We have shown and others that cysteine may act as PPO inhibitor or, by
adding sufficient amount, react with quinones, because of the high
nucleophilicity of the thiol group, to give colorless adduct. It should be noted that other enzymes contribute to the browning process e.g. phenylalanine ammonia-lyase (PAL) and peroxidase (POD); activities of both enzymes increase in stressed plants.