was papered over the sides, ends, and bot-
tom. After 17 hours at 60° all stalks (the entire trimmed plants)
showed freezing injury marked by loosening of the epidermis on
many of the outer leaf stems. The hearts were unfrozen. Loosen-
ing of the epidermis is best demonstrated by twisting the leaf
stem, or petiole.
Unblanched Pascal celery was held in a standard crate, which
had its ends, sides, and bottom papered, in a 24° F. room for 72
hours. After 3 days at 55° many outside leaf stems of bunches
found actually frozen when removed (about one-half of the con-
tents of the crate) were water-soaked and slightly slimy; a few
showed pink mold rot (watery soft rot). Many leaf stems that ap-
peared normal on the outside were water-soaked and discolored on
the inner surface. Many of the leaf stems, discolored and not
discolored, showed wrinkling of the epidermis when twisted.
No flabbiness or drooping of the leaf stems was observed in
either the blanched or the unblanched celery,
MARKET OBSERVATIONS
Celery found frozen on the market frequently shows flabby or
limp outer leaf stems after it thaws. Furthermore, the epidermis
not only becomes loosened but sometimes it peels off. The loosen-
ing is often more pronounced in the upper nodes or branches of
the leaf stem than in the main leaf stem itself,
GREEN CORN
Green corn was held in a bushel basket in an 18° F. room for 120
hours. The basket was papered around the sides and was covered
with a regular basket lid. After about 2 hours the outside husks
of the top-layer ears were found frozen but the inside husks were
not. After 48 hours only the outside husks of the uppermost ears
FREEZING INJURY OF FRUITS AND VEGETABLES 15
were frozen. After 96 hours all ears in the upper part of the
basket were frozen hard but those at the center were not. After
120 hours, when the basket was removed from the freezing room,
all ears were found frozen. After 24 hours at 45° the corn had a
slightly sour smell, the husks and cob were water-soaked, and the
grains were watery and much softened.
CUCUMBERS
Cucumbers in a half-bushel basket, papered over the sides and
bottom, were held in a 24° F. room for 7 hours. On their removal
from the freezing room no injury was observed, and there was
none after the cucumbers had been held at 60° for 24 hours.
Other cucumbers were held in a bushel basket in an 18° F.. room
for 48 hours. The basket was papered around the sides and was
covered with a regular basket lid. During the first 5 hours no
signs of freezing could be detected, but after 6 hours several
cucumbers around the edge of the lid showed numerous circular,
water-soaked spots on the surface, mostly about one-sixteenth to
one-eighth inch in diameter. A few water-soaked areas one-half
inch in diameter were seen. After 24 hours all the top-layer cu-
cumbers were frozen hard. After 26 hours cucumbers halfway
down in the basket, both at the sides and in the center, were not
frozen. On removal from the freezing room all the cucumbers
showed freezing but some at the center were not entirely frozen.
After 48 hours at 45° the cucumbers that had frozen were soft,
- water-soaked throughout, and much wrinkled lengthwise, espe-
cially toward the stem end.
Frozen cucumbers seen on the market are completely water-
soaked and flabby and are soon invaded by bacteria or fungi.
EGGPLANT
Eggplant was held in an unpapered bushel basket in a 24° F.
room for 78 hours. On removal from the freezing room some were
frozen solid and others only on the outside to a depth of one-
quarter to one-half inch. After 6 days at 60° none showed any
sign of injury. After being held 2 more days at 18° they were
found frozen solid. On thawing they were slightly browned inside,
soft, very leaky, and worthless.
On the market, browning of the flesh and slight shriveling are
recognized as signs of moderate freezing.
LEMONS
Lemons were held in a standard crate in an 18° F. room for 75
hours. The crate was not papered, but the lemons in one end were
individually wrapped and those in the other were not wrapped.
(For rate of cooling of these two lots see figure 2.) After 75 hours
at 18° some fruits in both ends of the crate, at or near the center
position, were not frozen. After 20 hours at 45° many of the
fruits in both ends of the box were found to be soft, watery, and
worthless. The percentage of such injured fruits was greatest in
16 CIRCULAR 7138, U. S. DEPARTMENT OF AGRICULTURE
the end where no wrappers were used. Inspection after the box
had been held at 36° for 3 weeks showed that the percentage of
soft lemons had increased greatly. Percentage figures for the
two inspections follow.
Description and treatment: Baasee Le
Unwrapped, after thawing: 22) 031032 Bost DAS Visinw Sto 55
Wrapped, atter™= thawin@s soo. eee ac ee 25
Unwrapped, atters weeks) atpeGe ig ee ee 84
Wrapped, aiterss weeks/atiserrh, ieee te eee ee ee 57
The fruits not classed as soft were firm and usable.
On the market, frozen lemons have a rind flavor but not the
strong, unpleasant, rancid flavor and odor found in watery break-
down. Watery break-down is found scattered throughout stacks
of boxes and in the center of packs as well as along the edges,
whereas freezing injury is likely to be localized. (See also
page 18.)
LETTUCE
Lettuce in an eastern crate (2 dozen heads) was held in a 20° F.
room for 54 hours. The crate was papered over the sides, ends,
and bottom and contained package ice. After 17 hours at 60° the
outer leaves of the heads around the outside of the crate had a
water-soaked appearance. When these heads were cut, the freez-
ing injury was found to extend through about three layers of
leaves. The solid part of the heads was uninjured and not frozen.
No freezing injury was found in the heads next to the outside
layers. There was still some package ice remaining.
Conditions similar to those just described were found in a crate
of lettuce held for 72 hours in a 24° F. room.
On the market, lettuce injured by moderate freezing shows only
a few outer leaves glassy. The heads usually remain firm unless
there is opportunity for subsequent drying.
ONIONS AND GARLIC
EXPERIMENTAL OBSERVATIONS
Onions in an unprotected net bag of 1-bushel capacity were held
at 21° to 24° F. for 54 hours. The bulbs varied greatly in size and ©
many of them had sprouted. Large bulbs of the Yellow Bermuda
variety also were held loose in a box in the same room during the
same period. On removal from the freezing room all the onions
in the box, except a few at the center, were found frozen. Of the
326 onions in the bag 117 (approximately 36 percent) were not
frozen. Most of these were at or near the center of the mass of
bulbs. The frozen onions in both the bag and the box were hard
and had a glassy, water-soaked appearance.
After being held for 3 days at 31° F. both lots showed the
symptoms described under Market Observations.
FREEZING INJURY OF FRUITS AND VEGETABLES 7
MARKET OBSERVATIONS 2
The average freezing point of onion bulbs is about 30° F. AIl-
though some onions may freeze at a fraction of a degree higher or
lower, this temperature may be taken as the danger point. Under
some conditions onions standing on track or in storage may be
undercooled to a temperature of 25° or lower for a short time with-
out becoming frozen or otherwise injured. A slight jar will cause
undercooled products to freeze immediately; therefore onions
rarely undercool during transit.
Individual onions vary considerably in their reaction to low tem-
perature. Some bulbs in a lot may freeze quickly when they reach
a temperature of 30° F. and show severe injury when they thaw.
Others may not freeze, or if they do freeze they may thaw out
without injury.
Onions injured by freezing show water-soaked, grayish-yellow
fleshy scales when cut. In slight freezing the outer fleshy scales
alone are affected, but when the bulbs are exposed to low tempera-
tures for a prolonged period the inner scales may also become
water-soaked and discolored. Usually the entire scale is injured
all the way around the bulb, but the neighboring scales inside and
outside may or may not show injury. Irregularly shaped opaque
areas occur in many of the watery transparent scales.
Garlic bulbs do not freeze until a temperature of approximately
25° F. is reached. The frozen tissues are discolored and water-
soaked in the same way as those of onions.
In cases of slight freezing injury of onions and garlic there may
be little loss if the bulbs are spread out so that the injured scales
can dry thoroughly. Experimental evidence indicates that frozen
onions will show less injury and remain in a better general condi-
tion if thawed out at 40° F.. rather than at a higher temperature.
ORANGES AND GRAPEFRUIT
EXPERIMENTAL OBSERVATIONS
One standard crate of oranges and one of grapefruit were held
in an 18° F. room for 25 hours. The crates were not papered, but
the fruits in one end of each crate were individually wrapped and
those in the other were not wrapped. Immediately after removal
from the freezing room all top-layer fruits were found frozen but
those halfway down and away from the sides and ends in both the
wrapped and the unwrapped lots felt normally soft. Evidently
they were not frozen. The crates were held at 45° for 24 hours
after removal from the freezing room; the only marked symptoms
in any of the fruits were a definite softening and slight off-flavor
of a few from the top layers, both wrapped and unwrapped.
After 18 days at 36° F. all fruits cut were slightly off in flavor,
but otherwise there was no change from the condition noted at the
time of the first inspection. No pitting of the peel and no one
or separation of segments was noted.
3 Adapted from RAMSEY, G. B., and WIANT, J. S. MARKET DISEASES OF FRUITS AND
VEGETABLES: ASPARAGUS, ONIONS, BEANS, PEAS, CARROTS, CELERY, AND RELATED
VEGETABLES, U, S. Dept. Agr, Misc. Pub. 440, 70 pp., illus, 1941, (See p. 10.)
18 CIRCULAR 713, U. S. DEPARTMENT OF AGRICULTURE
MARKET OBSERVATIONS *
Freezing injury in transit is likely to occur in the fruit next
to the side walls and along the floor of the car rather than in that
in the body of the load. It seldom shows in the form of drying out
so characteristic of fruit frozen on the tree. Oranges and grape-
fruit are often bitter in flavor for a time after thawing, but this
is not a consistent condition. If the freezing has been severe, the
rind may show effects ranging in severity from almost typical
brown stain to leaden-gray discolored areas of varying size, which
greatly resemble watery break-down. The affected rind tissues
may or may not be sunken, but when severely frozen they usually
become soft and mushy and are underlain by mushy pulp tissue.
Freezing damage is best seen by cutting off both ends of a fruit,
then cutting lengthwise through the rind of the central portion re-
maining, and pulling the segments apart. If the fruit has been
frozen the membrane between the segments will show a soaked
condition and usually a number of white specks, which are hes-
peridin crystals (naringin in grapefruit) resulting from the freez-
ing. However, the presence of crystals is not necessarily an indi-
cation that the fruit has been frozen. They may also result from
the application of heat to the fruit or from rapid drying out of the
tissues. In tangerines the hesperidin crystals occur in the pulp as
well as on the segment walls and are seen even more readily than
in oranges when a cross-section cut is made. Freezing damage
may be confined to a part of the fruit, in which case the signs sug-
gested will be found in the affected part. The method of examina-
tion just described is particularly useful for California oranges.
Florida oranges are not so easily examined in this way but are
more likely to show the mushy condition in cross section.
Lemons and grapefruit show the damage in cross section much
more plainly than oranges do, although it is desirable at times to
pull grapefruit sections apart as recommended for oranges. If
lemons have been seriously damaged the pulp becomes mushy at
once after thawing.
In addition to the symptoms just described grapefruit shows a
milky appearance of the pulp, which is in marked contrast to the
very light amber color and the almost transparent condition of un-
frozen pulp. The contrast is especially noticeable in fruits that
have been frozen in small spots or on one side only. The milky ap-
pearance of the pulp is also found in grapefruit that has been in
storage for 8 to 10 weeks, and it may be accompanied by a bitter
taste. However, such fruit is not mushy and watery unless af-
fected by watery break-down; in such a case a positive diagnosis
must depend on a consideration of the history of the fruit and the
conditions under which the injury is found.
Citrus fruits frozen on the tree show a number of symptoms
similar to those described under transit freezing if examined
soon after the freezing occurs. After a few days, however, addi-
tional symptoms appear. First and most characteristic among
these is a buckling of the partition walls at the stem end of the
4Adapted from Rosr, D. H., Brooks, C., BRATLEY, C. O., and WINSTON, J. R.
MARKET DISEASES OF FRUITS AND VEGETABLES: CITRUS AND OTHER SUBTROPICAL FRUITS.
U. S. Dept. Agr. Misc. Pub. 498, 57 pp., illus, 19438. (See pp. 27 and 28.)
FREEZING INJURY OF FRUITS AND VEGETABLES 19
fruit, with or without drying of the pulp. Small pits or pitted
areas may also develop in the rind on any part of the fruit. In
fruit on the market, picked several days after it had been frozen on
the tree, the injury is manifest by woodiness of the pulp or by
open spaces between the segments due to the collapse and drying
out of some of the juice sacs. Cavities usually appear in the
orange pulp before open spaces develop between the segment walls.
Later, when considerable drying out of the pulp has occurred,
small open spaces between the segment walls may be found. Some-
times only one or two segments will show drying-out effects and
all or only a part of the segment may be affected.
There are, of course, all degrees of dryness, from very slight to
total. In the practical handling of citrus fruit three degrees of
dryness are recognized: (1) Slightly open, when the cut surface
shows a slight open space between the segment walls and the juice
sacs but the surface of the pulp appears juicy. (2) Distinctly
open, when the cut surface shows large open spaces but the pulp
still appears juicy. (3) Dry, when the cut surface shows no large
open spaces but the fruit seems to have dried out evenly all through
and the color of the pulp shows it to be almost devoid of juice, or
when the fruit has dried out with some of the segments more or
less collapsed.
Drying in oranges usually progresses from the stem end,
whereas in grapefruit it may proceed from either or both ends or
it may begin around the outside of the pulp. Drying is not found at
the center of the fruit except in extreme cases. If an orange that
has been frozen is examined a few days after it thaws, it will
usually show the hesperidin crystals already mentioned on the
membrane or rag that separates the segments of the pulp. On
the other hand, the crystals are sometimes visible when examina-
tion is made within a few hours after the freezing, whereas if it is
not made until several weeks afterward they may not be so numer-
ous or conspicuous, probably because the more severely frozen
fruit falls soon after the freeze. When examined on the market
tree-frozen fruit that has remained on the tree for several weeks
after freezing has a rind that is thicker than normal, especially
over the damaged part of the fruit.
: As differentiated from granulation, which sometimes occurs in
fruit on the tree, dryness from freezing results through the empty-
ing and subsequent collapse of juice sacs due to disappearance of
the juice. In granulation the juice sacs do not collapse but become
filled with gelatinous or solid matter.
PARSNIPS
Parsnips were kept in a 5-bushel hamper in a 24° F. room for
28 hours. The hamper was papered over the sides and bottom.
On removal from the freezing room the roots around the sides and
on top were found to be frozen hard. Freezing was less severe at
the center. After 17 hours at 60° there was no sign of injury in
any of the roots in the hamper. After 10 days more at 60° the
roots were in good condition except for one small root affected
with rhizopus rot.
20 “CIRCULAR 713, U. S. DEPARTMENT OF AGRICULTURE
PEAS
Peas were kept in a bushel hamper in a 24° F. room for 50 hours.
The hamper was papered over the sides and bottom. On removal
from the freezing room many pods over the top and next to the
sides of the hamper were found frozen. After 24 hours at 60° no
evidence of freezing injury was found.
A few pods not in a container were placed at 24° F. and left for
24 hours. When thawed out at 40° the pods were found to be soft
and water-soaked; the shelled-out peas were. generally water-
soaked and darker green than unfrozen ones. Numerous whitish
spots were noted, but these were on the pods and evidently were
due to the drying out of bruised or rubbed areas and not to
freezing.
PEPPERS
Peppers were held in a bushel basket in a 24° F. room for 24
hours; the basket was papered over the sides and bottom. After 6
hours ‘at 60° freezing injury was spotty. Some specimens showed
no injury; in others the entire outer wall was soft, water-soaked,
and darker colored than that of unfrozen ones. There was no
browning of any part of the peppers. This same lot was returned
to a temperature of 24° for 48 hours more. At the end of that
time all the peppers were found frozen and, after they had thawed
out, a few around the outside of the basket showed browning of
cores and seeds, as well as water soaking. Such peppers were soft
and worthless.
POTATOES
MARKET OBSERVATIONS ®
The symptoms of freezing injury in potatoes vary, depending
upon the kind of tissues killed, the amount of tissue involved, and
the consistency and color of the affected regions. Which of the
symptom complexes will develop depends upon whether the injury
is due to (1) long exposure to temperatures just below the freezing
point, (2) moderately long exposure to very low temperatures,
(3) moderately short exposure to temperatures just at the freezing
point, or (4) short exposure to temperatures far below the freez-
ing point. In the first two cases, almost all the tissues may be
killed and both external and internal symptoms may appear
shortly after thawing. In the last two cases only the most sus-
ceptible tissues are killed and no external symptoms are apparent
for weeks or months. Short exposures generally produce slight
injury of the net or ring type or a combination of these; longer
exposures the blotch type, often in combination with the net and
ring types. The restricted or limited type of injury is known as
freezing or frost necrosis.
Before thawing, frozen potato tissue no longer possesses the
crispness of the normal tissue. It is abnormally firm, looks dull,
and does not cut readily or with snap. This is due to the presence
5 Adapted from LINK, G. K. K., and RAMSEY, G. B. MARKET DISEASES OF FRUITS
AND VEGETABLES: POTATOES. U. S. Dept. Agr. Misc. Pub. 98, 63 pp., illus. 1932.
(See pp. 18 and 19.)
FREEZING INJURY OF FRUITS AND VEGETABLES 3
of ice. When ice occurs in only part of the tuber, its presence can
be detected at times by a crunching sound if, while held to the ear,
the tuber is pressed between the fingers.
Tubers that are frozen solid collapse promptly upon thawing,
becoming soft and watery. Their wet surfaces make detection and
sorting out easy. The presence of moisture on a tuber is not neces-
sarily a sign of freezing injury, however, since cold tubers when
brought into moist air with a temperature higher than their own
very readily condense water from the air on their surfaces. Some-
times if only a part of a tuber is exposed to freezing temperature
that portion is frozen to death and upon thawing becomes turgid
and blisterlike and the skin frequently discolors. If tissues that
have been frozen to death are cut shortly after thawing, they may
appear dull and colorless at first. Upon exposure to the air they
frequently pass promptly through pink and red stages and ulti-
mately become brown, gray, or inky black. The rate of develop-
ment of these colors depends upon the temperature of the air. At
temperatures below 75° F. several hours may be required, while
only 14 to 1 hour may suffice at higher temperatures. The occur-
rence of these color changes is also determined by the extent to
which air reaches the affected tissues.
When extensive areas of tissues are killed by freezing they
usually become infected with bacteria, which cause foul-smelling,
slimy, or sticky rots if thawing takes place in a warm, humid
atmosphere. They dry down to a mealy or tough, leathery, granu-
lar, chalky mass of starch and tissue remnants if they thaw in cold
or dry air. Tubers that have only one side frozen frequently have
the killed portion sharply set off from the unaffected area by a
purplish or ultimately brown line of corky tissues. Often infection
by species of Fusarium sets in before the unaffected cells are
sealed off by the corky layer.
From experience with potatoes in the field, in storage, and in
transit and from experiments it is generally known that when a
lot of tubers has been exposed to low temperatures for a long
enough time for some to freeze solid, there frequently are others
which do not freeze at all and still others which seem to be unaf-
fected so far as external appearances indicate, but when cut they
may show the various types of symptoms known as freezing or
frost necrosis.
The mildest type of freezing necrosis is one marked by tissues
that are drier than the surrounding ones and that have a grayish-
white tint. When the injured tissues are cut soon after thawing,
they may turn red in a short time and finally become brown. In
uncut tubers, even after a long time, the only sign of injury is a
grayish, dull, dry, collapsed appearance of some of the tissues.
Generally, however, freezing necrosis is marked by decided dis-
colorations. These may not be apparent immediately after thaw-
ing, but they usually set in after 5 or 6 hours. Usually the color
changes from dull white to pink or brick red and finally to gray,
brown, or black.
There are several types of discoloration. One, the ring type, is
limited to the vascular (water-conducting) ring and immediately
adjoining tissues and is characterized by pronounced blackening.
22 CIRCULAR 7138, U. S. DEPARTMENT OF AGRICULTURE
Another, the net type, is marked by more or less blackening of the
vascular ring and the finer strands which extend from it into ad-
jacent tissue. Both types are frequently restricted to the stem
end. Finally, there is a blotch type, marked by irregular patches
ranging in color from an opaque gray or blue to sooty black. These
patches occur everywhere in the tuber, though most generally in
the vascular ring and the cortex. When these blotches are in the
cortex they may be apparent externally in clean tubers with white
skins. This is the only type of freezing necrosis that may be vis-
ible externally. If tubers affected with any or all of these types of
freezing necrosis are held in storage, they generally shrivel or wilt
more than unaffected tubers. Excessive shriveling alone, how-
ever, cannot be relied upon as a sign of freezing necrosis.
It is popularly believed that the symptoms here described as
freezing necrosis are not due to freezing but to chilling, since
freezing, it is contended, is always followed by a complete break-
down after thawing. This is not the case. Freezing necrosis is as
truly a freezing injury as the complete type, and it does not occur
unless ice is formed in the tissues. The difference lies in the fact
that not all cells of the tuber, but only those that become discolored,
have been killed.
RUTABAGAS
EXPERIMENTAL OBSERVATIONS
Rutabagas, with starting temperatures of about 36° F. and 64°,
in bushel bags were kept in a 25° room for 29 and 31 hours, re-
spectively. A third bag with a starting temperature of about 57°
was held for 24 hours in an 18° room. After thawing at 45° for
5 days neither the lightly frozen nor the severely frozen lots
showed injury of any kind. When the roots were cut while still
frozen there was a water-soaked ring about one-quarter to one-
half inch deep around the outside, but this disappeared after the
roots had thawed. A second freezing at 18° for about 24 hours
caused no detectable injury.
For a discussion of the rate of cooling of two other lots of ruta-
bagas see pages 26 and 27. The rate of cooling of one of these is
shown in figure 3.
MARKET OBSERVATIONS
Severely frozen roots remain fairly firm but are water-soaked
and light brown. Free water runs from cut surfaces. A pro-
nounced mustard odor is given off.
There are considerable variation among different lots of ruta-
bagas and considerable difference of opinion among market
handlers as to the importance of freezing injury in this crop. The
situation can be summed up as follows: Most lots show no damage
after thawing, but some become flabby and are predisposed to
decay. Some dealers believe that one or two freezes do not cause
injury but that more do; others believe that the extent of the in-
jury depends on the temperature and on whether the roots are
handled while frozen and soon after thawing.
FREEZING INJURY OF FRUITS AND VEGETABLES 23
SPINACH
EXPERIMENTAL OBSERVATIONS
Spinach in a bushel basket was held in a 24° F. room for 96
hours. The basket was papered over the sides and bottom. On
removal from the freezing room the mass of spinach was cut
through vertically from side to side of the basket. Freezing was
found to extend inward about 11% inches from the top and 314
inches from the sides. One-half of the lot was left in the basket
and this and the portion that had been removed were held in a 40°
room for 4 days. At that time the only injury found was in the
topmost leaves, which were bruised and water-soaked where they
had been in contact with the lid. The basket was then repacked
and returned to 24° for a second freezing. It was removed after 2
days and thawed at 60°. No injury was found except in the upper-
most leaves that were against the lid and had been bruised in
handling. After 5 days at 60° spinach that had frozen at 24°
developed slimy soft rot in the uppermost injured leaves.
MARKET OBSERVATIONS
The frozen leaves of spinacn are easily bruised during unloading
and handling. This bruising gives a water-soaked, collapsed ap-
pearance to plants in the tops of baskets. Plants that protrude
under the lid are particularly likely to be injured.
SQUASHES
Acorn squashes in a 54-bushel hamper were kept in a 24° F.
room for 6 days. The sides and bottom of the hamper were pa-
pered, and paper was also laid loosely over the top. Thirty-six
hours after removal from the freezing room (12 hours in 32° after
standing in a handling room for 24 hours at about 60°), the
squashes from the top of the hamper were soft and water-soaked
and had a granular appearance in the flesh and a fermented smell.
- Soft ones were found all the way down to the bottom of the ham-
per, but two that apparently had not frozen were noted. Many of
the injured squashes showed small white spots on the outside, pro-
duced by localized blistering of the skin. After thawing, all
squashes from this hamper were darker externally than unfrozen
ones saved for comparison.
Two unprotected Blue Hubbard squashes were held in a 24° F.
room for 6 days. On removal from the freezing room these
squashes showed small lens- or disc-shaped masses of ice through-
out the flesh but the flesh was not discolored. On thawing at 60°
for 24 hours the flesh contained many small crescent-shaped
cracks. There was no discoloration, no water-soaked condition,
and no off-odor.
The most common indications of freezing injury of Hubbard
squashes seen on the market are drying of the flesh after thawing
and a tendency to develop decay spots in the shell.
Summer squashes (Yellow Crookneck) were kept in a bushel
basket in a 24° F,. room for 24 hours. It was papered over the
24 CIRCULAR 7138, U. S. DEPARTMENT OF AGRICULTURE
sides and bottom. After 6 hours at 60° all specimens showed
some injury but only in limited areas over the surface. No injury
was noted in the inner flesh. The outer wall, where affected,
showed a dark, water-soaked appearance on the surface and a
similar condition extending inward about an eighth of an inch.
Under light pressure the injured tissues readily exuded a watery
juice. When the squashes were left for 24 hours at 60° the injured
areas on the surface began to show mold growth.
SWEETPOTATOES
Sweetpotatoes in an unpapered 5.-bushel hamper were held in
an 18° F. room for 51 hours. After 5 days at 45° many of the
roots were soft and when cut were very leaky. The cut surface
did not exude a milky juice as normal unfrozen roots do. All roots
that were cut had a darkened band around the outside of the cross
section, extending inward one-eighth to three-sixteenths inch.
The flesh was mottled light gray to brown or reddish and had a
sour, off-smell. All roots in the hamper showed injury.
Other tests showed that an exposure of only 3 or 4 hours to 22°
F. caused unprotected sweetpotatoes to turn brown inside. The
first region of the root to be injured is the ring of water-conduct-
ing tissue about one-sixteenth to one-eighth inch under the skin.
TOMATOES
Tomatoes in an unpapered, lidded 30-pound lug were held in an
18° F. room for 48 hours. After 6 hours no freezing could be de-
tected, but after 7 hours a few fruits next to the lid were begin-
ning to freeze. On removal from the freezing room all tomatoes
around the outside of the mass of fruits were found frozen hard,
but those in the center layer, away from the sides, ends, bottom,
and lid, were still unfrozen. These comprised about one-quarter
of the total number of tomatoes in the lug. When the frozen toma-
toes thawed at 45°, they become soft, watery, and worthless.
Unprotected tomatoes laid out singly in the same freezing room
began to freeze on the outside in about 3 hours. After 6 hours the
freezing extended inward about one-half inch from the surface of
each fruit.
TURNIPS
Turnips were held in a bushel basket in a 24° F. room for 144
hours. The basket was papered over the sides and bottom, and
paper was also laid loosely over the top. Freezing temperatures in
the turnips were recorded after 19 to 21 hours, but no injury was
found when frozen roots were removed and thawed. The basket
was, therefore, left in the freezing room for 6 days in order to
obtain injury.
Thirty-six hours after removal from the freezing room (12
hours at 32° F. after being held for 24 hours at about 60°) those
from the top of the basket were dirty gray on the outside, soft, and
water-soaked and had a fermented smell. The skin had a blistered
appearance produced by the formation of small lens-shaped masses
FREEZING INJURY OF FRUITS AND VEGETABLES 755)
of ice between the skin and flesh. Softened, watery turnips were
found all the way to the bottom of the basket, but there were a few
that showed no signs of injury. In some of the injured specimens
the flesh had turned from white to light brown.
RATES OF COOLING OF FRUITS AND VEGETABLES
The rates of cooling of wrapped and unwrapped apples and of
topped carrots are given in figure 1, of wrapped and unwrapped
lemons in figure 2, and of rutabagas and cabbage in figure 38.
CENTER OF
*— — FRUITS NOT BAGEED:
CENTER OF TOP LAYER,
FRUITS NOT WRAPPED
yoy CENTER OF BOX,
FRUITS WRAPPED
g——v CENTER OF TOP LAYER,
Tm FRUITS WRAPPED
o=— =o
TEMPERATURE (°F)
(i ES (AES FE Fi dS a a =
-—
eee ———eee
= aie ea oH
Mi wa CENTER OC OF TOP LAYER (ee a ce |
AM, NOON PM, PM. AM. NOON PM.
FIGURE 1.—Rate of cooling of topped carrots and of wrapped and unwrapped
apples in packages not papered over the outside in an 18° F. room:
A, Apples; B, carrots.
The following comments on the rate-of-cooling experiments and
the results will help to bring out facts shown by figures 1 to 3; they
also supplement the statements on apples (p. 5), cabbage (p. 1A
carrots (p. 13), lemons (p. 15), and rutabagas (p: 22).
(1) Wrapped and unwrapped apples in eastern boxes at a start-
ing temperature of approximately 32° F. were placed in an 18°
room (fig. 1). The most exposed apples in both boxes (along the
edges, on top) began to freeze in about 3 hours after the test was
started. Freezing began after about 7 hours at the center of the
box of unwrapped fruit but not until after about 24 hours at the
same position in the box of wrapped fruit.
(2) Topped carrots with a starting temperature of about 32° F.
in a lined standard crate with a pad under the lid were placed in an
18° room (fig. 1). Freezing began in exposed roots at the top in
about 3 hours but not until after about 6 hours at the center of the
crate.
(3) Lemons in a standard California lemon crate were placed
in an 18° F.. room (fig. 2). The lemons in one-half of the box were
wrapped, whereas those in the other half were without wraps.
Their average starting temperature was approximately 32°. After
3 hours exposed lemons in the top layer, both wrapped and un-
wrapped, began to freeze on the outside. After 7 hours a few
26 CIRCULAR 713, U. S. DEPARTMENT OF AGRICULTURE
a
‘
aN,
-
z
WwW
=x
4
Ke
.
w
a
x
°
oO
wW
c
°o
=
”
~oce
>
°o
=x
~
TEMPERATURE (°F.)
CENTER OF HALF BOX,
@——® FRUITS NOT WRAPPED
TOP OF BOX AT SIDE,
O---——-O FRUITS NOT WRAPPED
CENTER OF HALF BOX
A—A FRUITS WRAPPED ;
Re A TOP OF BOX AT SIDE,
FRUITS WRAPPED
8:45 9:30 10:00 11:00 12:00 1:00 1130 2:00 2:30 3:00 3:30 4:00 4:30 8:30
A.M. NOON PLM, A.M.
FIGURE 2.—Rate of cooling of wrapped and unwrapped lemons in a crate
not papered over the outside in an 18° F. room. Undercooling occurred in
the top layer at the side in both wrapped and unwrapped fruits, but the
rise to the true freezing point probably came between 4:30 in the afternoon
and 8:30 the next morning, during which time no temperature readings
were made.
wrapped and unwrapped fruits in the middle layer at the end of
the box were frozen, while others were not. At the center of the
half box, unwrapped fruit began to freeze after 7 to 8 hours but
wrapped fruit did not begin to freeze until after about 24 hours.
After 75 hours at 18°, some wrapped and unwrapped fruits in both
ends of the box at or near the center position in each half were still
not frozen.
(4) Two bags of rutabagas (1 bushel each) having starting
temperatures ranging from 54.9° to 59.3° F. were placed in an 18°
room. One of the bags remained motionless during the test; the
other bag was shaken continuously on a jolting machine, to simu-
late conditions in a moving railroad car. The roots at the top and
sides of both bags began to freeze in about 7 hours. There was no
evidence that the jolting made any difference in the length of time
required for the roots to freeze.
(5) New cabbage in bushel crates, having starting tempera-
tures ranging from 50.6° to 54.7° F., was held in an 18° room
under the conditions just described for rutabagas. Freezing oc-
curred at the top and around the sides of both crates in about 9
hours. There was no noticeable effect of the jolting.
(6) New cabbage in a bushel basket and rutabagas in a bushel
bag were placed in a 25° F. room. Their starting temperature
was approximately 64°. The rutabagas began to freeze in 18 to
FREEZING INJURY OF FRUITS AND VEGETABLES 2,
20 hours. Outside leaves on the uppermost heads of cabbage
showed freezing in about 7 hours, but even in these heads freezing
was only about one-half inch deep after 48 hours. None of the
heads that lay deep in the basket showed any freezing during the
48-hour period.
(7) New cabbage in a bushel basket, having a starting tempera-
ture of about 36° F., was placed in a 25° room. Freezing of the
outside of the uppermost heads began in 1 hour but did not occur
to the extent of one-quarter inch inward from the surface until
after 7 hours.
(8) Rutabagas in a bushel bag, having a starting temperature
of about 36° F., were placed in a 25° room. Freezing of the out-
A.M. neon a
PM.
FIGURE 3.—Rate of cooling of rutabagas in a burlap bag and of new cabbage
in a crate at different starting temperatures in an 18° F. room. Packages
were not protected by paper.
UES CIRCULAR 713, U. S. DEPARTMENT OF AGRICULTURE
side of the most exposed roots began in about 2 hours. Ata depth
of three-quarters inch it did not begin until after 7 hours. After
24 hours at 25° the most exposed heads of cabbage (example 7,
p. 27) were frozen to a depth of one-half to three-quarters inch but
the heads at the center of the basket were still unfrozen. After 24
hours all the rutabagas were frozen solid.
It should be remembered that in all the examples given, the test
lot consisted of only one or two commercial packages. Under such
circumstances the rate of cooling would be more rapid than in the
interior of a stack of packages equivalent to a truckload or a half
or a whole carload. The rates of cooling shown are about what
could be expected in the outside packages of large stacks or blocks.
The difference shown in figure 3, namely that cabbage, except
for the outermost leaves of exposed heads, froze much more slowly
than rutabagas, probably depended directly on a difference in the
physical characteristics of the two vegetables. The cabbage heads
were loose and were therefore protected internally by the insulat-
ing effect of numerous air spaces between the leaves. Rutabagas
are solid and therefore without protective internal air spaces.
Another important difference between the two vegetables is that
cabbage produces heat more rapidly (respires more rapidly) than
rutabagas. This extra heat in containers packed with cabbage
would give added protection against freezing.
A slow rate of freezing similar to that observed in cabbage has
been noted in masses of lettuce and spinach in crates or baskets
and to some extent in other leafy vegetables and in peas and snap
beans in hampers or baskets. The reasons for the retardation of
freezing in all these instances are undoubtedly those mentioned
for cabbage—air spaces within the mass of packed produce and a
rather high rate of production of heat by the produce.
In the consideration of examples 6, 7, and 8 an important fact to
be noted is that the cooler the vegetables when placed in the freez-
ing room the sooner they froze. This important fact must always
be kept in mind by those concerned with the handling and protec-
tion of fruits and vegetables that are likely to be subjected to
_ freezing temperatures.
FREEZING NOT ALWAYS INJURIOUS TO PRODUCE
At this point a word of caution is necessary. It should not be
assumed that produce is ruined and must be counted a total loss
merely because it is found frozen. Much depends on (1) what the
produce is, (2) how low its temperature goes during the freezing
period, (8) how long it remains frozen, and (4) how it is handled
after freezing occurs. Suggestions concerning this last point will
be found on page 38. (See also pages 2 and 4.) Careful attention
to them should result in saving produce that otherwise might be
thrown away or unjustifiably sacrificed on the market. Informa-
tion on items 2 and 3 may not always be available, but if it is it
should be taken into consideration. As stated previously (p. 2).
hard freezing, say at a temperature of 15° to 18° F., is more in-
jurious than freezing at a temperature only slightly below the
freezing point of the produce, especially if the low temperature is
FREEZING INJURY OF FRUITS AND VEGETABLES 29
maintained for several days. Under such extreme treatment,
fruits and vegetables may become frozen to death and do not re-
turn to normal appearance or condition when they thaw. Repeated
freezing and thawing are injurious to products, such as apples,
sprouting broccoli, brussels sprouts, cabbage, cauliflower, and
onions, that are injured very little if at all by only one light freez-
ing unless they are handled while frozen.
Certain fruits and vegetables are susceptible to chilling injury
by temperatures that are not low enough to‘ cause them to freeze.
The relative susceptibility of these products to’such injury is
shown in tables 2 and 38.
TABLE 2 Commodities susceptible to chilling OEE at only moderately low
temperatures (45° to 55° F.)1
Commodity Freezing Dangerous Limit of safe Character of injury
poin temperature exposure
OO oF,
Avocados....... 27 Below 422... 4 weeks to 2 months,
depending on variety...} Internal browning.
Bananas......... 30 Below 55 Only a few hours............. Dull color when rip-
OTibiGrs se. i ened.
Lemone........... 28 Below 50...... About 4 wWeekS.........ccccccce Pitting, membranous
stain, and red blotch.
Limes.............. 29 Below 45...... 6 tO 8 WeEEKS...........cccccsecese Rind spotting.
Pineapples.... GBR BERS dove an 1 to 3 weeksun........ eee Dull green color when
ripened.
1 Temperatures lower than these cause more serious injury.
2For commercially important varieties.
3 Mature green, 29.1° ; ripe, 29.9°
TABLE 3.—Commodities susceptible to chilling injury at temperatures
close to 32° F.
Commodity Freezing | Dangerous | Limit of safe Character of injury
point temperature exposure
° 4 of,
Cranberries......... 27 Below 34....| About a month.| Rubbery consistency and
diffusion of red _ =,color
through the flesh. /
Cucumbers.......... 30 Below 40..... About 2 weeks.) Water-soaked spots on sur-
face.
Eggplant............. SOR UTS dove... 2 About 10 days..| Internal browning: and
dried-out surface spots.
Potatoes, white.. Des lene s.. dots. 3.4 About 1 week..! Sweetening.
(AO Tee About 2 to 3
Sweetpotatoes.... 29 weeks. Decay.
50 to 55...| 6 weeks to 2 |
months. J
(SQ R Ee About 8 days | | Susceptibility to decay ; fruit
Tomatoes ........... 30 | of poor quality and color
ARH ate 2 to 3 weeks J when ripened.
From the results of experiments, the observation of produce
under commercial conditions, and the experience of commercial
operators various fruits and vegetables can be grouped as follows
on the basis of their susceptibility to freezing injury.
30 CIRCULAR 713, U. S. DEPARTMENT OF AGRICULTURE
Freezing Freezing
point (°F.)1 point (°F.)1
Most susceptible: Moderately susceptible—Continued.
Asparagus: 4.031: FeoU et 30 Grapes® LU 5 38 TOS. 25-28
Avecados() 2: 2I3L Oe eis 27 Bettugerhasrnh fin mage 31
Bananas ps csc. sie eile... 30 Qniens | ciAsactcl.-cpssteie 30
Beans, tima .1000 eee org 30 Oranges and grapefruit...... 28
BEANS, “SAD et sche te cee 30 Parsi¢y -0. 0a ee eee (?
BOTS ooo ooo ches panccsizecs ss ee 3C Peaches and plums .............. 29
Cuctimbers 02..0L 3G 0e yr 30 Pear’ (2.000. sii see: 28
Beeplant! :5ct265:44. 245 30 Peas: seecitbesish ieee cee 30
demons we Ris 2 eeu Ade a pauash, Winter ..ots0 5. 29
WEMIOS Go cakeweccpebanctant seahorse cosseabounes Least susce tible:
PeEPPeTs ......ssesse-csnesseeee eee 30 Beets 2s 5 25idae ah ay 27
Potatoes; white (2.0.20 .2.. 29 Brussels Sprouts ceccsccccsessese- ee
Squash, summer .................. 29 Cabbage (old and Savoy).. 31
Sweetpotatoes ...2.4.2....204 29 Carrots .................. 29
Tomatoes =. 30 Cauliflower... 230
Moderately susceptible: PAVE AE: BIS ON t seat ete (7)
Amples 2205 a ene ere 28 Parsaips to te 30
Broccoli, sprouting .............. 29 Rutaharas 3.005... ee 39
Gabbage (new) | ..2.230.2..2... 31 : Saisttg, WR eet oils a 29
“07s Per en Ramee scree eS Sn RAINE 30 PIMACI thet hc ee oe 30
(Cranberries. ..5: 8 eee oe a “at Tearnips, obe.5. 50h eee ee 30
1In most cases these are the approximate average freezing points; it should be
remembered that individual specimens may freeze at slightly higher or slightly lower
temperatures.
2 Exact freezing point not Known.
In this tabulation no attempt is made to arrange the commodi-
ties in order of susceptibility to freezing injury within each of the
three groups. It is felt that probably no two persons, no matter
how experienced, would agree on what the order should be.
Furthermore, the susceptibility of various fruits and vegetables to
freezing and to some extent to freezing injury will depend on
whether they are packed tightly or loosely in the containers,
whether they are wrapped or not, and whether they are in tight
boxes, baskets, or barrels or in slatted crates or hampers. Varia-
tions in any of these factors might make a difference in determin-
ing in which group a given commodity should be placed. This fact
would be of most importance if the classified list were used by
commercial operators as a guide in handling the various oe
ties in severely cold weather.
It is evident from the tabulation that the freezing point of a
commodity gives little indication of the damage to be expected
from freezing. For example, tomatoes and parsnips have the
same average freezing point (30° F.), yet parsnips can be frozen
and thawed several times without serious injury whereas toma-
toes are severely damaged or utterly ruined by one freezing.
Bananas freeze at 30°, avocados at 27°, and spinach at 30°.
Bananas, however, are chilled by exposure to temperatures below
55° to 56° for more than a few hours and do not color normally
afterward when ripened at 65° to 70°. Avocados are very sensi-
tive to low temperatures; some varieties are injured by exposure to
temperatures of 50° to 53° for 15 days, and even the most resistant
cannot safely be stored at a temperature as low as 37° for more
than about 4 weeks. Spinach will withstand repeated freezing
FREEZING INJURY OF FRUITS AND VEGETABLES 31
and thawing without sustaining serious injury if it is not handled
while in a frozen condition.
The freezing point of cabbage is 31° F. and of white potatoes
29°. Cabbage, however, can be frozen and thawed two or three
times without being permanently damaged, provided the tempera-
ture does not go so low that the cabbage is frozen to death and it is
not handled while frozen; potatoes, on the other hand, are perma-
nently, although only slightly, injured if some parts of the tubers
are in a frozen condition for only 15 minutes.
If the root crops listed in the third section of the tabulation
have tops, the bacterial soft rot organism is likely to attack the
tops if they become frozen and then thaw. The roots of beets,
carrots, salsify, and parsnips are not visibly damaged by moderate
freezing if not handled while frozen; after thawing they are usable
for food and should be salable if the tops are removed.
For sale by the Superintendent of Documents, Washington 25, D. C.
Wu. S. GOVERNMENT PRINTING OFFICE: 1944—608109
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