切開的蘋果放置在空氣中不久後，切面會變成褐色，主要是因為蘋果中的多酚氧化酶（PPO）催化了哪種化學轉變？Cut apples turn brown after being exposed to the air, mainly because polyphenol oxidase (PPO) in the apple catalyzes what type of chemical transformation?
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多酚化合物與空氣中氧氣的氧化反應 / Oxidation of polyphenols by oxygen in the air 細胞受損後多酚氧化酶接觸空氣中的氧氣，將多酚氧化成醌類物質，進一步聚合形成棕黑色的黑色素。 When cells are damaged, PPO contacts oxygen and oxidizes polyphenols into quinones, which polymerize into brown-black melanins.