Changes in concentrations of phenolic compounds and polyphenol oxidase activity in banana peel during storage

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Phenolic compounds were analyzed in the outer, middle and inner layers of the peel and in the pulp of bananas during ripening from the ripening-initiation. The major phenolic compound found was dopamine followed by 3, 4-dihydroxyphenylalanine (L-DOPA), tyramine and tyrosine in all portions of the bananas. These compounds located mainly in the outer layer of peel followed by the middle and inner layers of the peel and the pulp. The concentration of dopamine decreased during ripening in all portions of the fruit, particularly in the outer layer of the peel in which senescent spotting appeared at a late stage of storage. On the other hand, those of the other compounds, namely L-DOPA, tyrosine and tyramine increased during ripening. Polyphenol oxidase (PPO) activity was detected in all portions of the fruit ; however, it increased only in the outer layer of the peel. Short-term (24h) nitrogen-gas treatment of the ripening-initiated banana delayed the induction of senescent spotting and the decrease in dopamine content, while it increased the PPO activity of the nitrogen-gas-treated bananas to the same extent as that of air-treated bananas. These results indicate that dopamine is used as substrate of PPO for the formation of brown pigments of senescent spotting.