Structure and Function of Complex Carbohydrates in Produce 581
that affect the quality of the produce. The subject of physiological response of fruits
and vegetables to postharvest stresses and its resulting effects has been reviewed
[111–112,114]. Postharvest deterioration is particularly acute in tuber plants such
as cassava, where wounding and mechanical damage of the tuberous roots cannot
be prevented during harvesting. Research indicates that processes such as oxygen
stress, carbohydrate metabolism, protein metabolism, and phenolic compound syn-
thesis are all involved in the postharvest deterioration of produce [115].
Two notable changes that fruits and vegetables exhibit during postharvest han-
dling are (1) the accumulation of sugars (monosaccharides, oligisaccharides, and
polysaccharides) in cells and tissues and (2) the production of high levels of catalytic
enzymes of variable specificities. While some of these activities are part of the plant’s
normal biochemical processes that are designed to accomplish fruit maturity and
ripening, others are a direct response to postharvest stresses, which lead to produce
deterioration that affects its quality. For example, in apple fruits, a gradual accumu-
lation of soluble sugars occurs during the first 60 d of harvest, and this increase in
sugars can be correlated with an increase in the activity of sucrose-phosphate syn-
thase [116]. Interestingly, other investigators have found that during storage, cell-
wall polysaccharides in apples increased, while the total neutral sugar and protein
content decreased considerably. However, these activities could be reversed if the
fruit was infiltrated with CaCl 2 during storage [117]. Liu et al. found that the
carbohydrate content in avocado fruits fluctuated during the growth and ripening of
fruits [118]. More than half of the fruit’s total soluble sugars were composed of the
seven-carbon (C7) heptose sugar D-mannoheptulose in both its monomer and its
polyol form, perseitol. The balance was accounted for by the more common hexose
sugars, glucose and fructose. Results from this research suggest that the C7 sugars
play an important role not only in metabolic processes associated with fruit devel-
opment but also in respiratory processes associated with postharvest physiology and
fruit ripening [118]. Higher sugar contents and accumulation of alcohol-insoluble
solids containing pectins and other cell wall polysaccharides have also been reported
for ‘Valencia’ oranges and, particularly, for grapefruits [119,120]. Blackberry fruits
kept under different environmental conditions and storage periods showed changes
in their pH, total titratable acid, and in total soluble solids (polysaccharides) [121].
Scores of biosynthetic enzymes are produced in plants during their growth and
development as well during their maturity and ripening [6,7,13]. Numerous enzymes,
including amylases, cellulases, pectinases, and glucosidases of variable specificities
are also produced in response to the pre- and postharvest stresses in fruits and
vegetables. Many of these enzymes play a role in the cell wall polysaccharide
deterioration in produce. A number of carbohydrate degradation products such as
3-deoxypent-2-ulose, furan-2-carboxaldehyde and hydroxy-acetaldehyde have been
isolated; these serve as precursors of the browning process [122–125]. Asparagus
(Asparagus officinalis L.) spears, which contain immature tissues, are highly sus-
ceptible to deterioration. Postharvest deterioration in the spears is accompanied by
changes in respiration rate, soluble carbohydrates, and carbohydrate-degrading enzy-
matic activities. During initial quality loss, spear tips experience a rapid loss of
sucrose and a gradual decrease in glucose concentration, whereas fructose remains
unchanged [126]. Irving et al. also reported significantly higher enzyme activities