How Does an Apple Turn Brown?

Enzymes in Apples

The cells of apples, and many other fruits and vegetables, act as balloons, holding enzymes inside. Portions of the cells contain phenolic compounds, and other parts house enzymes. These enzymes, known as polyphenol oxidase enzymes, are found in the chloroplasts of the cells. When the cells remain intact, as when the apple stays whole, the fruit keeps its normal coloring, but cutting the fruit and exposing it to air changes this.

Oxygen Invasion

Anything that breaks the walls of an apple allows the polyphenol oxidase enzymes and the phenolic compounds to come together. When air enters the mixture, oxidation occurs, producing o-quinones. These mix with amino acids or proteins to create the browning seen on the cut surface of an apple.
All plants have the potential to brown when their cell walls are cut or bruised, but the speed at which it occurs depends upon the amount of both phenolic compounds and polyphenol oxidase in the cells.

Prevention

Commercial fruit-canning companies treat the cut fruit with sulfur dioxide. This, along with heating the cans of prepared fruit, deactivates the enzymes and prevents browning. Home cooks need to either kill the enzymes or prevent oxygen from coming into contact with the cut surfaces. If you're cutting apples at home, choose one of the following methods to slow the enzyme oxidation: cover the fruit with sugar or syrup, which reduces oxygen diffusion; submerge it in water mixed with lemon juice, which includes antioxidants; or place the cut fruit in boiling water to cook it.

References

• Cookwise; Shirley O. Corriher; 1997
• Why do apple slices turn brown after cutting? - Scientific American