Post harvest diseases fruits and vegetables - Xavier University ...

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
194 Postharvest Diseases of Fruits and Vegetables
The Effect of Heat on the Host
Heat can affect many of the processes within the tissues, such as the
rate of fruit ripening, the development of fruit color, sugar metabolism,
electrolyte leakage, ethylene production, respiration rate, pectic enzyme
activity, volatile production and susceptibility to pathogens
(Barkai-Golan and Phillips, 1991). Decay control together with ripening
inhibition was reported for plastic-wrapped nectarines treated by moist
air heating (Anthony et al., 1989). The inhibition of ripening by heat may
be mediated via its effect on the ripening hormone, ethylene, since heat
treatment has been found to inhibit ethylene synthesis within hours, in
both apples and tomatoes (Biggs et al., 1988; Klein, 1989). Elevated
temperatures can lead to the accumulation of endogenous ACC
(1-aminocyclopropane -1- carboxylic acid), the precursor of ethylene
synthesis, in apple and tomato tissue, concomitantly with the decrease in
ethylene. However, raising the temperature higher or extending the
exposure of the fruit to it will cause the disappearance of ACC as well
(Klein, 1989; Atta Aly, 1992). The inhibition of ethylene formation is
reversed when the fruits are removed from heat; this recovery requires
protein synthesis, and both mRNA and protein of ACC oxidase were
found to accumulate during recovery from heat treatment (Lurie et al.,
1996). A relationship between the inhibition of both ACC synthase and
ACC oxidase was detected in mango fruits during heat treatment (Ketsa
et al., 1999): following heating, ACC oxidase recovered full activity while
ACC synthase recovered only partially, but sufficiently to allow the
previously heated fruits to achieve an ethylene peak. During the heating
period, not only is endogenous ethylene production inhibited, but the
fruits also do not respond to exogenous ethylene (Yang et al., 1990).
Heated fruits often soften more slowly than non-heated fruits. Plums,
pears, avocados, and tomatoes have all been found to soften more slowly
when held at temperatures between 30 and 40°C than at 20''C. The rate
of softening increased when the heated fruits were returned to 20°C, but
it was still less than that of non-heated fruits (Klein and Lurie, 1991).
Holding apples at 38°C for 3-4 days prior to removal to prolonged cold
storage (0°C) resulted in maintenance of firmness even after 6 months of
storage at 6°C, and 10 shelf-life days at 20°C. The retardation of
softening was related to the high level of insoluble pectin in the heated
fruit as compared with that in the non-heated fruit, because of the
inhibition of the synthesis of cell-wall degrading enzymes.
A heat treatment can change the climacteric respiration peak as well
as advancing or delaying it after treatment; the extent to which it does so
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