Anaerobic respiration is a common event in the respiration of ripe fruits and especially
becomes significant when fruits are exposed to low temperature. Often, this may result
from oxygen-depriving conditions induced inside the fruit. Under anoxia, ATP production
through the citric acid cycle and mitochondrial electron transport chain is inhibited. Anaerobic
respiration is a means of regenerating NAD, which can drive the glycolyic pathway
and produce minimal amounts of ATP (Fig. 3.4). Under anoxia, pyruvate formed through
glycolysis is converted to lactate by lactate dehydrogenase using NADH as the reducing factor,
and generating NAD. Accumulation of lactate in the cytosol could cause acidification,
and under these low pH conditions, lactate dehydrogenase is inhibited. The formation of
acetaldehyde by the decarboxylation of pyruvate is stimulated by the activation of pyruvate
decarboxylase under low pH conditions in the cytosol. It is also likely that the increase in
concentration of pyruvate in the cytoplasm may stimulate pyruvate decarboxylase directly.
Acetaldehyde is reduced to ethanol by alcohol dehydrogenase using NADH as the reducing
power. Thus, acetaldehyde and ethanol are common volatile components observed in the
headspace of fruits, indicative of the occurrence of anaerobic respiration. Cytosolic acidi-
fication is a condition that stimulates deteriorative reactions. By removing lactate through
efflux and converting pyruvate to ethanol, cytosolic acidification can be avoided.