Why a cut apple turns brown

A cut apple turns brown because the cut mixes the enzyme polyphenol oxidase with phenolic compounds and with oxygen. The enzyme turns the phenols into quinones, and the quinones react onward into brown pigments.

The color is not rust, and it is not the apple merely drying out. The surface can still be wet when it darkens, and the reaction needs the enzyme, the phenols, and oxygen together rather than air alone.

The mechanism is more particular than the slogan. Intact cells keep the enzyme and the phenolic substrates from meeting freely. The cut ends that separation, oxygen gets in, and the copper-containing enzyme oxidizes the phenols to reactive quinones that then link into larger brown molecules.

Follow it one step further. Acid slows the enzyme, which is why a sour juice on the cut face delays the color, and cold slows it the way cold slows most chemistry. Keep the oxygen off and the same partners have nothing to react with, so the face stays pale longer.

The case worth keeping in mind is specific. A fresh slice pales and then browns on the cut face over minutes, while an uncut apple keeps the color of its flesh because the skin was still a barrier. The open face is where enzyme, phenols, and air finally share one place.

That case has a boundary. Cultivars brown at different speeds because they carry different amounts of enzyme and of phenolic substrate. A very acidic fruit can be a poor host for the enzyme even after the cells are broken, which is a difference in conditions rather than a different atmosphere.