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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Physical Means 195
depends on the temperature and the length of exposure. The response of
a particular fruit or vegetable is determined by a combination of factors:
the physiological age of the commodity, the time and temperature of
exposure, whether the commodity is removed from heat to storage or to
ripening temperature, and whether the heat treatment causes damage
(Lurie, 1998).
Decay Suppression through Increased Host Resistance
Along with the retardation of the ripening process which follows heat
treatments, heating may lead to the maintenance of fruit quality during
prolonged storage. In many cases no damage is caused to the fruit and, in
some fruits, enhanced resistance to pathogens is induced in parallel with
the maintenance of fruit firmness. In this respect, heat treatment could
be included among the means that suppress decay by maintaining host
resistance.
Spotts and Chen (1987) described decay suppression in injured pears,
obtained through enhanced resistance to postharvest diseases which
followed heating at 37°C; decay caused by Mucor and Phialophora was
reduced when the fruits were inoculated after heating - that is to say,
without the direct exposure of the pathogen to heat. In this case, the
heating was found to increase the resistance of the wound to fungal
infection. The mechanism by which heat treatment causes changes in
fruit ripening and induces host resistance may be tied to changes in gene
expression and protein synthesis (Sachs and Ho, 1986). Various studies,
however, related the increased infection resistance of the host, following
heating, to the accumulation and enhanced activity of antifungal
compounds.
1. Enhanced antifungal activity of the fruit
Heating green lemon fruits (36°C, 3 days) was found to inhibit the
reduction of the antifungal citral in the rind, which occurs naturally
during fruit ripening (Rodov et al., 1995b). This was suggested to be the
reason why heating the fruit prolongs the antifungal activity of the rind
and elicits a considerable reduction in decay development (Ben-Yehoshua
et al., 1995). The hypothesis is, therefore, that decay suppression
following heat treatments is related, at least in part, to the enhanced
activity of the antifungal compounds located in the peel (preformed
antifungal compounds).
Heat stress can also induce the synthesis of the enzyme, phenylalanine
ammonia lyase (PAL) in citrus fruit (Golomb et al., 1984). This is one of
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