Structural Deterioration of the Produce: The Breakdown of Cell Wall Components - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 168

Polyuronides in tomato and avocado exhibit progressive depolymerization from unripe
to midripe to full ripe, whereas in peach there was a little change in midripe fruit compared
with unripe but it showed a dramatic depolymerization at the full ripe stage. Depolymeriza-
tion of polyuronides has important consequences for the strength of cell-to-cell connections,
which affects softening and texture of the fruit, but the large differences in the extent of
polyuronide depolymerization among species during ripening indicates that altered inter-
cellular adhesion is more important component of textural changes in some species than
others. Induction of polyuronide depolymerization in a nonsoftening mutant tomato by
expressing polygalacturonase (PG) has shown that it alone is not the determinant of fruit
softening (Giovannoni et al., 1989). Depolymerization of polyuronides late in ripening is a
major contributor to weakening of the middle lamella and declining intercellular adhesion.
Softening often does not correlate with polyuronide depolymerization early in ripening,
although the more extensive depolymerization occurring late in ripening is associated with
a large decline in firmness.
A reduction in degree of pectin methylesterification is a common feature of most aspects
of plant development, which is most noticeable during fruit ripening. A large decrease in
the degree of pectin methylesterification has been reported during ripening of kiwifruit
(Redgwell et al., 1990), papaya (Paull et al., 1999), avocado (Wakabayashi et al., 2000),
peach (Brummell et al., 2004a), and grapes (Barnavon et al., 2001). A 40% drop in the
degree of pectin esterification was reported in fruit development in tomato cv VF145b-
7879 (Koch and Nevins, 1989), but was not observed in other tomato varieties (Tieman
et al., 1992). Demethylesterification of pectin not only affects the rigidity of the pectin
network and properties of the wall but also modifies the diffusion and activity of enzymes
within the wall spaces. The changing ionic conditions in the apoplast (alteration in pH,
increase in K ion, and osmoticum) may affect the activity of the cell wall hydrolases
(Grignon and Sentenac, 1991; Chun and Huber, 1998; Almeida and Huber, 1999) and
aggregation of pectin molecule (Fishman et al., 1989).
During ripening of avocado, water-soluble polyuronides increased dramatically, con-
comitant with marked downshifts in molecular mass. PG plays the central role in polyuronide
degradation in ripening avocado fruit cell walls, and partial deesterification is neces-
sary for the increase in the susceptibility of polyuronides to PG (Wakabayashi et al.,
2000).
Fleshy fruit of strawberry soften during ripening mainly as a consequence of solubi-
lization and depolymerization of cell wall components and is associated with an increment
of pectin solubility and a reduction of the molecular mass of hemicelluloses (Martinez
et al., 2004). The amount of water-soluble polymers (WSP) increased from small green
(SG) to white (W) stage, on the contrary, the hydrochloric acid-soluble pectins (HSPs)
decreased during ripening. The amount of hemicellulosic polysaccharides and cellulose
also decreased. A slight depolymerization was observed in hemicelluloses (Rosli et al.,
2004).

8.6 Matrix glycan solubilization

Galactan and arabinan play important role in cell wall structure and function. The majority
of Gal and Ara in the primary walls is present as the 1,4-

β α

- L -arabinan,
and branched arabinogalactans side chains of RG-I with smaller amounts in structural

- D -galactan, 1,5-