Chemical mechanism advances of enzymatic browning reaction in postharvest lychee and longan fruits

The pericarp tissues of lychee (Litchi chinensis Sonn.) and longan (Dimocarpus longan Lour.) turns brown easily immediately after harvesting, which greatly affects the storage and commercial values of two fruits. The postharvest browning of lychee and longan fruits was mainly due to the enzyme-catalyzed oxidation of polyphenol oxidase (PPO) and phenolic substrates in pericarp cells. Basing off of recent relevant researches, this paper comprehensively reviewed the enzymatic browning mechanism, composition, and chemical structure identification of enzymatic browning substrates, effects of storage, and transportation conditions, on substrate properties and changes in substrate contents during storage. It was discovered that except for anthocyanin, the properties of other phenols browning substrates in pericarp of lychee and longan were seldom reported; hence, the types and chemical structure identification of enzymatic browning substrates, effects of environmental factors on changes in PPO substrates and enzyme activity, relationship between environmental factors, differences in browning process in different varieties, and changes in PPO substrates content during storage and transportation still need to be further studied in order to provide bases for controlling pericarp enzymatic browning during storage and transportation of postharvest lychee and longan.