Temperature Effects on Produce Degradation 635
occurs as a result of protein denaturation occurring at temperatures above 35°C.
This results in loss of catalytic activity and a progressive decline in respiratory rate.^7
At the same time, the respiratory rate is increasing due to the direct effect of
temperature.
The most extensively studied heat-susceptible fruit is the tomato, which fails to
ripen properly at temperatures above 30°C. Increasing storage temperature from 21
to 30°C and to 37°C resulted in abnormal ripening due to inhibition of lycopene
development.^51 As storage temperature increased from 21 to 37°C, firmness was
more readily retained. Since softening is associated with increased pectolytic enzyme
activity, retardation of softening was attributed to inhibition of the activities of these
enzymes. Electrolyte leakage, observed on exposure of mature green tomatoes to
temperatures above 40°C, indicated that the temperature effect may be mediated
through membrane lipids.^52
Yang et al.^51 added exogenous ethylene to the storage atmosphere of fresh
tomatoes to determine whether inhibition of ripening at high temperatures (≥ 30°C)
was related to reduced ethylene production or reduced ethylene sensitivity. They
observed that exogenous ethylene did not overcome inhibition of ethylene formation,
ACC synthesis, color development, or softening during storage at 30 or 37°C. Based
on these findings, they concluded that ripening inhibition at higher temperatures is
not merely the result of decreased production of ACC and ethylene but also of
reduced sensitivity to ethylene.
20.5 TEMPERATURE EFFECTS ON SPECIFIC COMMODITIES
Temperature is the most important factor affecting the quality and storage life of all
fruits and vegetables. The mechanisms that trigger responses to temperature in
produce must be understood relative to the cellular and physiological responses that
keep plants alive. However, every species has somewhat unique responses to tempera-
ture and its interrelationship with moisture and air flow. The key to managing these
responses in handling and storing fresh fruits and vegetables until they are sold or further
processed is to understand the conditions needed for each species. The following
examples represent fruits and vegetables commonly marketed as fresh produce.
20.5.1 POTATOES, SWEET POTATOES, AND YAMS
Many root crops have a cork layer over the surface called the periderm.^20 This layer
serves to protect the interior against microbial invasion and excessive water loss.
Curing allows healing of breaks in the periderm that can result from damage due to
rough treatment during harvesting and handling. Curing roots to allow wounds to heal
before placing them in storage is an extremely important operation to reduce subsequent
moisture loss, increase resistance to disease, and help them recover from injury.
Microscopic examination of wounded tissue of sweet potatoes has shown that
the first observable change in the wound-healing process is the desiccation of several
layers of the outermost parenchyma cells exposed to air after wounding.^53 This is
followed by a progressive suberization of the parenchyma cells beneath the desic-
cated cells. The final stage of the wound healing process is the formation of a wound