Production Practices and Quality Assessment of Food Crops. Vol. 1

of more than 13 hours of night temperature below 15.6 °C during the week preceding
harvest would cause irreversible damage resulting in visible symptoms during
marketing. This treatment may, however, cause undesirable softening and increase
decay and may cause condensation to form on the product especially in storage
(Skog, 1998). Preconditioning can also be used. It is the stepwise cooling of the
commodity and can allow the vegetable to adapt to the cooler temperature and
minimize chilling injury development in storage.
Controlled or modified atmospheres (generally O 2 < 5%, CO 2 > 2%) can slow
plant metabolism and slow chilling injury development in certain crops. Controlled
atmospheres can also allow longer storage of chilling sensitive crops when stored
above their critical temperature (Skog, 1998). Modifying the atmosphere surrounding
certain fruits subject to a number of low temperature disorders has been reported
in some instances to alleviate or delay the symptoms usually associated with chilling
injury (Lyons and Breidenback, 1987). Reduced O 2 and elevated CO 2 can overcome
the impact of low temperature injury on the ripening process (Wade, 1979; Ilker
and Morris, 1975). Controlled atmospheres may in some cases further stress crops
and increase chilling injury susceptibility for crops such as cucumbers, tomatoes and
asparagus (Skog, 1998). Maintaining a high RH around the commodity during
storage, as occurs with film wraps and modified atmosphere storage, reduces the
severity of chilling injury symptoms by minimizing desiccation.
Proper pre-harvest nutrition can minimize chilling susceptibility. Calcium treatments
may stabilize cellular membranes and reduce chilling injury in certain
commodities. Ilker and Morris (1975) found that treatment with solutions of calcium
and potassium salts could afford some protection against chilling injury in okra. The
severity of chilling injury in squash (Cucurbita moschata) fruits, stored at 4 °C
was reduced by dipping in 1% CaCl 2 or 10 mM sodium benzoate at 20 °C for 30
minutes (Lee and Yang, 1999). Fruits treated with sodium benzoate showed a low
incidence of chilling injury (< 10 °C) following 30 days of storage at 4 °C.
Reports examining possible approaches to alleviating chilling injury conclude that
the most likely solution occurs through genetic modification (Graham and Patterson,
1982; Couey, 1982; Bramlage 1982). A better understanding of the primary causes
of chilling injury will provide information for the development of rapid methods
for screening potential germ-plasm and possibly control of this disorder. The debate
as to whether there is a single primary response and whether the membrane plays
a central role as the primary temperature sensor remains unresolved.
3.2. Freezing injury
Effect of Preharvest Factors 11
Commodities may be frozen in the field before or during harvest, in transit and
storage. Vegetables vary in their susceptibility to freezing injury. Some may be
able to go through freezing and thawing several times with little or no apparent
damage, while others are damaged by slight freezing. The relative susceptibility
of fresh vegetables to freezing injury is shown in Table 4. The inherent susceptibility
of tissues to freezing stress may be partially responsible for this variation in
sensitivity. Ice nucleating bacteria may also play an important role. Whether or
not ice formation in the tissues is accompanied by permanent injury and death