Temperature Effects on Produce Degradation 627
production, was stimulated by chilling but is soluble and not membrane-bound, it
is unclear whether increased ethylene production is related to the physical phase
transition of the membrane.
20.4.1.3.3 Changes in Respiratory Activity
Tissues that are tolerant to low temperatures have a decrease in respiratory activity
as temperature decreases.^32 Thus, the respiratory activity remains in balance with
glycolysis and other closely associated reactions, and there is little change in the
respiratory quotient. However, chill-sensitive products, including citrus fruits,
cucumbers, snap beans, and sweet potatoes, exhibit abnormal respiration when their
temperature falls below 10 to 12°C.^6 Typically, the respiratory quotient is much
higher at these low temperatures for chilling-sensitive crops than for chilling-tolerant
ones. Respiration may increase dramatically at the chilling temperature or when the
product is returned to nonchilling temperatures. The mechanism of this stimulation
is not known but is presumed to be due to uncoupling of oxidative phosphorylation.
The enhanced respiration may reflect the cells’ efforts to detoxify metabolic inter-
mediates that accumulated during storage and to repair damage to membranes and
other subcellular structures. Since enhanced respiration is one symptom of chilling
injury, respiratory response may be used as an index of the extent of chilling injury.
A sustained increase in respiratory rate after prolonged exposure to chilling temper-
atures may be indicative of irreversible metabolic disturbance and accumulation of
oxidizable intermediates.
In bananas that ripen after harvest, Pantastico et al.^41 found the respiratory
response to chilling was much different. Respiration at 20°C was depressed and the
climateric rise delayed following 1 or 2 d at 5°C. After 5 d at 5°C, respiration was
completely inhibited, and the climateric rise was absent. As a result, the fruit lost
the capacity to ripen after prolonged chilling.
20.4.1.3.4 Changes in Proteins and Enzyme Activity
Most of the changes in enzyme activity associated with chilling injury occur in
enzymes associated with membranes.^33 The Arrhenius plots of the membrane-bound
enzyme systems in sensitive plants show a “break” at the same temperature at which
the membrane undergoes a phase transition from the liquid-crystalline to the solid-
gel state. For chilling-resistant plants, the “break” is either nonexistent or occurs at
a lower temperature.
Lyons and Raison^37 and Raison et al.^42 reported that the mitochrondrial succinate
oxidase system, succinate dehydrogenase, and cytochrome oxidase of chilling-resis-
tant plant tissues showed constant activation energy over the usual range of biological
temperatures. There was an increase in activation energy below 9°C for these same
enzymes from chilling-sensitive plant tissues. Raison et al.^42 showed that the tem-
perature-induced change in activation energy observed in chilling-sensitive tissues
was due to configurational changes in the enzyme proteins caused by a temperature-
dependent phase change in the lipid component of the mitochrondrial membranes.
20.4.1.3.5 Effect on Protoplasmic Streaming
A difference in the protoplasmic streaming response of chilling-sensitive and chilling-
insensitive plants was one of the chilling responses first identified by early researchers.^32