Effect of pH and temperature on peroxidase and polyphenoloxidase activities of litchi pericarp

Abstract

After harvesting litchi, the red color of the fruit pericarp is rapidly lost resulting in discoloration and browning during storage and marketing. The loss of the red color is caused by the degradation or loss of stability of anthocyanins. The action of peroxidase and polyphenoloxidase is usually related to the browning and discoloration of fruits of various species. This study aimed to evaluate the influence of pH and temperature on peroxidase and polyphenoloxidase activities, in a partially purified preparation of pericarp of the litchi cultivar Bengal. Fruits were harvested at the ripe stage and polyphenoloxidase was partially purified by sequential saturation in 80% ammonium sulfate. At concentrations of 40-50% and 60-70% ammonium sulfate the activities of polyphenoloxidase and peroxidase were, respectively, 124 times and 158 times higher than in the crude extract. The activity of peroxidase and polyphenoloxidase was maximum at pH 6.5 and 7.0, respectively, and no activity was detected at pH 2.5 and 9.5. Pre-incubation of the enzyme extract for 45 min at pH 2.5 or 9.5 completely inactivated the enzymes, with the highest degree of efficiency at pH 2.5. Peroxidase activity was highest at 70°C and remained active for a period of 120 min at 70 and 80°C. Peroxidase became completely inactive when maintained at 90°C for 10 min or 1 min at 100°C. Polyphenoloxidase activity was highest at 20°C and remained active for a period of 120 min at 40 and 50°C and was inactivated after 10 min at 60°C. Due to the high temperature of inactivation of the peroxidase and polyphenoloxidase activities, the enzymes can be inactivated more easily in fruits using acid or alkaline solutions.