There are many fruits that do not turn brown after cutting, such as cherries, grapes and citrus fruits. Browning in fruits such as apples, bananas, pears and peaches is caused by the enzyme polyphenol oxidase (PPO), which catalyses the oxidation of phenolic compounds to quinones, which in turn react to form melanin, a brown pigment. PPO only works at a certain pH, which is why very acidic fruits like lemons do not turn brown. You can even sprinkle lemon juice on apple slices to prevent browning.
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