Food Biochemistry and Food Processing (2 edition)

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BLBS102-c04 BLBS102-Simpson March 21, 2012 11:59 Trim: 276mm X 219mm Printer Name: Yet to Come


4 Browning Reactions 57

that which occurs in plants in which the compounds produced
as a result of the polymerization of quinones, melanins, or
melanoidins exhibit both antibacterial and antifungal activities.
In addition, enzymatic reactions are considered desirable during
fermentation (Fennema 1976). Despite these positive effects,
enzymatic browning is considered one of the most devastating
reactions for many exotic fruits and vegetables, in particular
tropical and subtropical varieties. Enzymatic browning is espe-
cially undesirable during processing of fruit slices and juices.
Lettuce, other green leafy vegetables; potatoes and other starchy
staples, such as sweet potato, breadfruit, and yam; mushrooms;
apples; avocados; bananas; grapes; olive (Sciancalepore 1985);
peaches; pears (Vamosvigyazo and Nadudvarimarkus 1982);
and a variety of other tropical and subtropical fruits and
vegetables are susceptible to browning. Crustaceans are also
extremely vulnerable to enzymatic browning. Since enzymatic
browning can affect color, flavor, and nutritional value of these
foods, it can cause tremendous economic losses (Marshall et al.
2000, Queiroz et al. 2008).
A better understanding of the mechanism of enzymatic
browning in fruits, vegetables, and seafood; the properties of
the enzymes involved; and their substrates and inhibitors may
be helpful for controlling browning development, avoiding
economic losses, and providing high-quality foods. On the basis
of this knowledge, new approaches for the control of enzymatic
browning have been proposed. These subjects will be reviewed
in this chapter.

Properties of PPO

PPO (EC 1.10.3.1;o-diphenol oxidoreductase) is an oxidore-
ductase able to oxidize phenol compounds, employing oxygen

as a hydrogen acceptor. The abundance of phenolics in plants
may be the reason for naming this enzyme PPO (Marshall et al.
2000). The molecular weight for PPO in plants ranges between
57 and 62 kDa (Hunt et al. 1993, Newman et al. 1993). PPO
catalyzes two basic reactions: hydroxylation to theo-position
adjacent to an existing hydroxyl group of the phenolic substrate
(monophenol oxidase activity) and oxidation of diphenol too-
benzoquinones (DPO activity; Figure 4.1).
In plants, the ratio of monophenol to DPO activity is usually
in the range of 1:40–1:10 (Nicolas et al. 1994). Monophenol
oxidase activity is considered more relevant for insect and crus-
tacean systems due to its physiological significance in conjunc-
tion with diphenolase activity (Marshall et al. 2000).
PPO is also referred to as tyrosinase to describe both
monophenol and DPO activities in either animals or plants. The
monophenol oxidase acting in plants is also called cresolase be-
cause of its ability to employ cresol as substrate (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 pres-
ence of oxygen is catalyzed by DPO activity (Figure 4.1). The
p-DPO catalyzed reaction follows a Bi Bi mechanism (Whitaker
1972). Diphenolase activity may be due to two different en-
zymes: catecholase (catechol oxidase) and laccase (Figure 4.1).
Laccase (p-DPO,EC 1.10.3.2) is a type of copper-containing
PPO. It has the unique ability to oxidizep-diphenols. Phenolic
substrates, including polyphenols, methoxy-substituted phenols,
diamines, and a considerable range of other compounds, serve
as substrates for laccase (Marshall et al. 2000). Laccases occur
in many phytopathogenic fungi, higher plants (Mayer and Harel
1991), peaches (Harel et al. 1970), and apricots (Dijkstra and
Walker 1991).

OH

OH OH

OH

RR

O
Polymer
brown
melanoidin

o-Dihydroxyphenol o-Quinone

p-Dihydroxyphenol

Monohydroxyphenol

p-Quinone

O

R

OH

R

OH

Diphenol oxidase
(laccase)

Diphenol oxidase
(catecholase or catechol oxidase)

Monophenol oxidase
(cresolase)

OH

R

Figure 4.1.Polyphenol oxidase pathway.
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