640 Produce Degradation: Reaction Pathways and their Prevention
Chilled apples that are suddenly transferred into warm air are likely to “sweat”
(i.e., water vapor in the air will condense on them).^63 Sweating also occurs when
the doors of a cold storage room are opened, allowing warm, moist air to enter.
Sweating itself does not harm the fruit, but it causes wetting, which encourages the
growth of fungi and bacteria. Chilled apples should not be allowed to warm and
then rechill. To prevent sweating, chilled apples should be allowed to warm gradually
inside the storage area, if possible, before they are moved into the open air.
The timing of the climacteric and ripening of apple fruit is advanced by exposure
to ethylene.^62 Prevention or slowing of ethylene production, by affecting ethylene
synthesis or perception, is a strategy for increasing fruit storability. This delay is
achieved primarily by use of low storage temperatures and application of controlled
atmosphere storage technologies.
Some varieties of apples are sensitive to low-temperature disorders. Sensitive
varieties can develop a number of problems after storage for several months at 0 to
4°C.^63 Brown core, also known as core flush, low-temperature breakdown and
internal browning, soft scald, and soggy breakdown are all conditions that may
develop in apples. Because each of these conditions is a distinct disorder and each
occurs in apples that have been kept for a prolonged time at less than 5°C, it is
assumed that each disorder must be derived by a different metabolic route.^10 How-
ever, these disorders typically develop only in fruit kept for several months, so risks
of low-temperature injury are low for fruit kept in short-term storage (2 to 3 months).
In addition to avoiding long-term storage at low temperatures, other treatments
may be used to prevent chilling injury symptoms. For example, low-temperature
breakdown is a physiological disease that affects some varieties of apples when
stored at temperatures not sufficiently low to freeze the tissue.^46 As a result of this
disorder, the fruit develops browning of the cortical tissue, which may eventually
affect the whole fruit. It has been observed that low-temperature breakdown is
preceded by an accumulation of oxaloacetic acid. Both the oxaloacetic acid accu-
mulation and low-temperature breakdown development can be reduced by interrupting
the cold storage with a short period at warmer air temperature (5 d at 15°C after 6 to
8 weeks at 0°C). Hulmes and coworkers^46 suggested this effect occurred because the
warming period was sufficient to allow the Krebs cycle to operate “normally,” thereby
removing keto-acids that accumulated during the slower metabolism of cold storage.
20.5.5 BANANAS
The banana is very susceptible to chilling injury and in some cultivars this injury
may occur at temperatures as high as 13°C.^7 Chilling injury of bananas is mainly a
peel disorder. Symptoms include subepidermal discoloration visible as brown to
black streaks in a longitudinal cut (Figure 20.10), a dull or grayish (smoky) cast on
ripe fruit, or failure to ripen. In severe cases the peel turns dark brown or black, and
even the flesh can turn brown and develop an off-taste. The pulp of chilled fruit
softens unevenly and may be acid, astringent, and lacking in flavor and sweetness.
Less severe chilling may have little effect on the green fruit or on the pulp of the
ripe fruit, but color development during ripening may be affected, leading to skin
that is dull yellow, grayish-yellow, or gray.