Study on mechanism of senescent spotting and its control in ripe Kluai Khai

'Kluai Khai' (Musa AA Group) is an important economic fruit crop of Thailand which is grown for both local and oversea markets. 'kluai Khai' has senescent spotting whereas Kluai Hom (Musa AAA Group) and Kluai Namwa (Musa AAB Group) do not have when they are ripe. Senescent spotting is a serious problem of postharvest quality for 'Kluai Khai'. Consumers are reluctant to buy 'Kluai Khai' with senescent spotting, because they look like infected bananas. The objective of the study was to investigate the mechanism of senescent spotting and its control. The experiment was divided into 3 parts as follows: 1) to study the primary cause of senescent spotting if it is caused by infection or physiological disorder, 2) to study how senescent spotting occurs and what is the decisive factor and 3) to study the control of senescent spotting. Preharvest spray of carbendazim and prochloraz to banana bunches with and without bagging, postharvest dipping of bananas in benomyl and prochloraz solutions and ripening bananas under aseptic condition did not reduce the severity of senescent spotting of 'Kluai Khai'. Epidermal cells of pericarp of ripe 'Kluai Khai' were thinner than that of ripe 'Kluai Hom' and 'Kluai Namwa'. The pericarp of Kluai Khai had lower phenolic content and PAL activity than that of 'Kluai Hom' and 'Kluai Namwa' during ripening. Modified atmosphere packaging (MAP), surface coating and heat treatment of 'Kluai Khai' ripened at color index 4-5 inhibited senescent spotting, but did not affect phenolic content and activities of PAL and POX. MAP and surface coating increased polyphenol oxidase (PPO) activity, while heat treatment reduced PPO activity. This suggests that senescent spotting of 'Kluai Khai' may be related to the activity of PPO and/or the thickness of epidermal cells of fruit pericarp, not total phenolics and activities of PAL and POX. Senescent spotting of 'Kluai Khai' can be controlled by 1) placing 'Kluai Khai' ripened at color index 4-5 into twist-tied polyethylene bags with perforation, 2) wrapping bananas with PVC plastic film, 3) waxing bananas with Sta-Fresh 7055, 4) exposing bananas to 42 deg C for 24 hours and 5) keeping bananas at 12-18 deg C. These methods did not inhibit ripening processes of bananas and eating quality of 'Kluai Khai' was not altered.