Enzymes in Quality and Processing of Tropical and Subtropical Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 43

other nonenzymatic reactions (Maillard and Strecker mech-
anisms) (Cano et al., 1997).
Miesle et al. (1991) demonstrated that POD has various
functions in the ripening and postripening processes,
including changes in cell wall plasticity and anthocyanin
breakdown. It may also promote lipid oxidation with
consequent off-flavor formation. Bruemmer et a1. (1976)
associated POD activity with the loss of flavor quality in
orange juice.
Peroxidases have been investigated and characterized in
a variety of fruits, including avocado (Sharon-Raber and
Kahn, 1983), banana (Cano et al., 1997), mango (Ndiaye
et al., 2009), and papaya (Cano et al., 1998).

CELLULASE

Nomenclature and reactions catalyzed
Cellulases (EC 3.2.1.4) cleave the -1,4 linkages of cel-

lulose or its chemically modified forms, in addition to de-
grading cellodextrin or cellobiose. They will also hydrolyze
1,4-linkages in β -D-glucans also containing 1,3-linkages.
Typically, they are multienzyme complexes bearing endo-
1,4- β -glucanases, cellobiohydrolase, and

β

β -glucosidase

activity (Goyal et al., 1991).

Occurrence and activity of cellulase
in tropical and subtropical fruits
Cellulase activity has been detected in different fruits,
where it may exist in multiple forms (Pesis et al., 1978;
Awad and Young, 1979). In fruits, cellulase activity is linked
to physiological processes associated with fruit softening
during the ripening stage (Rose and Bennett, 1999). Soft-
ening of fruits is the result of texture modifications that take
place during ripening (Abu-Goukh and Bashir, 2003). Dur-
ing softening, fruits are more susceptible to physical injury
as well as diseases. As mentioned elsewhere in this chap-
ter, fruit softening is attributed to cell wall disassembly and
modifications to pectin, involving polygalacturonase (PG)
and cellulase (White, 2002).
Cellulase activity is implicated in fruit softening, pre-
sumably by contributing to cell wall degradation (Pesis
et al., 1978; Awad and Young, 1979). This is supported
by the high correlation between the increased cellulase ac-
tivity and the reduced firmness of fruit flesh (Abu-Goukh
and Bashir, 2003). In avocados, several authors have in-
vestigated the relationship between softening and the ac-
tivity of cell wall-degrading enzymes. Pesis et al. (1978)
found a direct correlation between cellulase activity and
softening, respiration, and ethylene production. Similarly,
Awad (1977) found a strong correlation between the rapid
increase in cellulase content after harvest, the climacteric
rise in respiration, and softening of the avocado fruit, where
edible softness occurred before maximum cellulase levels
were reached.
Cellulase also plays an important role in fruit and juice
processing. The combination of cellulase and pectinase en-
zymes has been shown to be very effective in the enzymatic
hydrolysis of polysaccharides in orange peel (Grohmann
and Baldwin, 1992). In pineapple, pulp treated with cellu-
lase and pectinase enzymes yielded significant increases in
juice recovery (Sreenath et al., 1994). Also, improved filtra-
tion, clarity, and flow of pineapple juice during processing

Control of POD in tropical and subtropical fruits
POD is known to be one of the most heat-stable enzymes
present in plants; therefore, if this enzyme is destroyed dur-
ing blanching, then it is unlikely that other enzymes would
have survived deactivation (Halpin and Lee, 1987). Hence,
POD has been the most widely used indicator enzyme for
determining the adequacy of blanching processes (Halpin
and Lee, 1987).
Heat treatment through blanching is one of the most
commonly used methods of controlling POD activity in
vegetables and has been used in fruits (Ndiaye et al., 2009).
Efficient blanching treatments, however, require knowledge
of several factors, including enzymatic distribution within
the tissue, inactivation kinetic parameters, and relative pro-
portions of heat-labile and heat-resistant fractions (Adams,
1991).
Since some of the isozymes are highly heat stable, com-
plete inactivation during blanching may cause high losses
of nutrients, flavor, color, and texture (Ganthavorn and
Powers, 1988). Thus control of blanching to leave residual
POD activity, almost equivalent to the heat-stable isozyme,
has been recommended (Williams et al., 1986). In banana,
blanching has been demonstrated as an effective method
to inactivate 96% to 100% of POD and PPO, where im-
mersing peeled bananas in boiling water has been shown
to control browning of frozen sliced bananas (Cano et al.,
1990). Steam blanching and microwave blanching have also
been employed and found effective for bananas, but steam
blanching was found to be significantly more effective in
POD inactivation (Cano et al., 1997). Total inactivation of
POD has also been achieved with nonthermal methods, like
supercritical carbon dioxide and high-pressure treatment
(Tedjo et al., 2000), which may be effective in retaining the
freshness and flavor profile.