582 Produce Degradation: Reaction Pathways and their Prevention
during the ripening of buttercup squash, which led to starch degradation and accu-
mulation of high levels of sucrose in cell walls [127].
Pectin depolymerization or degradation appears to influence both the ripening
and sweetening process and helps develop the flesh to a desirable texture in many
fruits. The pectinolytic enzymes, which include pectin methylesterases, polygalac-
turonases, and pectate lyases, are described in a previous section along with their
mechanisms of pectin degradation. While maximum hydrolytic activities are present
during tissue disruption or wounding, pectin depolymerization, interestingly, differs
significantly between different fruits, even when the levels of enzymatic activity in
these fruits are similar. Both polygalacturonase and pectin methylesterases cause
extensive solubilization and depolymerization of cell wall polysaccharides during
the ripening of the avocado fruit. In particular, polygalacturonase is important for
polyuronide degradation in the ripening avocado cell walls (partial de-esterification
was necessary for the increase in susceptibility of polyuronides to polygalacturonase)
[128,129]. Glycosidase and pectin methylesterase activities have been reported dur-
ing postharvest ripening of apricots [130]. Modifications in cell wall structure during
senescence include fragmentation of pectic polymers and hemicelluloses, solubili-
zation of long-chain pectin and loss of pectic sugars. Within hours of harvest, produce
cellular responses lead to altered metabolism, which causes reduction of proteins
and lipids, accumulation of free amino acids [131,132], and degradation of chloro-
phyll [133]. Arenas-Campos et al. [134] also noted that textural changes occurred
in the preclimacteric stage of ripening of sapote mamey fruit but concluded that the
fruit pulp softening was not dependent on pectin methylesterase, polygalacturonase,
or β-galactosidase enzymatic activities.
The occurrences of pectic hairy regions in various plant cell wall materials have
been noticed. Rhamnogalacturonan oligomers liberated during the degradation of
pectin have been isolated and characterized [135]. Many high-molecular-weight
pectic polysaccharide fractions isolated from a variety of sources showed consider-
able resistance to degradation by pectolytic, hemicellulolytic, and cellulolytic
enzymes [136]. Enzyme-assisted degradation of the surface membranes of harvested
fruits and vegetables improves the water permeability of these surface membranes,
which facilitates dehydration and absorption of substances such as sweeteners,
stabilizers, preservatives, and flavor enhancers [137–139]. In vitro fermentation of
two dietary fibers with distinct structural features (pea hull and apple fibers) showed
different compositions and physiochemical properties. Fermentation of apple fiber
led to a higher production of short-chain fatty acids compared to fermentation of
pea fiber. Production of major short-chain fatty acids, acetate, propionate, and
butyrate, occurred in both fibers. Uronic acid and arabinose were the most exten-
sively fermented sugars, while xylose and glucose were least fermented [140].
It is interesting to note that nonenzymatic breakdown of pectin has also been
proposed. Huber et al. [141] have reported that radical oxygen species, generated
either enzymatically or nonenzymatically, might also participate in the scission of
pectin and other polysaccharides during ripening and other developmental processes.
Another group of enzymes that are abundantly distributed throughout starchy
fruits and vegetables are starch-degrading enzymes. These enzymes have been exten-
sively studied [142–149]. Their mode of action is depicted in Figure 19.12. These