Enzymes in Quality and Processing of Tropical and Subtropical Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 42

Figure 3.2. The catalytic cycle of peroxidase with ferulate as reducing substrate. The rate constants k 1 , k 2 ,
and k 3 represent the rate of compound I formation, rate of compound I reduction, and rate of compound II
reduction, respectively (source: Veitch, 2004).

activity has been shown to increase in the roots of NaCl-
stressed plants (Botella et al., 1994). PODs play a key
role in controlling plant differentiation and development.
Various authors suggest the involvement of plant PODs
not only in biosynthetic processes related to wall develop-
ment, such as lignification (Lagrimini et al., 1987), suber-
ization (Roberts and Kolattukudy, 1989), and polymeriza-
tion of hydroxyproline-rich glycoproteins (Bradley et al.,
1992), but also in the regulation of cell wall elongation
(Greppin et al., 1986) and wound healing (Lagrimini and
Rothstein, 1987).
POD is of great interest in food technology because of
its influence on the quality of raw and processed fruits and
vegetables (Robinson, 1991). In the presence of peroxide,
POD produces phytotoxic free radicals, which react with
a wide range of organic compounds like ascorbic acid,
carotenoids, and fatty acids, leading to losses in the color,
flavor, and nutritional value of raw and processed foods
(Robinson, 1987).

Activity of POD in tropical and
subtropical fruits
Together with lipoxygenase, catalase (EC 1.11.1.6), and
polyphenol oxidase, POD has been considered one of the
principal enzymes responsible for quality deterioration in
frozen fruits (Whitaker, 1985). However, though activ-
ities of these other enzymes diminish with lower tem-
peratures, POD is very stable at low temperatures (Cano
et al., 1998). Freezing and subsequent storage of frozen
fruits and vegetables usually lead to only a partial in-
activation of this enzyme, where regeneration often oc-
curs and enzyme activity is frequently observed after
long periods resulting in quality deterioration (Marin and
Cano, 1992).
POD can contribute to adverse changes in the flavor,
texture, color, and nutritional value of raw and processed
fruits (Vamos-Vigyazo, 1981). Browning in banana ( Musa
cavendishii cv. Enana) products, for example, is a result
of phenol oxidation catalyzed by PPO and POD or of