Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

26 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
of the fruit, which entails the development of ideal organoleptic characters such as taste,
color, and aroma that are important features of attraction for the vectors (animals, birds,
etc.) responsible for the dispersal of the fruit, and thus the seeds, in the ecosystem. Human
beings have developed an agronomic system of cultivation, harvest, and storage of fruits
with ideal food qualities. In most cases, the ripening process is very fast, and the fruits
undergo senescence resulting in the loss of desirable qualities. An understanding of the
biochemistry and molecular biology of the fruit ripening process has resulted in developing
biotechnological strategies for the preservation of postharvest shelf life and quality of
fruits.
In response to the initiation of ripening, several biochemical changes are induced in the
fruit, which ultimately results in the development of ideal texture, taste, color, and flavor.
Several biochemical pathways are involved in these processes as described next.
3.3.1 Carbohydrate metabolism
3.3.1.1 Cell wall degradation
Cell wall degradation is the major factor that causes softening of several fruits. This involves
the degradation of cellulose components, pectin components, or both. Cellulose is degraded
by the enzyme cellulase or β-1,4-glucanase. Pectin degradation involves the enzymes pectin
methylesterase, polygalacuronase (pectinase), and β-galactosidase. The degradation of cell
wall can be reduced by the application of calcium as a spray or drench in apple fruits. Calcium
binds and cross-links the free carboxylic groups of polygalacturonic acid components in
pectin. Calcium treatment therefore also enhances the firmness of the fruits.
The activities of both cellulase and pectinase have been observed to increase during
ripening of avocado fruits and result in their softening. Cellulase is an enzyme with a relative
molecular mass of 54.2 kDa and formed by extensive posttranslational processing
of a native 54-kDa protein involving proteolytic cleavage of the signal peptide and glycosylation
(Bennet and Christofferson, 1986). Further studies have shown three isoforms of
cellulose ranging in molecular masses between 50 and 55 kDa. These forms are associated
with the endoplasmic reticulum, the plasma membrane, and the cell wall (Dallman et al.,
1989). The cellulase isoforms are initially synthesized at the style end of the fruit at the
initiation of ripening, and the biosynthesis moves toward the stalk end of the fruit with the
advancement of ripening. Degradation of hemicelluloses (xyloglucans, glucomannans, and
galactoglucomannans) is also considered as an important feature that leads to fruit softening.
Degradation of these polymers could be achieved by cellulases and galactosidases.
Loss of pectic polymers through the activity of polygalacturonases (PG) is a major
factor involved in the softening of fruits such as tomato. There are three major isoforms of
polygalacturonases responsible for pectin degradation in tomato, designated as PG1, PG2a,
and PG2b (Fischer and Bennet, 1991). PG1 has a relative molecular mass of 100 kDa, and
is the predominant form at the initiation of ripening. With the advancement of ripening,
PG2a and PG2b isoforms increase, becoming the predominant isoforms in ripe fruit. The
different molecular masses of the isozymes result from the posttranslational processing and
glycosylation of the polypeptides. PG2a (43 kDa) and PG2b (45 kDa) appear to be the same
polypeptide with different degrees of glycosylation. PG1 is a complex of three polypeptides:
PG2a, PG2b, and a 38-kDa subunit known as the β-subunit. The 38-kDa subunit is believed
to exist in the cell wall space where it combines with PG2a and PG2b, forming the PG1