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