Temperature Effects on Produce Degradation 631
stored below 10°C when they are given a step-down temperature treatment. These
regimens allow the produce to acclimate and resist low-temperature stress.
Controlled atmospheres, which are depleted in oxygen and/or enriched in carbon
dioxide relative to air, are often used in conjunction with cool storage.^30 The effects
of these altered storage conditions on chilling injury are variable. With some products
atmospheric changes serve to intensify the inhibition of metabolic pathways caused
by chilling and lead to increased injury.
The beneficial effects that are sometimes observed when controlled atmospheres
are combined with chilling are more difficult to explain.^30 For example, peaches
cool-stored in air at 1°C lose the ability to ripen normally when removed from
storage. However, peaches stored in 20% volume/volume carbon dioxide retain the
ability to ripen. Although controlled experiments have verified that the effect is due
to the carbon dioxide, there are many possible sites of carbon dioxide action in the
cell and the exact mechanism of this effect is still unclear.
Chemical treatments have been found to reduce the injurious effects of chilling
on some produce.^28 Antioxidants such as diphenylamine, ethoxyquin, and butylated
hydroxytoluene have been useful in reducing the development of scald in apples.
Similar results were seen when oil was applied to the fruit and when treatments of
gibberellic acid and phorone were used. The fungicide thiabendazole reduces surface
pitting in cool-stored grapefruit. The mode of action for these responses is unknown.
Good correlations have been found between tissue calcium content and the suscep-
tibility of the produce to chilling injury.^30 Produce with relatively high calcium is less
likely to suffer from problems due to cool storage than is produce with low calcium.
The application of calcium to fruit after harvest reduces the incidence of disorders such
as low-temperature breakdown of apples. Variations in tissue calcium may explain why
various units of the same commodity do not respond uniformly to chilling.
20.4.2 FREEZING INJURY
The lower limit for normal metabolism is the freezing point of the tissue. Because
of the concentration of soluble solids dissolved in the water in the cell sap, freezing
of produce occurs at temperatures below 0°C.^20 Soluble solid concentration varies
not only with the type of fruit and vegetable but between individuals and even parts
of the same fruit or vegetable. It is difficult to cite precise values for freezing points
of a particular commodity since the actual freezing point will vary between cultivars,
and with such factors as the conditions in which the crop is grown and previous
storage history. In general, leafy vegetables, which are fairly low in sugar, freeze at
about –0.5°C, while high-sugar fruits generally freeze at –2 to –5°C.
All fruits and vegetables can be classified into three groups based on their
sensitivity to freezing^29 :
- most susceptible: those that are likely to be injured by even one light
freezing - moderately susceptible: those that will recover from one or two light
freezings - least susceptible: can be lightly frozen several times without serious injury