Why do apples turn brown after cutting?

Answer: Enzymes react with oxygen

Vitamins break down in air — Wrong. While vitamins do degrade over time, that's not what causes browning. The brown color comes from enzymatic oxidation of phenolic compounds, not vitamin breakdown. Vitamin loss is a separate, slower process.

Enzymes react with oxygen ✓ — Correct! Apples contain enzymes called polyphenol oxidase and compounds called phenols. When you cut an apple, these enzymes mix with oxygen in the air and oxidize the phenols, creating brown melanin pigments. This is the same process that makes bruises brown! Lemon juice prevents this by lowering pH.

Cells die and decay — Wrong. While cells are damaged when cut, the browning is specifically from enzymatic oxidation, not decay. The process happens in minutes, far too fast to be decay. It's a chemical defense mechanism, not decomposition.