Purification of polyphenol oxidase and the browning control of litchi fruit by glutathione and citric acid

Abstract

Polyphenol oxidase (PPO, EC 1.10.3.2) was purified to homogeneity from litchi peel yielding a single protein with a molecular weight of about 75.6 kD by Sephadex G-100 gel filtration, and a 108-fold purification of PPO achieved. The enzyme was determined to be composed of two similar subunits. Glutathione, L-cysteine and citric acid suppressed PPO activity markedly, whereas ascorbic acid and n-propyl gallate showed a little inhibition. Moreover, the effect was enhanced by the addition of citric acid. On the basis of the inhibition of PPO activity in vitro, the use of 10 mmol l −1 glutathione and 100 mmol−1 l citric acid was found to give good control of the browning of litchi fruit, and an 80–85% inhibition of PPO activity was observed. It is suggested that application of glutathione in combination with citric acid may slow down the browning of litchi fruit. © 1999 Society of Chemical Industry