630 Produce Degradation: Reaction Pathways and their Prevention
high temperature causes little damage. Since subjecting plants to one form of stress
can sometimes confer greater tolerance to other stresses, it has been proposed that
HSPs may remain in the tissue and provide thermotolerance during cold storage.^3
Because high temperatures increase membrane leakage but the tissue recovers
after the heat stress is removed, it has been postulated that high-temperature treat-
ment also may cause alterations in the cell membrane that induce tolerance to chilling
injury.^13 Comparison of the lipid composition of apple plasma membrane and total
tissue lipids of tomato showed that samples heated prior to cold storage had more
phospholipids and greater fatty acid unsaturation than did unheated fruit. These
differences in fatty acid composition would indicate more fluid membranes in the
heated fruit. This condition would correspond with reduced indiscriminate leakage
in the tissue of heated fruit.
Chilling injury was reduced in some commodities by interrupting cold storage with
one or more short periods of warm temperature. This treatment was found effective
with apples, citrus, cranberries, cucumbers, nectarines, okra, peaches, plums, potatoes,
sweet peppers, and tomatoes.^33 Ben-Arie et al.^49 reported control of woolliness in
peaches if the fruit was warmed to room temperature (not specified) for 2 d every 2
weeks during a 6-week storage at 0°C. Lill^50 showed that warming peaches to 12°C
was effective in preventing injury for a longer time than was warming to 20°C.
Although intermittent warming has been shown to be effective in reducing
chilling injury of some fruits and vegetables, the mechanism of this effect is not
well understood. The warming treatment may allow the tissues to metabolize toxic
substances that were accumulated during chilling, or warming may allow tissues to
restore materials that were depleted during chilling.^30
In work with apples, Hulme et al.^46 observed that low-temperature breakdown
was preceded by an accumulation of oxaloacetic acid. Removing the apples from
0°C storage to 15°C storage for 5 d then returning them to 0°C could reduce both
the oxaloacetic acid content and the subsequent intensity of the low-temperature
breakdown. They suggested that the low-temperature breakdown was caused by
interference in the operation of the Krebs cycle in the tissue.
Reduction of woolliness in peaches and nectarines by intermittent warming
seems to be related to the warming treatments leading to more normal ethylene
production by the fruit after storage.^3 Chilling injury resulted in reduced ethylene
production and altered cell wall hydrolytic enzyme activities. In healthy fruit, polyg-
alacturonase was the major enzyme leading to softening and juiciness. In chill-
injured fruit, this enzyme activity was absent, and softening seemed to depend on
endoglucanase. At the same time, pectin molecules bound the extracellular juice,
causing the woolly condition.
Another method to prevent chilling injury in some fruit is a dual-temperature
method. For example, plums are rapidly cooled and stored at 0 to –1°C for 10 to
14 d.^3 The temperature is then raised to 7°C for another 10 to 14 d. This prevents
the storage disorders that would develop at 0°C, yet 7°C is still low enough to prevent
over-ripening. A reverse procedure in which the fruit is stored first at a warmer
temperature and then the temperature is gradually lowered has been shown to prevent
injury of some produce. Lemons can be stored at 6 to 8°C if the temperature is
gradually lowered over a week to the storage temperature. Avocados can also be