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Heat treatment also affects flavor characteristics of fruits and vegetables. For
example, titratable acidity declines in apples held for 3 or 4 d at 38°C, while the
concentration of soluble solids is not affected by heating. The same effect has been
shown in other fruits such as nectarines and strawberries, while in other fruits, such
as tomatoes and grapefruits, the effect was not seen. Heat treatment also increases
sugar content of some fruits and vegetables. For example, when squash fruits were
subjected to 3-h heat treatment at 30°C, their sucrose content increased significantly.
The heat-treated squash were perceived as sweeter by a test panel (Bycroft et al.,
1997). Likewise, heat-treated Golden Delicious apples treated for 4 d at 38°C were
perceived as crisper, sweeter, and overall more acceptable than unheated fruit. In
the apples, the sweetness was due more to a decrease in acidity than to an increase
in sugar content.
8.9.3 CHILLING TREATMENTS
Overexposure of produce before and during harvest to high environmental temper-
atures might be harmful to the quality of fruits and vegetables, such as when produce
is harvested during the summer. Precooling is one of the methods used to remove
heat from fruits and vegetables when they are taken from the field. The aim of
precooling is to quickly slow down the respiration rate, minimize microbial growth,
and reduce transpiration rate. Various methods are used for chilling produce. These
include air cooling, hydrocooling, and vacuum cooling. Air cooling involves forced
cold air (~ 1.5°C) flow through the produce in cold rooms. Hydrocooling involves
submerging the fruits or vegetables in ice-cold water to immediately lower their
temperature. This is done for only a limited time and is followed by another cooling
method such as air cooling. Vacuum cooling is a rapid method of cooling vegetables.
The vacuum cooler consists of a large autoclave with steam injectors that works by
evaporative cooling. In this case, since water can be removed from the produce, it
must be properly packaged in nonporous material to minimize wilting (Salunkhe et al.,
1991).
Improper chilling treatments can result in chilling injuries in both fruits and
vegetables. A chilling injury is a disorder induced by low, but not freezing, temper-
atures that occurs in certain susceptible fruits and vegetables. Usually this damage
can occur in tropical fruits and vegetables when they are stored at low refrigerated
temperatures. Chilling injury may occur most often in sweet potatoes, bananas,
apples, olives, oranges, tomatoes, avocados, cucumbers, lemons, limes, mangos,
melons, papayas, eggplants, and other tropical and subtropical fruits and vegetables.
Chilling injury induces decay by enhancing tissue breakdown, which would lead to
nutrient losses. Some of the chilling injury effects observed in various fruits and
vegetables are internal browning (apples, avocados, olives, pineapples), pitting
(oranges, avocados, lemons, limes, melons, papayas, tomatoes), brown stains
(oranges, bananas, cucumbers, lemons, mangos), surface scalds (apples, eggplants)
and soft rots (apples, tomatoes, melons) (Salunkhe et al., 1991). Chilling, similar to
heat treatment, can potentially have negative physical and nutritional effects that can
be minimized by strict temperature control.