Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE BREAKDOWN OF CELL WALL COMPONENTS 183
ordered arrangement of cell wall and middle lamella polysaccharides occur. As the fruit
ripens, a substantial portion of its cell wall pectins are converted to a water-soluble form
affecting the texture (Labavitch, 1981).
The delay in ripening of cold-stored peaches has been associated with reduced ability of
fruits to convert insoluble pectic substances into soluble pectin (Lurie et al., 1994), and with
the inhibition of PG activity (Artes et al., 1996). The major changes involved in softening
and chilling injury in peaches are the catabolism of cell walls and the development of an
intercellular matrix containing pectins (Harker and Maindonald, 1994). Gel-like structure
formation in the cell wall due to the deesterification of pectins without depolymerization
leads to the development of woolliness in peach (Lurie et al., 2003).
Peach fruits develop mealiness (pastiness), which is associated with separation of middle
lamella without extensive degradation of cell wall. Mealiness has been attributed to
the presence of insoluble low methoxyl pectic substances of high molecular weight that
are formed by the action of PE during chilling. The affected cells showed larger primary
walls separated, forming a continuous extracellular matrix. The intracellular spaces were
characterized by the presence of amorphous pectic substances and polysaccharides. At the
ultrastructural level, dissolution of the middle lamella, cell separation, irregular thickening
of the primary wall, and plasmolysis of the mesocarp parenchyma cells were seen as internal
breakdown progressed (Luza et al., 1992).
Uronic acid content was higher in both water-soluble and -insoluble pectin fractions in
sound peach fruit compared to fruit with internal breakdown symptoms. The chilling-injured
fruits were characterized by 26% higher content in total neutral sugars compared to sound
fruit, which was mainly attributed to increased galactose, arabinose, and glucose contents,
whereas tissue derived from sound fruit had a 27% higher cellulose content compared to
chilling-injured tissue. Decreased activities of both PG and PME, accompanied by decreased
levels of cation binding in the cell walls, primarily of calcium, were recorded in the brownfleshed
tissue (Manganaris et al., 2006).
Ruoyi et al. (2005) showed that combination of chitosan coating, calcium chloride, and
intermittent warming partially inhibited PG activity, slowed down the increase in soluble
pectinefic substances. Addition of calcium chloride and intermittent warming could keep
the intactness of cell wall and reduce fruit sensitivity to injury in peach.
Endo-PG, PE, and endoglucanase (EGase) activities of delayed-storage nectarines fruit
were same as the control fruit at the beginning of storage, although exo-PG was higher.
Endo-PG activity was lower in control than delayed-storage fruit at the end of storage,
while PE activity was higher, and exo-PG and EGase activities were similar. Prevention
of chilling injury by delayed storage (DS) appears to be due to the ability of the fruit
to continue progressive and slow cell wall degradation in storage, which allows normal
ripening to proceed when the fruits are rewarmed (Zhou et al., 2000).
8.12.5 Modified atmosphere
Fruit softening is associated with the disassembly of primary cell wall and middle lamella
structure. The changes in cell wall structure and composition result from the composite
action of hydrolytic enzymes produced by fruits, which include PG, PE, PL, β-GAL, and
cellulases (Brummell and Harpster, 2001). High-oxygen atmosphere retards the decrease in
firmness in grapes (Deng et al., 2005), sweet cherries (Tian et al., 2004), fresh-cut carrots