Food Technology and Nutrition Mimeo Report 59-1
VEGETABLE LIFE AFTER HARVEST
R. K. Showalter
Florida Agricultural Experiment Stations
A paper presented in the symposium on
"Recent Research on Post-Harvest Physiology"
at the joint session of the American Society for Horticultural
Science and the American Society of Plant Physiologists sections
Association of Southern Agricultural Workers
Memphis, Tennessee, February 3, 1959
VEGETABLE LIFE AFTER HARVEST
R. K. Showalter
A number of advances have been made recently in our knowledge of
factors affecting the storage life of vegetables. During their grow-
ing period vegetables manufacture and store food. After harvest the
process is reversed and stored foods supply energy to maintain life.
Length of storage life is governed chiefly by transpiration, respira-
tion, and attacks by decay organisms. Water loss by transpiration
with consequent wilting and shriveling is most easily controlled by
maintaining a high relative humidity.
POST HARVEST RESPIRATION
Respiration of fresh vegetables can not be stopped except by
killing as in heat sterilization. However, respiration rates can be
retarded by lowering the temperature or modifying the atmosphere.
Several investigators have suggested that the cumulative CO2 produc-
tion may be a measure of the storage life. Thus apples, cucumbers,
and bananas each produce a total of 18 to 20 grams CO2 per kg. of
fruit. This assumes that a given amount of respirable substrate is
available for respiration and that the produce reaches the end of
its useful life when this substrate is exhausted. Even though as-
paragus respires 59 times faster than potatoes (21), the storage
life of both can be prolonged by lowering their temperatures. Many
vegetables have a more or less predictable increase in storage life
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at decreasing temperatures down to 320 F.
Vegetables with high respiration rates, such as asparagus, sweet
corn, broccoli, and peas, are often the most perishable, and their
storage life is consumed rapidly after harvest. For example, Lip-
ton (14) in a recent study of asparagus deterioration at 10 tempera-
tures, found that the total storage life ranged from 2 1/2 days at
860 to 44 days at 36. The high rates of deterioration were gener-
ally accompanied by high rates of respiration.
VACUUM COOLING PROCESS
Since respiratory activity is usually high if the vegetable
temperature is high following harvest, the rapid removal of field
heat is important. Vaclum cooling is a recent development for ex-
tracting heat quickly from lettuce, sweet corn, cabbage, celery and
other leafy vegetables. The temperature reduction is produced by
extremely rapid evaporation of water from these vegetables when they
are subjected to an almost complete vacuum. When the air pressure
is reduced to about 4.6 mm. of mercury, the boiling point of water
is 320. Evaporation within the vegetable tissue requires heat to change
water to vapor. During the change from a liquid to a vapor phase
much field heat is removed and the evaporative cooling is continued
until the vegetable temperature is slightly above 320. The vapor-
ization of one pound of water lowers the temperature of 100 lbs. of
leafy vegetables about 100 F.
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A. Vacuum Cooling and Moisture Loss
Since there is a water loss of about one percent for each 100 F.
of cooling, one might expect excessive wilting. However, no visible
wilting or injury is usually noted immediately after vacuum cooling.
After shipment and storage of vacuum cooled lettuce there is slightly
more wilting of the outer leaves as compared with ice-packed lettuce.
However, more bruising injury and decay are usually found in ice-
packed lettuce so that vacuum cooling yields a larger quantity of
Early studies were made by Barger (1) to determine if minute in-
juries occurred on the surface of lettuce heads, grapes, and straw-
berries during the vacuum cooling. No differences in appearance or
weight loss were found between vacuum cooled and non-treated checks
during subsequent storage. Thus it appears that the water vapor
molecules originating from all parts of the fruit or vegetable eva-
porate without injuring the cells. Also when the moisture loss occurs
from all parts of the produce, wilting is not as apparent on the
I have found in current studies that vacuum cooled sweet corn
had temperatures of 350 400 compared with 55 600 temperatures
obtained by hydrocooling. Fifteen percent more total sugars were
retained by the vacuum cooled corn during storage for 2 to 8 days.
The loss of five percent moisture during vacuum cooling resulted in
objectionable denting of kernels during storage. This denting was
eliminated by adding water to the ears before and after the cooling
process (29). Thus, in connection with this more effective precool-
ing method, new water relationships became apparent.
B. Wetting with Water Maintains Sweet Corn Moisture
Many vegetables absorb enough water during hydrocooling, icing,
or sprinkling to prevent wilting during marketing. Even though corn
kernels are covered by lignified pericarp, several workers have found
that all surfaces readily absorb water (30). Since corn can be vac-
uum cooled and other vegetables with relatively impervious skins can
not be cooled by this method, it may be assumed that water also is
readily transpired from the kernel surfaces. The moisture content
of the kernels after several days storage was not as high in those
ears which were dipped in water only subsequent to vacuum cooling
as in those dipped before and after cooling. To prevent denting it
appears that surface water is necessary when the diffusion pressure
deficit is greatest at the moment the vacuum is released.
STORAGE LIFE AT LOW TEMPERATURES
Recent studies have been made by Parsons (17, 18, 19) on the
storage life of three vegetables which responded favorably to low
temperatures. Lettuce was in much better condition at 320 than at
380 after 6 weeks storage. After 8 weeks storage there was 18 per-
cent more edible celery at 320 than at 38, and none remained at 45.
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Cabbage stored well at 32 and 380 for 8 weeks, but the heads were
greener and more turgid at 32. Storing these vegetables in film
lined crates or film packages greatly reduced the moisture losses and
retained more of the original green color.
Not all vegetables have an increase in storage life as their
temperature is lowered to 320. Some vegetables are susceptible to
chilling injury at 320 to 500, while others may be frozen and thawed
without permanent injury. In the latter group we find parsnips with
a freezing point of 300. Tomatoes also have a freezing point of
300, but they may be injured at temperatures below 50. Instead
of the water soaked areas caused by freezing, the symptoms of chil-
ling injury are circular pits on the surface, discoloration, failure
to ripen properly and susceptibility to decay. Such vegetables as
cucumbers, peppers, egg plants, watermelons, sweet potatoes, squash,
snapbeans and tomatoes are subject to chilling injury. Several re-
cent investigations dealing with the deterioration, respiration and
biochemistry of chilling injury have been made, but the actual mech-
anism and true cause are still unknown.
A. Physiology of Chilling Injury
Eaks and Morris (7) evaluated the physiology of chilling injury
by the responses of cucumbers to chilling and non-chilling tempera-
tures. cucumbers stored at five temperatures from 550 to 860 all
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produced approximately 20 grams CO2 per kg. of fruit during their
entire storage life. Cucumbers held at chilling temperatures produced
the following amounts of CO2: 50 18 grams, 410 6 grams, 320 -
3 grams. Cucumbers exposed to chilling temperatures and transferred
to 77, produced less than 20 grams CO2, the amount depending upon
the severity of chilling. At non-chilling temperatures the rate of
CO2 production decreased with duration of storage, whereas at 500
and below, the rate increased with time to a plateau and then de-
creased. The increasing rate occurred at the same time as the de-
velopment of chilling injury as measured by the degree of surface
pitting, and the decline occurred as the tissue died.
The storage life of cucumbers at non-chilling temperatures in-
creased from 16 days at 86 to 62 days at 55. Lower temperatures
reduced the storage life instead of increasing it as for the vegeta-
bles previously discussed. The average storage life was 48 days at
500, 18 days at 41O, and 24 days at 32.
B. Symptoms of Chilling Injury
High relative humidity has been found to delay pitting of cu-
cumbers and peppers (16). The severity of the localized desiccated
areas was inversely proportional to the relative humidity of the
storage atmosphere. Chilling injury develops slowly. Cucumbers
held at 320 appeared field fresh for about two weeks. Eaks and
Morris (11) then reported small droplets of exudation on the surface
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which indicated a disturbance in physiological balance. It is not
known whether desiccation of epidermal cells is the cause or effect
of pitting. Histological studies showed that cells near the pitted
area had lost most of their water content.
Differences in cell permeability resulting from chilling injury
of sweet potatoes was found by Lieberman et al (13). The leakage
from cut slices of sweet potatoes stored at 450 increased rapidly
during a 3 to 10 week period. Leakage from potatoes held at 600
remained constant. There was approximately 5 times much leakage
from the chilled tissue slices as from the non-chilled slices, and
almost all of the leakage consisted of potassium ions.
Chilling temperatures also produce color differences in vege-
tables. Cut sweet potatoes darken rapidly after prolonged storage
at low temperatures. Lieberman et al recently found that the dark
pigment formed in sweet potatoes may be associated with the formation
of chlorogenic acid and the decrease in ascorbic acid after 5 to 6
weeks chilling at 45. Tissue extracts from sweet potatoes chilled
for a shorter period did not turn black. Chemical analyses for the
major constituents of vegetables have generally failed to give spe-
cific information on the mechanism of chilling injury.
The localization of chilling injury was studied by Eaks and
Morris (8) by exposing one half of intact cucumbers to 350 and the
other half to 55. When the cucumbers were transferred to 770 after
8 days, the chilled ends appeared slightly fresher than the non-
chilled ends. However, severe pitting developed on the chilled ends
after 3 days and decay after 4 days. Decay failed to develop on
the non-chilled portions after 8 days. If a toxic substance was
responsible for the injury, it was not translocated, or it was des-
troyed in the warmer end.
In 1941 Ramsey and Wiant (22) wrote that nothing was known re-
garding the cause of russeting in green beans. These brown surface
lesions were frequently a serious marketing problem, and no fungi or
bacteria could be found in them. In 1958, Lewis (12) reported that
russeting was influenced by temperature, length of storage, and
sprinkling. Beans held at 320 or 400 for 5 or 10 days became severe-
ly russeted and discolored during the succeeding 1-day period at
750- 800. Unlike the chilling injury of cucumbers and peppers which
was inhibited by high humidity, this injury of beans was more pro-
nounced on sprinkled than on non-sprinkled beans. Russeting develops
slowly, since beans held 4 days or less at 320 did not show much
injury when moved to higher temperatures. This indicates that chil-
ling injury is reversible up to a certain point. Eight days of
continuous refrigeration were required to produce visible symptoms
C. Mechanism of Chilling Injury
When cucumbers were stored 2 days at 320, 4 days at 41, and
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8 to 16 days at 500, followed by transfer to 77, no visible symp-
toms of chilling appeared, but some change had occurred which affected
the respiratory rates to varying degrees.
The activity of mitochondria from sweet potatoes stored at 450
and 60 were compared over a 10 week period (13). There was very
little difference in activity during the first 4 to 5 weeks, but
after the 8th week phosphorylation was about eliminated. This data
indicates that the chilling effects on the mitochondria were rever-
sible during the first 4 weeks.
In another recent study Lewis (11) showed that temperatures
affect the protoplasmic streaming differently in chilling-sensitive
and insensitive plants. By observing individual cells with a micro-
scope he noted that the streaming in tomato, watermelon, honeydew
melon and sweet potato plants practically ceased after 1 or 2 minutes
at 500, and it ceased entirely at 40 or 32. In contrast, the
streaming in radishes and carrots proceeded at 360 and 320. Proto-
plasmic streaming proceeded slowly for 3 days at 320 in the chilling
insensitive plants and resumed rapid streaming in 1 to 2 minutes when
returned to 680. However, for the tomato, increasing the exposure
time at 320, increasingly delayed the resumption of streaming when
transferred back to 68. If the exposure exceeded about 24 hours the
streaming failed to resume.
Tomato fruits are also susceptible to chilling injury before
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ripening when held at temperatures of 320to 500. The symptoms, which
are not usually apparent when the tomatoes are at chilling tempera-
tures, consist of susceptibility to Alternaria rot, mottled, off-
color red ripening, and dark, sunken surface scars. After prolonged
holding at low temperatures the physiological processes may be so
impaired that ripening will not occur when the fruits are returned
to favorable ripening temperatures.
Chilling of tomatoes may occur before or after harvest and both
are cumulative. Several years ago McColloch pointed out the high
correlation between growth of Alternaria lesions and the number of
days that mature-green tomatoes were stored at chilling temperatures.
It is interesting to note that the increased susceptibility to in-
fection takes place slowly and is also reversible as was the injury
to cucumbers, beans, and sweet potatoes. Thus, tomatoes held at
320 to 400 for 3 to 5 days ripened satisfactorily at higher tempera-
tures. Whereas, after 9 to 12 days chilling the ripening was un-
satisfactory, and after 17 to 21 days all the tomatoes decayed with-
out ripening (23).
The reversible nature of chilling in tomatoes was recently con-
firmed by Lewis (10) who reduced the harmful effects of 320 storage
by interrupting the chilling periods with periods of non-chilling
D. Maturity Affects Tomato Chilling Injury
This discussion of tomatoes has dealt only with those harvested
before ripening. Recently, increasing attention has been focused on
vine ripened tomatoes. Difficult handling problems were presented
by the following factors:
1. Long time required to market tomatoes from distant produc-
2. Rapid ripening and softening rates after harvest.
3. Consumers' rejection of soft ripe tomatoes.
A partial solution to these problems may be found in precooling,
controlled transit temperatures, and possibly modified atmospheres.
In 1957 McColloch (15) found that pink tomatoes in cartons could
be precooled from 930 to 450 500 in 21 hours by circulating 320
air. When they were harvested showing 25 30 percent color, pre-
cooled to 500, and held for 3 days at 500, the tomatoes had 40 to
70 percent red color. If held 3 days at 550, they were 63 to 75
percent colored. Thus, it is evident that temperatures must be con-
trolled rather accurately to retard ripening and maintain tempera-
tures above 500.
There is some evidence that pink and ripe tomatoes can be stored
successfully at lower temperatures. Scott and Hawes (27) showed
that pink and ripe tomatoes could be stored at 320 for 6 days, and
then ripening completed at 720, with no evidence of chilling injury
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or impairment of flavor. Storage for 12 days at 32 before the ripen-
ing was complete did produce off flavor. These findings approximate
those of McColloch for mature green tomatoes as stated above where
3 to 5 days chilling was not injurious at subsequent higher tempera-
In 1958 McColloch (4) reported that firm ripe tomatoes could be
stored for 42 days at 320 with a decay loss of only 3 percent. Ripe
tomatoes stored better at 320 than at 380 in these tests. The toma-
toes were edible, had a good appearance, but had softened. It was
recommended that these tomatoes be eaten within a few hours after
removal from 320. These results were somewhat striking. It appears
that chilling injury develops much slower, or not at all, in ripe toma-
toes, and that they can be handled at temperatures as low as 320 to
prevent over ripening.
Recent studies (24) in California have shown that the ripening
rate of vine ripened tomatoes can be decreased by increased CO2 and
decreased 02 during storage at 680 for 2, 4, and 8 days. The use of
modified atmospheres to control ripening during transit periods of
4 days or less seemed to hold promise, but longer periods reduced the
number of marketable tomatoes.
E. Summary of Chilling Injury
In summarizing the subject of chilling injury it may be defined
as physiological injury resulting from low storage temperatures above
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the freezing point of the tissue. Some of the first evidences of
chilling injury are prompt ceasing of protoplasmic streaming, changes
in respiration rate and leakage from tissues. During later stages
pitting or color changes occur, chlorogenic acid increases and as-
corbic acid decreases. Chilling effects are reversible only to a
certain breaking point beyond which vegetables soon decay or die.
It appears to me, on the basis of the papers reviewed, that pitting,
desiccation of epidermal cells, and decay are secondary symptoms
caused by increased cell permeability, leakage, plasmolysis, and
production of a toxic material. Since pitting can be delayed by
high relative humidity, pitting must be one result of rapid water
Mitochondria from chilled and non-chilled tissue showed equal
activity during the 1st four weeks of chilling and then the activity
in the chilled tissue rapidly declined. It appeared that mitochondria
were not affected by temperature, but rather by the accumulation of
a toxic substance. The fact that chilling injury can be reduced by
intermittent periods of non-chilling temperatures further suggests
the production and elimination of a harmful substance.
This discussion of factors affecting post harvest vegetable
life would not be complete without including irradiation. The ef-
fects of irradiation on deterioration by microorganisms and on normal
living processes must both be considered. It has been known for many
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years that ionizing radiation would kill bacteria, but only during
the last 15 years has this approach been considered for extending the
storage life of fresh vegetables. When radiation passes through mic-
roorganisms or vegetable tissue it causes ionization in the proto-
plasm of the cells. The number of ionizations are increased by in-
creased amounts of radiation, but the actual number occurring in
living tissue have not been measured. Estimates of the number of
ionizations produced by one rep of radiation in an average sized
cell vary from 2 to 2,000. It is known that high doses produce
considerable alteration of the chemical composition, normal physi-
ology, and even structure of living cells (6).
A. Effects of Irradiation on Microorganisms
Many studies have been made to determine the effects of radia-
tion on various spoilage organisms. It has been difficult to meas-
ure any specific type of cell damage even though the organism may
be lethally injured upon entering division. Thus, bacteria may show
no drop in respiration rate after exposure to 60,000 reps, and yet
only one in 10,000 of these bacteria will be able to multiply.
Doses of 100,000 to 500,000 reps will kill most of the common
spoilage organisms. However, to kill the spores of the same species
and get complete sterilization 2 to 5 million reps are required.
Thus two classifications of radiation treatments for controlling
deterioration by microorganisms have been established (28). The
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lower doses, below 500,000 reps, may be considered pasteurization
since they require post-irradiation refrigeration to prevent the
growth of residual organisms.
B. Effects of Irradiation on Vegetable Physiology
Another application of radiation has been sprout inhibition in
root crops. Much research has been done on irradiating potatoes.
Sparrow (32) reported in 1956 that irradiated potatoes were stored
satisfactorily for 10 months at room temperatures and 2 years at
420 without sprouting, with low weight loss, and with little soften-
ing or wrinkling. Sawyer and Dallyn (25) found that 8,000 reps of
gamma irradiation entirely inhibited sprouting of sweet potatoes and
onions, and 10,000 reps were equally effective for Irish potatoes.
In 1956 when a large quantity of potatoes were irradiated and
stored under commercial conditions it was discovered that storage life
was not actually increased because of a high incidence of decay. The
sprouting was inhibited by several high levels of radiation, but the
potatoes darkened after peeling and had off odors. The increase in
decay was attributed to a delay in suberization following irradia-
tion and the prevention of periderm formation. Since the function
of periderm is protection from drying of the tissues, the increased
weight loss was understandable.
Heiligman (9) evaluated doses of 5,000 to 200,000 reps for
potatoes, and found that a minimum of 10,000 reps inhibited sprouting,
while the higher levels increased weight loss and decay. Schwimmer
et al (26) studying some physiological responses to potato irradia-
tion found an increase of 210 percent in sugars, darker colored chips
after processing, and a slower rate of greening than in non-irradiated
tubers. The increase in sugars may be related to an increased res-
piration rate found in irradiated tubers or an enzymatic hydrolysis
of starch. In 1957 Sparks (31) confirmed the minimum dose to be
10,000 reps for sprout control of potatoes. However, his non-irrad-
iated potatoes, which had sprouted, had lost less weight after 3
months than any of the irradiated treatments. He also observed that
cells in the periderm of irradiated tubers were dead and plasmolyzed
after 6 months. Chamberlain (3) says potato irradiation is neither
ready for commercial acceptance nor the scrap heap.
The lack of enzyme inactivation is a serious disadvantage in
using radiation for increasing fresh produce storage life. Enzymes
may not be inactivated by radiation doses of 10 to 20 times the
magnitude required for sterilization. Thus, long-time storage with-
out refrigeration will probably require heat destruction of the en-
zymes. Microwave heating for uniform temperatures throughout the
material may have possibilities for enzyme inactivation.
C. Storage Life After Irradiation
Fruits and vegetables vary widely in their response to irradia-
tion; some are damaged seriously by low doses, while others will
withstand doses that considerably lengthen their storage life.
Pentzer (20) states that most fruits were injured by 200,000 to
300,000 reps, but peaches withstood 400,000 reps and grapes and to-
matoes 500,000 reps. Radiation looked promising for tomatoes, peach-
es, grapes and strawberries. Dennison (5) has shown that storage life
of lychees can be extended by cobalt-60 irradiation which reduced
deterioration by fungi.
Alternaria decay of tomatoes has been inhibited enough to leng-
then the storage life for 7 to 10 days at room temperatures (33).
Retention of sweetness and tenderness of fresh corn on the cob was
also reported after irradiation of 200,000 reps and 3 weeks storage
at 40. The softening effects of gamma irradiation on carrots, beets,
and apples were measured by Boyle et al (2), and tentatively related
to degredation of pectin and cellulose in the tissues.
Research on irradiation of many kinds of foods, both fresh and
processed, is being conducted in more than 100 laboratories in the
United States. Many of the results have been unfavorable from a
practical viewpoint because of the development of off flavors, colors
and odors. However, radiation research requires many new approaches,
and much progress has been made on the causes and remedies for the
It appears now that irradiation will only supplement refrigera-
tion for many fresh fruits and vegetables. Irradiation will probably
take its place in the future along with modified atmospheres, anti-
biotics, chemicals, and low temperatures for extending vegetable life
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