Role of Polyphenol Oxidase in Anthocyanin Degradation of Lychee Pericarp

【Objective】 It is well established that anthocyanin and polyphenol oxidase (PPO) are involved in pericarp browning of harvested lychee fruit. However, the mechanism by which PPO catalyzes the anthocyanin degradation and pericarp browning has not been well understood yet. 【Method】To understand the role of PPO in anthocyanin degradation, the in vitro degradation of anthocyanins was analyzed in the reactions containing the purified anthocyanin, purified PPO or its immobilized style, exogenous phenol, anthocyanidin or its degraded products.【Result】Purified PPO did not catalyze the degradation of purified anthocyanin. However, in the presence of catechol, anthocyanin was degraded rapidly by PPO and then formed to be brown by-products. PPO directly catalyzed the oxidation of anthocyanidin or its degraded by-product. A sequential reaction system of PPO-phenol-anthocyanin was established by application of the immobilized PPO. The study indicated that the anthocyanin degradation was independent of PPO, but dependent of the quinones generated by PPO.【Conclusion】It was suggested that, during the pericarp browning of lychee fruit, PPO first catalyzes the oxidation of phenols, anthocyanidin and/or its degraded products, then forms quinones, and finally quinones oxidize anthocyanin to brown by-products, leading to the decoloration, browning and reduced anthocyanin content.