608 Produce Degradation: Reaction Pathways and their Prevention
a significant effect on refrigeration requirements.^6 Some commodities have high respi-
ration rates and require considerably more refrigeration to hold them at a specified
temperature. Tables such as Tables 20.1 and Table 20.2 that present respiration rates
of fruits and vegetables are useful in calculating refrigeration requirements of various
products. Vital heat also must be considered in selecting proper methods for cooling,
packaging, stacking packages, and refrigerated storage facility design.
20.2.2 RIPENING AND ETHYLENE PRODUCTION AND UTILIZATION
Normal ripening generally occurs within a particular temperature range (commonly
10 to 30°C), although some fruits, for example, some pear varieties, will ripen slowly
and satisfactorily at temperatures below 10°C.^2 The best quality of ripe fruit generally
develops at about 20°C, the temperature generally considered optimum for ripening
most fruits.
Ethylene is a natural product of plant metabolism and is produced by all tissues
of higher plants.^8 As a plant hormone, ethylene regulates many aspects of growth,
development, and senescence. There is no consistent relationship between the eth-
ylene production capacity of a given commodity and its perishability; however,
exposure of most commodities to ethylene accelerates their senescence.
Climateric and nonclimateric fruits differ in their response to applied ethylene
and in their ethylene production during ripening. Climateric fruits produce much
larger amounts of ethylene during ripening than do nonclimateric fruits. Applying
low concentrations of ethylene will hasten ripening of climateric fruits, but the
magnitude of the climateric is relatively independent of the concentration of applied
ethylene.
Temperature affects the rate of ethylene production.^1 The ethylene synthesizing
enzymes, 1-aminocyclopropane carboxylic acid (ACC) oxidase and ACC synthase,
are sensitive to low temperatures. Typically, as temperatures are lowered, less eth-
ylene is produced.^3 Temperature also affects the sensitivity of products to ethylene.
At lower temperatures longer exposure to a given concentration of ethylene is
required to initiate ripening. Figure 20.2 shows the effect of temperature on ethylene-
induced floret yellowing of broccoli.
20.2.3 MOISTURE LOSS
As fresh commodities lose water to the surrounding environment, this water loss
may lead to shriveling and reduced quality. Three environmental factors affect the
rate of produce shrinkage from water loss: (1) temperature of the produce;
(2) temperature and relative humidity of the air; and (3) air velocity past the product.^9
The first two factors are related since together they determine the vapor pressure
deficit between the fruit and the surrounding air.
Relative humidity (RH) indicates the amount of water vapor in the air as a
percentage of the maximum amount of water vapor the air can hold at that temper-
ature. It has a critical effect on the moisture lost from a product. Product moisture
loss is rapid at low RH and slower at higher RH.^10 If the RH in a room is 100%,
the air in the room is saturated, as is the interior of fruits and vegetables. Thus, there