Enzymatic Property of Mulberry Leaf Polyphenol Oxidase and Peroxidase

Polyphenol oxidase( PPO) and peroxidase( POD) are the main factors that lead to browning of mulberry leaf and its derived products. Understanding enzymatic properties of PPO and POD will facilitate inhibition of enzyme-mediated browning in mulberry leaf products through adjusting processing conditions. Studies on stability of PPO and POD under different temperature,pH value,and sonication conditions showed that both enzymes had an optimum temperature of 40 ℃,and high temperature( 70 ℃) treatment for short time( 2 min) could inhibit activity of both enzymes obviously.The optimum pH value for PPO and POD activity was both 7. 0,and weak acid( pH 6. 0) or weak base( pH 8. 0) treatment for short time( 5 min) had little inhibitory effect on their activity. Ultrasonic treatment at power of 500 W and frequency of 4 kHz for 30 min could not inactivate the enzymes either. In addition,effect of chemical inhibitors on mulberry leaf PPO and POD activity was tested. It was found that ascorbic acid,sodium thiosulfate and sodium sulfite could inhibit activity of the two enzymes in a clear dose dependent manner within the concentration range of 0. 1 to10. 0 mmol/L. Therefore,in order to preserve nutritional active substances and improve quality of mulberry leaf products,it is recommended to use high temperature( 70℃) treatment for short time( 2 min) or to add suitable amount of chemical inhibitors for controlling enzyme-medi-ated browning of mulberry leaf products.