Identification of procyanidin A2 as polyphenol oxidase substrate in pericarp tissues of litchi fruit

Postharvest browning of litchi fruit results in short shelf life and reduced commercial value. Experiments were conducted to separate, purify and identify polyphenol oxidase (PPO) substrates that cause litchi fruit to brown. PPO and its substrate were extracted from the pericarp tissues of litchi fruit. The litchi PPO substrate was purified using polyamide column, silica gel column and Sephadex LH-20 column chromatography. The browning substrate was selected by a 0.5% FeCl3 solution and then identified using a partially purified litchi PPO. Analyses of ultraviolet spectrometry, nuclear magnetic resonance and electrospray ionization mass spectrometry indicated that the PPO substrate was procyanidin A2. The substrate can be oxidized to o-quinones by litchi PPO and then fibrin brown-colored by-products, resulting in pericarp browning of harvested litchi fruit.