Food Biochemistry and Food Processing

4 Browning Reactions 73

oxidase activity (Fig. 4.1). The p-diphenol oxidase–
catalyzed reaction follows a Bi Bi mechanism (Whi-
taker 1972). Diphenolase activity may be due to two
different enzymes: catecholase (catechol oxidase)
and laccase (Fig. 4.1). Laccase (p-diphenol oxidase,
EC 1.10.3.2)(DPO) is a type of copper-containing
polyphenol oxidase. It has the unique ability to oxi-
dize p-diphenols. Phenolic substrates, including poly-
phenols, methoxy-substituted phenols, diamines, and
a considerable range of other compounds, serve as
substrates for laccase (Marshall et al. 2000). Lac-
cases occur in many phytopathogenic fungi, higher
plants (Mayer and Harel 1991), peaches (Harel et al.
1970), and apricots (Dijkstra and Walker 1991).
Catechol oxidase and laccase are distinguishable
on the basis of both their phenolic substrates (Fig.
4.1) and their inhibitor specificities (Marshall et al.
2000).
Catecholase activity is more important than cres-
olase action in food because most of the phenolic
substrates in food are dihydroxyphenols (Mathew
and Parpia 1971).

This bifunctional enzyme, polyphenol oxidase

(PPO), containing copper in its structure has been

described as an oxygen and four-electron transfer-

ring phenol oxidase (Jolley et al. 1974). Figure 4.2

shows a simplified mechanism for the hydroxylation

and oxidation of phenols by PPO. Both mechanisms

involve the two copper moieties on the PPO.

Polyphenol oxidase, active between pH 5 and 7,

does not have a very sharp pH optimum. At lower

pH values of approximately 3, the enzyme is irre-

versibly inactivated. Reagents that complex or re-

move copper from the prosthetic group of the en-

zyme inactivate the enzyme (Fennema 1976).

SUBSTRATES

Although tyrosine is the major substrate for certain

phenolases, other phenolic compounds such as caf-

feic acid and chlorogenic acid also serve as sub-

strates (Fennema 1976). Structurally, they contain

an aromatic ring bearing one or more hydroxyl

groups, together with a number of other substituents

Figure 4.2.Simplified mechanism for the hydroxylation and oxidation of diphenol by phenoloxidase.