Apples brown after cutting because exposed enzymes react with oxygen in the surrounding air.

The longer version

Inside apple cells, enzymes known as polyphenol oxidases sit apart from the phenolic compounds they act on. Cutting breaks the compartments and exposes both to oxygen, and the enzymes oxidize the phenols into quinones that then link together into brown pigments called melanins.

The reaction is fastest at moderate temperatures and slows when the enzyme is denatured by heat or when oxygen is blocked. Acidic conditions also slow it, because the enzyme works poorly at a low pH. Browning is purely cosmetic; the compounds involved are part of the fruit's normal chemistry and appear in many other produce items too.

Where this entry sits

Everyday ingredients turn out to be full of quiet chemistry and long history. This collection gathers short, plain explanations of how bread rises, why onions sting, where common spices originate, and how coffee, tea, and chocolate reached the modern kitchen. The full list sits on the Food and Drink page, where every entry is listed in order.