Look a little closer
A cut apple turns brown for more than simple drying. A knife ruptures cells and brings an enzyme and phenolic compounds, normally kept apart, into contact with oxygen from the air. A chain of oxidation reactions begins.
In intact tissue, polyphenol oxidase and its potential substrates occupy separated cellular compartments. Cutting, peeling or bruising breaks those boundaries. The ingredients mix, and oxygen gains easy access through the damaged surface.
The enzyme uses oxygen while converting some phenolic compounds into reactive molecules called quinones. Quinones then join with one another or with other components to form larger, dark-colored products. The visible result is called enzymatic browning.
The rate varies with apple variety, ripeness, temperature and acidity. Refrigeration slows enzyme activity. Acidic lemon juice shifts conditions away from the enzyme's preferred range, while vitamin C can help reduce some oxidized compounds back to earlier forms.
Submerging slices can limit contact with air, although the result and texture depend on the liquid and timing. Too much lemon changes the flavor, and slowing discoloration does not make cut fruit safe to leave unrefrigerated indefinitely.
Brown color is a chemical record of damaged tissue meeting oxygen. Related reactions occur in pears, bananas and potatoes. Controlling oxygen exposure, temperature and acidity can delay the change without altering what caused it.
