Food Biochemistry and Food Processing (2 edition)

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.