Biochemistry of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 35

range from 2 to 20%. Ascorbate is another major component of citrus fruits. Ascorbate
levels can range from 20 to 60 mg/100 g juice in various citrus fruits. The orange skin may
possess 150-340 mg/100 g fresh weight of ascorbate, which may not be extracted into the
juice.
3.3.1.6 Anaerobic respiration
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. Anaer-
obic 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 fac-
tor, 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.
Anaerobic respiration plays a significant role in the respiration of citrus fruits. During
early stages of growth, respiratory activity predominantly occurs in the skin tissue. Oxygen
uptake by the skin tissue was much higher than the juice vesicles (Purvis, 1985). With
advancing maturity, a decline in aerobic respiration and an increase in anaerobic respiration
was observed in Hamlin orange skin (Bruemmer, 1989). In parallel with this, the levels
of ethanol and acetaldehyde increased. As well, a decrease in the organic acid substrates,
pyruvate and oxaloacetate, was detectable in Hamlin orange juice. An increase in the activity
levels of pyruvate decarboxylase, alcohol dehydrogenase, and malic enzyme was noticed in
parallel with the decline in pyruvate and accumulation of ethanol. In apple fruits, malic acid
is converted to pyruvate by the action of NADP-malic enzyme, and pyruvate subsequently
converted to ethanol by the action of pyruvate decarboxylase and alcohol dehydrogenase.
The alcohol dehydrogenase in apple can use NADPH as a cofactor, and NADP is regenerated
during ethanol production, thus driving malate utilization. Ethanol is either released as a
volatile or can be used for the biosynthesis of ethyl esters of volatiles.
3.3.1.7 Pentose phosphate pathway
Oxidative pentose phosphate pathway (PPP) is a key metabolic pathway that provides
reducing power (NADPH) for biosynthetic reactions as well as carbon precursors for the
biosynthesis of amino acids, nucleic acids, secondary plant products, etc. The PPP shares
many of the sugar phosphate intermediates with the glycolytic pathway (Fig. 3.5). The PPP
is characterized by the interconversion of sugar phosphates with three (glyceraldehyde-
3-phosphate), four (erythrose-4-phosphate), five (ribulose, ribose, xylulose phosphates),