Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

174 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
endotransglycosylase (XTH, EC 2.4.1.207), the endo-β-1,4-glucanases (EGases, EC
3.2.1.4; previously known as cellulases), endo-β-mannanase (EC 3.2.1.78) (Bewley et al.,
2000), β-D-xylosidases (EC 3.2.1.37) (Itai et al., 2003), and expansins. Mannan transglycosylases
regulated during ripening are very recent additions to the list of candidates for roles
in glycan network disassembly (Schroder et al., 2004). Ripening-associated modifications
in hemicellulose-cellulose structure appear to greatly influence the accessibility of multiple
enzymes to their substrates. Expansins target components of the glycan superstructure
in ripening fruits, indirectly facilitating hydrolysis of the HGA substrate by one or more
pectinase (Brummell et al., 1999).
Efficient distribution of C-14-starch into glucose, fructose, and sucrose revealed considerable
sugar interconversions indicating active gluconeogenesis during mango fruit ripening
(Yashoda et al., 2006). But the texture of Kent mango is most likely moderated by changes in
solubility of insoluble pectin or by nonpectin components in the cell wall as the temperature
gradient infusion of fresh-cut mangoes with PME and/or calcium chloride had no impact
on hardness and adhesiveness (Banjongsinsiri et al., 2004).
Biochemical changes during ripening of capsicum are characterized as an increase in
free sugar levels, an increase in in situ hydrolysis of some hemicellulose fractions (Hem a,
b, and c), and a general increase in the activity of cellulase, α-mannosidase, laminarinase,
polygalacturanase, galactanase, mannanase, β-galactosidase, and hemicellulase (HCe) activity
on Hem b and c. But the activity of xylanase, PME, and HCe on Hem a decreased
during ripening (Prabha et al., 1998). Jagadeesh et al. (2004a) observed ripening specificity
for β-hexosaminidase and an interrelationship between this and α-mannosidase activities,
which may be responsible for the textural softening associated with capsicum fruit
ripening.
The amount of cellulose does not decline during ripening in most fruits (Maclachlan
and Brady, 1994; Sakurai and Nevins, 1997), and it is presumed that depolymerization of
cellulose is not a major feature of ripening. In avocado, a loss of fibrillar material from the
wall occurred during ripening that was due to the depolymerization of large unbranched
molecules of cellulose and an increase in the proportion of crystalline cellulose, suggesting
a preferential degradation of peripheral amorphous cellulose chains (Platt-Aloia et al., 1980;
O’Donoghue et al., 1994).
During peach fruit development and ripening, cell wall undergoes several structural
and biochemical changes driven by several hydrolases (Bonghi et al., 1998). Among these,
the endo-β-1,4-β-glucanase (EGase), or cellulase, may play a crucial role in cell wall
hydrolysis. During the four stages of peach growth, EGase activity was high during S1 and
early S2, declined during S3, and increased with the onset of ripening (S4), implying that
the EGases is involved in early fruit growth and the initial phases of softening. The presence
of two isoforms and the dual effect of propylene on enzyme activity suggest that different
EGase genes operate during the early and late developmental stages in peach.
Detectable endo-β-1,4-glucanases (EGases) enzyme activity is first observed in large
green fruits of strawberry, but a steep increase occurs in white fruits when the ripening
process starts (Trainotti et al., 1999). This process is then accompanied by a further increase
in EGase activity, which appears to be doubled in red ripe fruits.
An increase in cellulase (endo-1,4-β-D-glucanase) accompanies progressive softening,
loss of skin strength, and a breakdown of cell walls in the mesocarp of raspberry, indicating
its involvement in fruit separation as well as softening (Sexton et al., 1997). The initial