Cut or bruised apple flesh browns because of enzymatic browning. Slicing ruptures cells and lets the enzyme polyphenol oxidase meet natural polyphenols and oxygen in the air; the enzyme oxidizes those compounds into quinones that polymerize into brown melanin-like pigments.
Food scientist Rich Hartel at UW–Madison notes the reaction starts almost as soon as cells break, and similar browning appears in avocados and potatoes. Acidic coatings such as lemon juice can slow it by limiting the enzyme’s activity and oxygen exposure. Source: UW–Madison News.