Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE ROLE OF POLYPHENOLS 271
OH
OH
O
OH
O 2 O 2
Cresolase
Catecholase
O
R
R
R
(a)
Monophenol
o-Diphenol (Catechol)
o-Benzoquinone
OH
O
O 2
Laccase
(b)
OH
p-Diphenol
O
p-Benzoquinone
Fig. 12.6 Polyphenol oxidation reactions catalyzed by polyphenol oxidase (PPO): (a) monophenol oxidation
pathway catalyzed by cresolase and o-diphenol oxidase (catecholase); and (b) p-diphenol oxidation catalyzed by
p-diphenol oxidase (laccase). (Adapted from Marshall et al., 2000.)
secondary reactions (nonenzymatic reactions) to form higher-molecular-weight polymers;
to form macromolecular complexes with amino acids or proteins; and to oxidize compounds
of lower oxidation–reduction potentials (Vamos-Vigyazo, 1981). The third types of reactions
are considered to be most destructive as quinones can be reduced back to dihydroxyphenols.
Hence, these continue to provide fresh substrate for PPO until it gets inactivated by reaction
products or the compounds of lower oxidation–reduction potentials such as ascorbic acid
that gets depleted (Pifferi and Cultrera, 1974). This is one of the mechanisms responsible
for the inhibitory action of ascorbic acid in enzymatic browning (Baruah and Swain, 1953;
deMan, 1990; Ozdemir, 1997). Laccase is also a type of PPO, which has the unique ability
of oxidizing p-diphenols, which is not shown by o-diphenol oxidases such as catechol
oxidase; however, laccase does not act on monophenols. It is less frequently encountered
in fruits and vegetables except some peach cultivars (Harel et al., 1970), mushrooms, and
tomatoes.
POX, just as PPO, belong to the same group of enzymes, oxidoreductases (Vamos-
Vigyazo, 1981). POX is widely distributed in nature and catalyzes the decomposition of
hydrogen peroxide (H 2 O 2 ) in the presence of a hydrogen donor. A great variety of compounds
may act as hydrogen donors, including phenols (p-cresol, guaiacol, and resorcinol),
aromatic amines (aniline and benzidine), reduced nicotinamide-adenine dinucleotide, and
reduced nicotinamide-adenine dinucleotide phosphate (Vamos-Vigyazo, 1981). It has been
observed that the wounding of fruits results in an increase of POX activity besides PPO
activity (Cantos et al., 2002). The generation of H 2 O 2 by POX during the oxidation of phenolics
catalyzed by PPO also suggests the role of POX in enzymatic browning processes
(Subramanian et al., 1999). However, the content of H 2 O 2 is very less in the plant tissue, but