What enzyme is responsible for the browning of cut apples when exposed to oxygen?

The enzyme responsible for the browning of cut apples exposed to oxygen is polyphenol oxidase, or PPO.

Cutting or bruising an apple breaks cellular compartments that normally keep enzymes and phenolic compounds apart. Once oxygen reaches the damaged tissue, PPO catalyzes the oxidation of those phenolic compounds into o-quinones. The quinones then react and polymerize into darker pigments, creating the familiar brown surface known as enzymatic browning.

PPO is a copper-containing enzyme found in many plants, not just apples. The reaction is not the same as caramelization or the Maillard reaction, which are heat-driven processes. Browning can vary with apple variety, ripeness, temperature, acidity, and the amount of phenolic material present.

Lemon juice slows browning because its acidity lowers pH and its vitamin C can reduce oxidation products. Submerging slices in water, limiting air exposure, chilling them, or briefly heating them can also slow the reaction by restricting oxygen or reducing enzyme activity.