Study on enzymatic browning control of lychee fruits during pretreatment of freeze drying

The effect of several pretreatments on polyphenol oxidase ( PPO) and peroxidase ( POD) activity were studied for Huaizhi lychee. PPO and POD activity were decreased after being immersed in 1mmol /L glutathione solution but the treatment had no effect on PPO and POD inactivity.After being immersed in 1mmol /L glutathione solution and ascorbic acid solution in vacuum condition ( 0.08MPa ) , PPO and POD activity were also not passivated.The treatment of blanching at 90℃ for over 3min could result in POD passivation. The treatment of blanching in 0.3% ascorbic acid solution at 90℃ for over 3min could lead to passivation of PPO. The optimal blanching time was 5min by considering the results of color and crispness of freeze dried lychees.