Why do apples turn brown?

Apples turn brown because cutting them exposes the fruit to oxygen, and an enzyme called polyphenol oxidase starts turning the apple’s phenolic compounds into brown pigments. That is enzymatic browning, not immediate spoilage, which is why a brown apple slice can still be edible if it still smells and tastes normal.

The easiest way to slow it down is to cut off oxygen or slow the enzyme. Acid helps, which is why lemon juice or ascorbic acid works so well on cut fruit, and refrigeration slows the reaction by slowing enzyme activity. If you want the shortest version: brown apples are a chemistry problem, not a sign that the apple suddenly went bad.

That is also why a tight lid, plastic wrap pressed against the fruit, or a quick dip in an acidic solution works better than just hoping the color holds. You are either reducing oxygen exposure or changing the conditions the enzyme needs to keep making those brown compounds.