If you've ever cut or peeled an apple to save for later, you may return to find, in dismay, that the snowy white flesh has turned an unappealing brown. The apple peel serves as a protective layer that prevents the flesh from being injured. Peeling, cutting and bruising are all forms of injury to the plant's tissue.
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.
Spritzing a peeled apple with an acidic ingredient, such as lemon juice, deters the browning process by deterring the PPO enzymes from activating. Pineapple juice has a similar effect on peeled apples. A sugar-syrup coating protects a peeled apple from oxygen and thus deters the oxidation process, too. You can also blanch peeled apples by submerging them in boiling water for 2 to 4 minutes; this brief exposure to high heat deactivates the PPO enzymes, but also leaves you with a mushier product.