Effect of Grape Leaf Extract on Phenolic Profile and Browning of Fresh-Cut Lettuce (Lactuca sativa)

Abstract

This study aimed to investigate the effects of grape leaf extract (GLE) on the inhibition of lettuce polyphenol oxidase (PPO) and on the loss of phenolic compounds in fresh-cut lettuce during storage. The addition of GLE that had been heated at 100C for 15 min exhibited a higher inhibitory effect on the lettuce PPO activity than that of the fresh extract. It was found that heat-treated GLE inhibited the lettuce PPO noncompetitively. The addition of GLE prevented loss of lightness in lettuce monitored during storage. Loss of phenolic compounds was measured during the enzymatic browning process by high-performance liquid chromatography. Protocatechuic acid, chlorogenic acid, ferulic acid, coumaric acid and caffeic acid were identified in lettuce. Degradation of phenolics followed the first-order kinetic pattern during storage. Synergistic effects of GLE, in combination with ascorbic acid or citric acid, were also tested for the protection of phenolics. Practical Applications Fresh-cut produce, especially lettuce, deteriorates faster than intact produce because of internal and external browning of the cut surface. The addition of chemicals, pH adjustment, refrigeration, thermal treatments and exclusion of oxygen are among the most effective methods for the inhibition of browning. Several authors have stated the necessity to evaluate the inhibitory effect of natural and nontoxic compounds on polyphenol oxidase (PPO) activity and its potential applicability to the food industry. The result of the present study revealed that grape leaf extract (GLE) had an inhibitory effect on PPO and effectively prevented enzymatic browning in fresh-cut lettuce. Our results suggest that GLE could potentially be used as a natural inhibitor of PPO, and its utilization is possible because of its wide occurrence, low cost and nontoxicity, but further feasibility studies are needed to increase its effect as an anti-browning agent in its application in the food industry.