Flavor and Volatile Metabolism in Produce 169
The fact that fruits and vegetables are living, respiring tissues has major conse-
quences related to shelf life and storage. Product respiration, transpiration, and
ethylene production are major factors responsible for the deterioration of fresh
produce. Temperature may have the greatest effect on respiration rate and thus
deterioration rate of produce. Other factors that can be controlled to maximize
storage period include relative humidity and gas composition. All of the above
parameters are commodity-specific; each fruit or vegetable is unique and is affected
by both genetic factors (e.g., species, cultivar, etc.) and stage of maturation or
ripeness. Postharvest practices prior to storage may also affect the behavior of
produce during storage. Understanding the physiology of fresh produce is funda-
mental to understanding its stability and likely shelf life. Factors that affect stability,
shelf life, and quality attributes such as appearance, texture, and flavor of fruits and
vegetables have been reviewed [1,165]. The following discussion addresses some
important intrinsic and external factors that affect the flavor quality of fruits and
vegetables.
6.4.1 TEMPERATURE
Temperature affects respiration rate, and likewise volatile production is known to
increase with increasing temperature within a moderate range (e.g., between 0 and
30°C in the case of apples) [166]. However, high-temperature storage is not condu-
cive to prolonged shelf life for most produce. In general, respiration rate is controlled
by lowering the temperature during storage or by controlled or modified atmospheric
storage with or without active packaging technology. Humidification also is neces-
sary for some produce especially susceptible to moisture loss (e.g., leafy vegetables),
but the impact on flavor is not well documented. Another advantage of low-temper-
ature storage is the reduced growth rate of postharvest pathogens and spoilage
microorganisms. Ideal storage temperatures varies from product to product and
recommended conditions for over 50 fruits and vegetables have been tabulated [167].
Most fruits and vegetables cannot survive cold temperature storage (chilling
injury) at near-freezing temperatures, and many are intolerant to low temperatures
well above freezing.
Refrigeration may also slow, limit, or halt (e.g., in the case of chilling injury)
the development of aroma volatiles in ripening fruits. Temperature conditioning can
affect the response of some produce. For example, tomatoes are susceptible to
chilling injury when stored below about 12 to 13°C [168], resulting in several
metabolic disorders leading to decline in soluble solids [169] and flavor loss [170].
However, chilling injury can be prevented in mature green tomatoes by high-tem-
perature conditioning (36 to 40°C for 3 d) before cold storage (2 to 3°C) for up to
3 weeks [171].
6.4.2 ETHYLENE
The best-quality fruits are produced when the concentration of ethylene, CO 2 , and
O 2 in the atmosphere and the duration of exposure, temperature, and humidity are
carefully controlled and maintained at optimal levels [172]. The detrimental effects