Why does a cut apple turn brown?

What is happening?

In intact apple tissue, cellular structures keep many components separated. Cutting damages those compartments and exposes the interior to oxygen.

Polyphenol oxidase enzymes can then catalyse oxidation reactions involving phenolic compounds. The products undergo further reactions that form brown-coloured pigments.

What changes it?

Acidic conditions, lower temperature and reducing exposure to oxygen can slow browning. Lemon juice is useful because its acidity and reducing compounds interfere with the reaction network.