Chlorogenic acid (CQA) is one of the major polyphenols in apple and a good substrate for the polyphenol oxidase (PPO) in apple. Apple contains catechins as well as CQA, and the role of CQA quinone and its interaction with catechins in the enzymatic browning of apple were examined. Browning was repressed and 2-cysteinyl-CQA was formed when cysteine was added to apple juice. CQA quinone was essential for browning to occur. Although catechins and CQA were oxidized by PPO, some catechins seemed to be non-enzymatically oxidized by CQA quinone.

Mung bean sprouts turn brown during cold storage. Here we showed that heat-shock treatment inhibited this browning by repressing the induction of phenylalanine ammonia-lyase (PAL) activity and the biosynthesis of polyphenols during cold storage. Mung bean sprouts were soaked in warm water at 50℃ for 60 s as heat-shock treatment and then stored at 8℃ for 6 days. Heat-shocked mung bean sprout turned less brown than that of the control sprout. The PAL activity in the control significantly increased during cold storage, while that in heat-shocked mung bean sprouts did not. The polyphenol content in heat-shocked mung bean sprouts was lower than that in the control at 3 and 6 days. There was no significant difference in polyphenol oxidase activity between the control and heat-shocked mung bean sprouts during cold storage. These results show that mild heat treatment inhibited the enzymatic browning of mung bean sprouts by repressing the induction of PAL activity during cold storage and the accumulation of polyphenols.

We screened for inhibitors of enzymatic browning among fermented broths of lactic acid bacteria (58 strains), extracts of spices (7 samples), and food additives (34 samples) from the standpoint of practical use. We found that hinokitiol, a preservative, definitely inhibited the browning of apple sections. An apple section sprayed with a 0.02% hinokitiol solution did not turn brown for 2 h at room temperature. Hinokitiol competitively inhibited apple polyphenol oxidase (PPO). The Ki value of hinokitiol against apple PPO was 5.5 μM, while the Km value of chlorogenic acid was 2.5 mM. Furthermore hinokitiol non-competitively inhibited tyrosinase and the discoloration of shrimp caused by tyrosinase during cold storage.