Apples, like most other plant tissue, contain a compound called polyphenol oxidase in their cells, explains Scientific American. These PPO enzymes change the fruit's phenolic compounds to o-quinones when exposed to oxygen in the air. O-quinones turn the apple brown when they come in contact with amino acids or proteins present in the apple. The process is the fruit's attempt to preserve its vulnerable flesh and delay premature rotting.
Not all apples contain the same amount of PPO enzymes, and thus discolor at different rates. Granny Smith varieties, for example, tend to maintain their whiter color longer after being peeled. The conditions in which an apple was grown and its age can also affect the amount of PPO and the rate at which it turns brown. Temperature also slows down the oxidation rate -- a peeled apple placed in an oven browns quickly, while a peeled apple stored in the refrigerator browns more slowly.