Fruit turns brown when damaged or rotting
because chemical compounds in the fruit (called
phenolic compounds) are oxidized (interact with
oxygen). It is easier for oxidation to occur
when the fruit is damaged, because the fruit
cell walls and membranes break, allowing oxygen
in. When the cell´s compounds react with oxygen,
they tend to turn brown. If you coat acid (e.g.
lemon juice) on a broken fruit (e.g. a sliced
avocado or apples) this will slow down the
browning (oxidation) because acids react
preferentially with oxygen, thereby reducing how
quickly oxygen reacts with the fruit
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