Inhibition of loquat enzymatic browning by sulfhydryl compounds

The effectiveness of a series of sulfhydryl compounds in inhibiting polyphenoloxidase (PPO) activity in a model system (chlorogenic acid solution) and in loquat juices was evaluated. Application of different concentrations of sulfhydryl compounds to chlorogenic acid solution and fresh loquat juice showed that l-cysteine appeared to be an effective browning inhibitor. The required concentration of l-cysteine for 90% browning inhibition depended on loquat cultivars, and ranged from 0.6 mM for “Nagasaki” to 2.0 mM for “Yukawa” or “Toi”. The difference may be due to the concentration of endogenous phenolic compounds inherent in each cultivar. The results from oxidized product analysis by loquat PPO implied that the mechanism of browning inhibition by thiols involved the formation of a thiol-conjugated reaction product.