Food Biochemistry and Food Processing

72 Part I: Principles

and juices. Lettuce and other green leafy vegetables;
potatoes and other starchy staples such as sweet po-
tato, breadfruit, and yam; mushrooms; apples; avo-
cados; bananas; grapes; olive (Sciancalepore 1985);
peaches; pears (Vamosvigyazo and Nadudvari-
markus 1982); and a variety of other tropical and
subtropical fruits and vegetables are susceptible to
browning. Crustaceans are also extremely vulnera-
ble to enzymatic browning. Since enzymatic brown-
ing can affect the color, flavor, and nutritional value
of these foods, it can cause tremendous economic
losses (Marshall et al. 2000).
A better understanding of the mechanism of enzy-
matic browning in fruits, vegetables, and seafood;
the properties of the enzymes involved; and their
substrates and inhibitors may be helpful for control-
ling browning development, avoiding economic loss-
es, and providing high quality foods. Based on this
knowledge, new approaches for the control of enzy-
matic browning have been proposed. These subjects
will be reviewed in this chapter.

PROPERTIES OFPOLYPHENOLOXIDASE(PPO)

PPO (EC 1.10.3.1; o-diphenol oxidoreductase) is an
oxidoreductase able to oxidize phenol compounds
employing oxygen as a hydrogen acceptor. The ab-

undance of phenolics in plants may be the reason for

naming this enzyme polyphenol oxidase (Marshall

et al. 2000). The molecular weight for polyphenol

oxidase in plants ranges between 57 and 62 kDa

(Hunt et al. 1993, Newman et al. 1993). Polyphenol

oxidase catalyzes two basic reactions: (1) hydroxy-

lation to the o-position adjacent to an existing hy-

droxyl group of the phenolic substrate (monophenol

oxidase activity) and (2) oxidation of diphenol to o-

benzoquinones (diphenol oxidase activity) (Fig. 4.1).

In plants, the ratio of monophenol to diphenol

oxidase activity is usually in the range of 1:40 to

1:10 (Nicolas et al. 1994). Monophenol oxidase ac-

tivity is considered more relevant for insect and

crustacean systems due to its physiological signifi-

cance in conjunction with diphenolase activity (Mar-

shall et al. 2000).

Polyphenol oxidase is also referred to as tyrosi-

nase to describe both monophenol and diphenol oxi-

dase activities in either animals or plants. The mon-

ophenol oxidase acting in plants is also called

cresolase due to its ability to employ cresol as a sub-

strate (Marshall et al. 2000). This enzyme is able

to metabolize aromatic amines and o-aminophenols

(Toussaint and Lerch 1987).

The oxidation of diphenolic substrates to quinones

in the presence of oxygen is catalyzed by diphenol

Figure 4.1.Polyphenol oxidase pathway.