Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

28 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
chain, whereas the endopolygalacturonases can cleave the pectin chain at random within
the chain. The activities of these enzymes increase during the ripening and softening of
the fruit. Two exo-PG isozymes have been identified in peach, having a relative molecular
mass of near 66 kDa. The exo-acting enzymes are activated by calcium. Peach endo-PG is
observed to be similar to the tomato endo-PG. The peach endo-PG is inhibited by calcium.
The freestone peaches possess enhanced activities of both exo-PG and endo-PG leading to a
high degree of fruit softening. However, the clingstone varieties with low levels of endo-PG
activity do not soften as the freestone varieties. In general, fruits such as peaches, tomatoes,
strawberries, and pears, which soften extensively, possess high levels of endo-PG activity.
Apple fruits, which remain firm, lack endo-PG activity.
3.3.1.2 Starch degradation
Starch is the major storage form of carbohydrates. During ripening, starch is catabolized
into glucose and fructose, which enters the metabolic pool where they are used as respiratory
substrates or further converted to other metabolites (Fig. 3.2). In fruits such as banana, the
breakdown of starch into simple sugars is associated with fruit softening. There are several
enzymes involved in the catabolism of starch. α-Amylase hydrolyzes amylose molecules
by cleaving the α-1,4-linkages between sugars, providing smaller chains of amylose termed
as dextrins. β-Amylase is another enzyme that acts on the glucan chain, releasing maltose,
which is a diglucoside. The dextrins as well as maltose can be further catabolized to simple
glucose units by the action of glucosidases. Starch phosphorylase is another enzyme, which
mediates the phosphorolytic cleavage of terminal glucose units at the nonreducing end of
the starch molecule using inorganic phosphate, thus releasing glucose-1-phosphate. The
amylopectin molecule is also degraded in a similar manner to amylose, but also involves
the action of debranching enzymes, which cleaves the α-1,6-linkages in amylopectin and
releases linear units of the glucan chain.
In general, starch is confined to the plastid compartments of fruit cells, where it exists as
granules made up of both amylose and amylopectin molecules. The enzymes that catabolize
starch are also found in this compartment and their activities increase during ripening. The
glucose-1-phosphate generated by starch degradation (Fig. 3.2) is mobilized into the cytoplasm
where it can enter into various metabolic pools such as that of glycolysis (respiration),
pentose phosphate pathway, or for turnover reactions that replenish lost or damaged cellular
structures (cell wall components). It is important to visualize that the cell always tries to
extend its life under regular developmental conditions (the exceptions being programmed
cell death that occurs during hypersensitive response to kill invading pathogens, thus killing
both the pathogen and the cell/tissue, formation of xylem vessels, secondary xylem tissues,
etc.), and the turnover reactions are a part of maintaining the homeostasis. The cell ultimately
succumbs to the catabolic reactions during senescence. The compartmentalization
and storage of chemical energy in the form of metabolizable macromolecules are all the
inherent properties of life, which is defined as a struggle against increasing entropy.
The biosynthesis and catabolism of sucrose is an important part of carbohydrate
metabolism. Sucrose is the major form of transport sugar and is translocated through the
phloem tissues to other parts of the plant. It is conceivable that photosynthetically fixed carbon
from leaf tissues may be transported to the fruits as sucrose during fruit development.
Sucrose is biosynthesized from glucose-1-phosphate by three major steps (Fig. 3.3). The first
reaction involves the conversion of glucose-1-phosphate to UDP-glucose, by UDP-glucose