Temperature Effects on Produce Degradation 611
20.2.5 VITAMIN LOSSES
Produce is the most important source of many vitamins in the diets of most people.
Consumers generally regard any fresh produce as being extremely nutritious and far
superior to processed products.^14 This belief may not be true, however, since storage
conditions, especially storage temperature, can have a significant effect on vitamin
content.
Vitamin C is generally considered the most important vitamin in fruits and
vegetables for human nutrition. More than 90% of the vitamin C in human diets is
supplied by fruits and vegetables (including potatoes).^15 Vitamin C is most sensitive
to destruction when the commodity is subjected to adverse handling and storage
conditions. Losses are enhanced by extended storage, higher temperatures, low
relative humidity, physical damage, and chilling injury. The review paper by Lee
and Kader^15 provides an overview of the effects of preharvest conditions, harvesting,
and postharvest handling procedures on vitamin C content of fruits and vegetables.
Lee and Kader^15 reported that delays between harvesting and cooling can result
in losses of vitamin C due to water loss and decay. Holding temperature was found
to affect the vitamin C content of a variety of produce including tomatoes, kale,
spinach, cabbage, snap beans, and broccoli, and all citrus fruits lost vitamin C if
stored at high temperatures. The range of temperatures and the extent of vitamin C
loss depended on the type of citrus fruit. In general, the extent of loss in ascorbic
acid content, the principle biological form of vitamin C, in response to elevated
temperatures was greater in vegetables than in acidic fruits, such as citrus, because
ascorbic acid is more stable under acidic conditions.
Horticultural crops such as sweet potatoes, bananas, and pineapples that suffer
from chilling injury at low temperatures lose significant amounts of ascorbic acid
during storage.^15 Destruction of ascorbic acid can occur before any visible symptoms
of chilling.
In studies with kale, it has been demonstrated that increased storage temperature
and therefore accelerated wilting led to greater losses of vitamin C and carotene.16,17
Although these data were obtained from studies with kale, it has been accepted that
many other horticultural commodities would have similar responses.
20.2.6 SUGAR–STARCH BALANCE
Some produce contains appreciable amounts of carbohydrates that are susceptible
to changes in the constituent substances.^7 A prime example of such change is ripening
of fruit in which reserve carbohydrates in the form of starch are turned to sugar. At
any given temperature starch and sugar are in dynamic equilibrium, and some sugar
is degraded to carbon dioxide during respiration^2 :
starch ↔ sugar → CO 2In some vegetables (e.g., sweet corn and peas) a high sugar content is desirable.
These vegetables are harvested immature, when the sugar content is highest, and
stored at low temperatures to retard the conversion of sugar to starch. High sugar