244 Produce Degradation: Reaction Pathways and their Prevention
elements are included in the fertilizer, while riboflavin content also increases with
high light intensity and trace elements in the soil. Thiamin synthesis in plants is
stimulated by light and generally occurs in the leaves and increases in concentration
until the plant matures (Winsor and Adams, 1975; Salunkhe and Kadam, 1998).
Therefore, the nutrients that seem to be most affected by season or light exposure
are carbohydrates, carotene/provitamin A, ascorbic acid, thiamin, and niacin.
8.9.2 POSTHARVEST TREATMENT
8.9.2.1 Heat Treatments
During the last few years, there has been increasing interest in the use of postharvest
heat treatments to control insect pests, prevent fungal rot, and affect ripening. Part of
this interest is due to consumer demand for chemical-free produce. The demand for
restricted use of chemicals has increased the interest in postharvest heat treatments that
can be used as nondamaging physical treatment substitutes for chemicals. There are
three methods commonly used to heat commodities: hot water, vapor heat, and hot air.
8.9.2.1.1 Hot Water Blanching
Hot water dips and sprays were originally used for fungal control, but their use has
now been extended to include disinfection of insects. Hot water dips are effective
for fungal pathogen control because fungal spores and latent infections are either
on the surface or in the first few cell layers under the peel of the fruit or vegetable.
Postharvest dips to control decay are often applied for only a few minutes at
temperatures higher than heat treatments designed to kill insect pests located at the
interior of a commodity. Only the surface of the commodity requires heating. Many
fruits and vegetables tolerate exposure to water temperatures of 50 to 60°C for up
to 10 min, but shorter exposure at these temperatures can control many postharvest
plant pathogens (Lurie, 1999). In contrast, hot water dips for fruits require 90 min
of exposure to 46°C. A combination of fungicide and heating may be applied to the
produce simultaneously to increase pathogen destruction. For example, the use of
the fungicides thiabendazole and imazalil with hot water has been very effective in
citrus fruits (Lurie, 1999).
Hot water dips have been used for disinfecting fruits from insects as well. Since
hot water is a more efficient heat transfer medium than hot air, when properly
circulated through a load of fruits a uniform temperature profile is established in
the bath. For disinfection, a longer treatment is necessary than for fungal control
because the total fruit, not just the surface, has to be brought to the proper temper-
ature. Procedures have been developed to disinfect a number of subtropical and
tropical fruits from various species of fruit fly (Paull, 1994). The time of immersion
can be as long as 1 h or more at temperatures below 50°C, in contrast to many antifungal
treatments, which are for less time at temperatures above 50°C (Lurie, 1998).
A recent extension of the hot water treatment has been the development of a
hot water spray machine (Fallik et al., 1996). This is a technique designed to be part
of a sorting line, whereby the commodity is moved by means of brush rollers through
a pressurized spray of hot water. By varying the speed of the brushes and the number
of nozzles spraying the water, the commodity can be exposed to high temperatures