Relationship between browning and the activities of polyphenoloxidase and phenylalanine ammonia lyase in banana peel during low temperature storage

Kluai Khai (Musa AA Group) and Kluai Hom Thong (Musa AAA Group) bananas were stored at 6 and 10 °C. Visible chilling injury (CI) in the peel, mainly browning, occurred at both temperatures, but more so at 6 °C, and without significant differences between the cultivars. At the time of harvest, total free phenolics in the peel were three times lower in Kluai Khai than in Kluai Hom Thong fruit, the polyphenol oxidase (PPO) activity in Kluai Khai being considerably higher and phenylalanine ammonia lyase (PAL) activity much lower. As CI developed, PAL and PPO activities in the peel increased, and total free phenolics decreased. The decrease in total free phenolic compounds and the increase in PAL and PPO activities occurred more rapidly at 6 °C than at 10 °C, in both banana cultivars. Correlations between visible CI and the level of total free phenolics, and between CI and the activities of PPO and PAL, were all highly significant. The results indicate that low temperature stress induced concerted activities of PAL and PPO, which resulted in browning. Since the concentrations of free phenolic compounds and the rate of PAL and PPO activities varied considerably between the two cultivars, but browning did not, the changes in the biochemical parameters rather than their absolute levels were correlated with peel browning.