Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

166 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
8.4 Ripening-associated cell wall changes
Fruit ripening is associated with extensive structural alterations within its semisoluble
pectin matrix, the pectin-rich middle lamellae that cement walls of adjacent cells, and the
hemicellulose–cellulose network. Pectins, hemicelluloses, cellulose, and starch are the common
polysaccharides that undergo modifications during ripening. Degradation of pectins
that represent over 50% of total carbohydrate in the fruit cell walls has been implicated in
tissue softening of a number of fruits. Pectin chemistry plays important roles in cell wall
hydration status and the mobility of resident enzymes by changing cellular porosity and ionfacilitated
gel formation. Even before the onset of fruit ripening, a loss of pectic galactan side
chains and the depolymerization of matrix glycans are usually initiated, which is followed
by a loss of pectic arabinan side chains. Fruit ripening initiates the depolymerization of
pectins, which becomes very prominent during the late ripening. The relationship between
the pectic material degradation and the decrease in the firmness of fruit tissues has been
extensively documented. In addition to the pectin degradation, xyloglucan breakdown has
been reported in the early stage of softening, but it is relatively limited and more consistent.
The degradation of both xyloglucans and polyuronides is cooperatively involved in fruit
softening processes; the xyloglucan breakdown may contribute to the initiation process,
while the polyuronide degradation to the excessive softening process (Wakabayashi, 2000).
Slippage between hydrogen-bonded hemicellulose and cellulose polymers also contributes
to softening (Brummell et al., 1999; Powell et al., 2003). In part, the expansin family of
cell wall proteins (Cosgrove, 2005) mediates such cell wall “creep,” and it is postulated that
the hydrogen-bonded interface between cellulose and xyloglucan may act as a substrate for
these bindings (Rose and Bennett, 1999; Brummell and Harpster, 2001).
Significant differences in ripening-associated cell wall polysaccharide modifications
exist among species of the same genus and even among different cultivars of same species
(Hiwasa et al., 2004; Rosli et al., 2004). These differences along with the marked diversity
in cell wall changes between species greatly contribute to the vast variability in firmness,
texture, and juiciness characteristics observed among different ripe fruits. Emerging evidences
have begun to support hypothesis that different cell wall changes are a result of
variations in the abundance, activity, timing, and type of gene family members of various
polysaccharide-modifying enzymes expressed during ripening. This is likely responsible
for the different cell wall processes observed between soft and crisp fruits. In spite of the
similar proportions of cell wall material, the melting and structure of the cell walls in these
fruits are very different at the ripe stage. Solubilization of the middle lamella and a restructuration
of the primary cell walls arising from the cells separation were observed in crisp
fruits, whereas the middle lamella of the soft fruits is better preserved and the primary cell
walls are thin. The changes in texture of cherries during ripening are linked to cell wall
degradation, involving synthesis and degradation of polymers, and the fruit texture depends
on the extent of the links between cell wall polymers. An increase in pectin solubility leads
to cell sliding and an elastic aspect of tissues (Batisse et al., 1996). Figure 8.1 shows changes
in relative solubilization of various major cell wall polysaccharides during ripening of some
of the fruits.
8.5 Pectin solubilization
During fruit softening, pectin typically undergoes solubilization followed by depolymerization
that is thought to contribute to wall loosening and disintegration resulting in textural