634 Produce Degradation: Reaction Pathways and their Prevention
the eradication of pests. The temperature of the product also may be increased by
heat generated during respiration by the product itself. In most cases, increasing the
heat of the product, regardless of the source of the heat, may result in produce
degradation through a variety of mechanisms. For example, increased temperature
can result in more rapid product respiration that can decrease shelf life.
High-temperature injury can result from both chronic exposure for days or weeks
to temperatures above typical room temperature (ranging from 30 to 40°C) and acute
short-term exposures to higher temperatures.^31 Much less is known about high-
temperature injury than about chilling injury since high-temperature injury generally
can be prevented with normal handling practices. Short-term injury can be induced
during harvesting operations in warm climates and during storage at ambient tem-
peratures in developing nations in the tropics. If the produce is stored at low
temperature after exposure to high temperature, the heat damage can be confused
with chilling injury, which has similar symptoms.^13
Elevated product temperatures can have some beneficial effects. These include
curing root crops, drying bulb crops, and controlling disease and pests.^20 Many fruits
are exposed to high temperatures, often in combination with ethylene or another
suitable gas, to initiate or improve fruit ripening or skin color. Lurie^13 presented an
in-depth discussion of the mechanisms of postharvest heat treatments for ripening,
inducing thermotolerance, reducing chilling injury, and disinfestation of fruits and
vegetables.
Three methods are used to apply heat to commodities: hot water, vapor heat,
and forced-air treatment.^13 Hot water treatment was originally used for fungal control
since fungal spores are either on the surface or in the first few cell layers under the
peel of the fruit or vegetable. Hot water dips also have been used for insect disin-
festation. Many fruits and vegetables tolerate water temperatures of 50 to 60°C for
up to 10 min, but shorter periods at elevated temperatures can control many post-
harvest plant pathogens.
Vapor heat was developed specifically for insect control and involves heating
produce with water vapor at temperatures between 40 and 50°C.^13 Heat transfer is
by condensation of hot water vapor on the cooler fruit surface. Hot air treatments
can be applied by placing fruits and vegetables in a heated chamber with a ventilating
fan or by applying forced hot air. Forced-air heating is slower than hot water or
water vapor heating but is faster than a regular heating chamber. It has been used
for both insect and fungal disinfestation as well as to study the response of com-
modities to high temperatures.
The activity of enzymes in fruit and vegetables declines at temperatures above
30°C, but the temperature at which specific enzymes become inactive varies.^2 Many
are still active at 35°C, but most are inactivated at 40°C.
When produce is held above 35°C, metabolism becomes abnormal and results
in a breakdown of membrane integrity and structure, with disruption of cellular
organization and rapid deterioration.^2 The changes are often characterized by a
general loss of pigments, and the tissues may develop a watery or translucent
appearance.
The short-term effect of elevated temperature on produce respiration is difficult
to predict because it is the result of two opposing influences. Enzyme inactivation