Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

176 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
In peach cell wall, the xyloglucan, a dominant constituent of hemicellulose, gradually
degraded throughout the fruit-softening period. Initially, the xyloglucan degrades during
which the pectin remains insoluble, and later, pectin becomes soluble concurrently with
continuous xyloglucan degradation. However, Saladie et al. (2006) suggested that xyloglucan
endotransglucosylase/hydrolases (XTHs) do not represent primary cell wall–loosening
agents in tomatoes.
Mannanase activity was found to increase during the later stages of ripening in tomato,
but no corresponding increase in LeMAN4 mRNA was noticed, suggesting that either this
gene is subjected to posttranscriptional regulation or the LeMAN4 protein remains inactive
or sequestered until the later stages of ripening (Carrington et al., 2002). It may also be
possible that the natural substrate of tomato fruit mannanase is not a cell wall mannan.
Reduction in the mannose content of the hydrolyzed polymeric fractions of ripe mango
revealed the possible involvement of an endomannase and α-mannosidase, the two major
enzymes, in mango fruit softening and ripening (Yashoda et al., 2007).
The increase in endo-β-mannanase activity is greatest in the tomato skin and less in
the outer and inner pericarp regions. This enzyme is probably bound to the walls of the
outermost cell layers of the fruit during ripening. Endo-β-mannanase may be produced
and sequestered in a mature-sized inactive form during early ripening. Most nonripening
mutants of tomato exhibit reduced softening and lower endo-β-mannanase activity, but it
may not be responsible for softening as some cultivars that ripen normally do not exhibit
any endo-β-mannanase activity in the fruit (Bewley et al., 2000).
In olive fruit cell walls, decreases in arabinose, xylose, glucose, and uronic acid levels
were observed, together with a slight increase in mannose on ripening. At the beginning of
ripening, pectic polymers were the major constituents. Between the green and cherry stages
of ripening, a significant loss of homogalacturonans was observed. Between the cherry and
black stages of ripening, rhamnogalacturonan side chains were also released in addition to
homogalacturonans (Jimenez et al., 2001).
In pears, cell wall degradation is correlated with a decrease in firmness during ripening,
and the modification of both pectin and hemicellulose are essential for the development of a
melting texture. Different softening behaviors during ripening among the pear fruits may be
caused by different endo-PG activity and different expression of PG genes (Hiwasa et al.,
2004). The increase in the activities of β-galactosidase (β-Gal) and α-L-arabinofuranosidase
(α-Af) during pear ripening correlated well with a concomitant decrease in flesh firmness.
The β-Gal and α-Af may not mediate difference in fruit softening between two pears, but
that they could play some role(s) in cell wall changes, perhaps in cooperation with other
cell wall–modifying enzymes such as PG (Mwaniki et al., 2007).
The textural changes were most noticeable at the preclimacteric stage in ripening sapote
mamey fruit. The water-soluble pectin increased at a different rate than firmness decreased.
No correlation between PG or PME activity and changes in firmness was observed in
ripening fruits, though a low correlation was seen between β-GAL activity and softening.
Fruit pulp softening was not dependent on a single enzyme activity (Arenas-Ocampo et al.,
2003).
The modification of cell wall polysaccharides during softening of grape berries is a
complex process involving subtle changes to different components of the wall, and in many
cases only small amounts of enzyme activity are required to effect these changes (Nunan
et al., 2001).