Why does an apple turn brown after slicing it?

• This browning effect is caused by an enzyme—Polyphenol oxidase or PPO—that is naturally present in the apple itself.
• When an apple is sliced, tissue cells located along the cut surface become ruptured or broken. This cell rupture allows PPO enzymes and other cell constituents to combine and react to create the brown discoloration. Though the apple becomes less appetizing—it is still safe to eat.