Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

180 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
have failed to establish their roles in softening (Lashbrook et al., 1998; Brummell et al.,
1999; Woolley et al., 2001; Harpster et al., 2002a, b). Transgenic fruit firmness was not
significantly altered when PG or expansin action was suppressed individually, but increased
when genes encoding both proteins were simultaneously downregulated (Powell et al.,
2003).
8.12 Postharvest factors affecting structural deterioration
The major postharvest problem with storage of fruits and vegetables is the excessive softening.
Ripening of many fruits is mainly orchestrated by biosynthesis of ethylene that triggers
a serial biochemical and physiological process inducing the softening in texture. In case of
Capsicum annuum fruits, softening during ripening is associated with alteration in pericarp
cell wall and the breakdown of middle lamella pectins (Sethu et al., 1996).
8.12.1 Processing
In olives, the lye treatment and wash causes an exchange of arabinans from carbonatesoluble
and 4 M KOH-soluble fractions to the water-soluble fraction. The main change
in pectins was a movement of homo- and rhamnogalacturonans from water-soluble and
carbonate-soluble fractions to the imidazole-soluble fraction, but a partial solubilization
of alkali-soluble and cellulose-linked pectins during lye treatment, wash, and fermentation
was also observed (Jimenez et al., 1998).
California Black Ripe processing of olives was accompanied by general solubilization
of polysaccharides, and pectins and a noncellulosic glucan component were most clearly
affected. Soluble polysaccharides accumulated in processing liquids. Analysis of polysaccharides
extracted from cell walls suggests that the polymer most extensively solubilized
and eluted during processing is relatively unbranched pectin (Araujo et al., 1994).
In mechanically injured tissues (fresh-cut) of papaya the PG, cellulase and β-GAL
activities increased within 24 h of cutting and remained significantly higher during storage
as compared to intact fruits. These enzyme activities were accompanied by an increase
in both 1-aminocyclopropane-1-carboxylate synthase (ACS) and 1-aminocyclopropanel-carboxylate
(ACC) activities raising the possibilities of enhanced ethylene production,
thereby stimulating ripening (Karakurt and Huber 2003). In tomatoes, wounding resulted
in reduction or complete cessation of PG synthesis (Chung et al., 2006). The increase in
PG activity during ripening is due to de novo synthesis (Tucker et al., 1980; Bird et al.,
1988; Biggs and Handa, 1989), and reduction in PG gene expression was observed after
wounding. Chung et al. (2006) also reported that wounding might also impair the ability of
ripening tomato tissues to recover PE activity and β-galactanase activity.
Carbohydrate analysis of partially defatted almond seeds revealed important changes in
cell wall polysaccharides. At low extraction percentages (up to 33%), pectic polysaccharides
and hemicellulosic xyloglucans were the main type of polymers affected, suggesting the
modification of the cell wall matrix, although without breakage of the walls. At higher
extraction rates (up to 64%), a major disruption of the cell wall occurred as indicated by
the losses of all major types of cell wall polysaccharides, including cellulose. At higher
extraction rates, fatty acid chains are able to exit the cells either through unbroken walls, or
the modification of the pectin-hemicellulose network might have increased the porosity of