Identification of (−)-epicatechin as the direct substrate for polyphenol oxidase isolated from litchi pericarp

Postharvest browning of litchi fruit results in a short life and a reduced commercial value. The experiments were conducted to separate, purify and identify the polyphenol oxidase (PPO) substrates that cause litchi fruit to brown. PPO and its substrates were, respectively, extracted from fruit pericarp tissues. The substrates for litchi PPO were separated and purified using polyamide column chromatography, silica gel column chromatography and preparative thin layer chromatography. The substrate was further identified by 0.5% FeCl3 solution and enzymatic reaction with litchi PPO. On the basis of UV, 1H NMR, 13C NMR, and ESI-MS data, the direct substrate for the PPO from litchi fruit pericarp tissues was identified to be (−)-epicatechin.