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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
190 Postharvest Diseases of Fruits and Vegetables
had already been reported in the 1920s (Fawcett, 1922), and during the
following decades, hot water treatments were tested on and applied to
various fruits and vegetables, although the intensive use of chemicals
pushed them aside, along with other means of control. Today, with the
trend toward less reliance on chemical treatments, the interest in
postharvest use of heat treatment has revived. A list of commodities
heat-treated to control postharvest decay is presented in the reviews by
Barkai-Golan and Phillips (1991) and Coates and Johnson (1993).
Short-term and Long-term Heat Treatments
Short-term heating, where the fruit or vegetable is dipped in hot water
at temperatures above 40°C (generally 44-55°C) for a short time (from a
few minutes to 1 h), has been the main heat treatment studied over the
years. The principle is in the use of temperatures that are high enough to
inactivate the pathogen without causing significant changes to the host.
Early studies had already confirmed that fruits and vegetables commonly
tolerate such temperatures for 5-10 min, and that even shorter exposures
to these temperatures is sufficient to control many of the postharvest
pathogens (Smith, W.L., Jr. et al., 1964).
Recent studies reveal increased interest in long-term heat treatments,
in which the commodity is exposed to temperatures lower than those
mentioned above (usually 38-46°C), but for a longer duration (12 h to 4
days) (Fallik et al., 1996; Klein et al., 1997). Both short-term and
long-term heating, aimed at suppressing storage decay, could act directly
by inactivating the pathogen, or indirectly via physiological and
biochemical changes in the host, which enhance the resistance of the
tissues to the pathogen.
The Effect of Heat on the Pathogen
Genetic differences among fungi are expressed in differences in their
sensitivities to high temperatures and, therefore, in differences among
levels which kill them or inhibit spore germination and hyphal growth
(Sommer et al., 1967) (Fig. 30). For a given species, spore inactivation
increases with both temperature and duration of treatment; conidia of
Alternaria alternata may be inactivated equally by treatment for 2 min
at 48°C or for 4 min at 46°C (Barkai-Golan, 1973) (Fig. 31).
Spore sensitivity to heat is also dependent on their physiological state.
Germinated fungal spores are markedly more sensitive to heat than
non-germinated spores. It was found that water at 42°C does not affect
dormant conidia of A, alternata but does inactivate many germinating
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