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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
198 Postharvest Diseases of Fruits and Vegetables
membrane and is known to cause a temporary increase in membrane
leakage (Lurie and Klein, 1990; Salveit, 1991). The ability of heat
treatments to enhance resistance to low-temperature injury is an
example how one stress - heat stress - may protect the plant tissue
against another stress - low-temperature stress (Lurie et al., 1994). One
should remember that the defense of the fruit against chilling injury may
result, in addition to the repair of the physiological damage, in a
reduction in the decay that typically develops in chilled tissues. Thus,
following heat treatment, it is possible to store sensitive fruits at low
temperatures without inducing chilling injury and subsequently
increased decay development.
Efficiency of Heat Treatments
Heat is delivered to the commodity by means of contact with hot air or
water. The water content of the air greatly influences heat transfer, and
greater pathogen inactivation is usually achieved by treatment with
moist heated air than with dry air at the same temperature (Teitel et al.,
1989). The reason for this may be the more vigorous physiological
activity of moist spores than of dry spores, or the fact that moist air
transfers heat more effectively than dry air. When dry air is applied,
condensation does not form on the target commodity, and the rate of heat
transfer depends largely on the temperature of the air passing over the
surface of the commodity and the heat conductivity of the commodity.
When the air is saturated (vapor heat), condensation forms on surfaces
that are cooler than the air and heat is transferred rapidly to the surface
(Edney and Burchill, 1967).
The complex structure of a given host may greatly influence the rate of
heat transfer. A grape berry, for example, transfers heat faster than the
tissues of an apple. Also, heat transfer from tissue to tissue can vary
greatly within the leaf, stem, root or fruit. Furthermore, heat transfer
may differ among different tissues of the fruit itself. The colored outer
layer (flavedo) of the citrus rind, which may have few and small
intercellular spaces, can transfer heat faster than the underlying white
spongy tissue (albedo) (Barkai-Golan and Phillips, 1991).
Since heat treatments may act directly on the spore population
infesting the host surface, a short exposure to heat may sometimes be
sufficient to reduce decay incidence markedly. However, heat treatment
is also based on the gradual penetration of heat into the host tissues;
thus the extent of pathogen progress within the tissues may determine
the success or failure of the treatment. Immersion of lemons in hot water
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