610 Produce Degradation: Reaction Pathways and their Prevention
Air velocity must be high during cooling to bring about a low VPD as quickly
as possible.^11 However, the rate of moisture loss from a product in storage is directly
related to air velocity. Therefore, in order to minimize moisture loss, air velocity
during storage should be reduced to that needed to maintain desired temperature.
20.2.4 DECAY ORGANISMS
Temperature affects the rate of growth and spread of fungal organisms in and on
fruits and vegetables. Postharvest pathogens generally grow best at 20 to 25°C,
depending on the fungus species.^12 The maximum temperatures for growth are
generally 32 to 38°C, but some species can grow at higher temperatures.
Lower temperatures may be used for the control of fungal disease in one of
three ways^12 :
- They may be used to achieve storage conditions below the minimum
temperature for growth of fungus. At –1 to 0°C, the recommended storage
temperature for non-chilling-sensitive commodities, only a few fungi
grow. Botrytis cinerea and Pencillium expansum are widely known excep-
tions since they can grow and cause product rot at 0°C. Some fungi have
a minimum temperature for growth of –5 to –2°C. The growth of these
fungi cannot be stopped by lowering product temperature since most
produce would freeze before it reached a temperature low enough to halt
the fungi. However, although these fungi are active at refrigerator storage
temperatures, their growth rates are significantly slowed from those
observed at higher temperatures. - The time between a fungal spore landing on a suitable growth medium
and the development of a tiny fungal colony is referred to as the lag phase.
On fruit, the lag phase is usually longer than on culture medium since the
spore must not only germinate, but also must initiate growth in highly
resistant living tissue. Lower temperature may extend the lag phase of the
fungal growth so that it has not been completed before the product is
consumed. Depending on the fungus species, the lag phase may lengthen
from a few hours to several days at optimum temperatures to weeks or
months at temperatures near minimum for fungal growth. - Lower temperatures may kill the spores of cold-sensitive fungi while they
are germinating.
Heat treatments to control pathogens have been the subject of extensive research.^13
The response of fungi to heat treatments varies with such factors as the organism, its
metabolic activity, the type of heat used (dry vs. moist), and the duration of the treatment.
Heat treatments have been reported to be effective in controlling Botrytis cinerea on
apples, peppers, tomatoes, and strawberries. Combining fungicides and hot water treat-
ments has been shown to be useful on citrus since the two give better protection than
either alone and on tomato and pepper since there are no approved postharvest chemicals
for decay control. However, Penicillium spp. have been increased in grapefruit as a
result of hot water treatment to control Caribbean fruit fly.^13