Therefore, browning
inhibition mechanisms of most inhibitors generally fall in two
main categories; first, direct PPO inhibitors either by forming
enzyme-inhibitor complex because of structural similarity to the
enzyme substrates e.g. 4-hydroxy resorcinol, kojic acid, benzoic
acid cinnamic acid and ferulic acid, or by chelating the copper
ion from the enzyme active site e.g. sorbic acid, oxalic acid, citric
acid, tartaric acid, quercetin, hexametaphosphate, amino acids
and peptides or proteins. In addition, organic acids deactivate the
enzyme by lowering the pH than that required for optimum activity.
Second, browning inhibitors that either react with quinones by
reduction to give back the uncolored catechols e.g. sulfites and
ascorbic acid, or form a colorless adduct e.g. cysteine