In
the presence of oxygen, PPO catalyses the hydroxylation
of monophenols to O-diphenols and the oxidation of
O-diphenols to their corresponding O-quinones. These
in turn are polymerised to undesirable brown, red or
black pigments (Mason, 1955). Peroxidase is another
oxido-reductase enzyme involved in enzymatic browning
as diphenol may function as the reducing substrate
in the reaction (Robinson, 1991). Lee et al. (1990)
reported that the higher the PPO activity, the higher the
rate of browning.