Why Do Apple Slices Turn Brown?

Apples and other produce (e.g., pears, bananas, peaches) contain an enzyme called polyphenol oxidase or tyrosinase. When you slice open or bite into a piece of fruit, this enzyme reacts with oxygen in the air and iron-containing phenols that are also found in the fruit. This oxidation reaction causes a sort of rust to develop on the surface of the fruit. You will notice browning whenever a fruit is cut or bruised because these actions damage the cells in the fruit, allowing oxygen in the air to react with the enzyme and other chemicals inside.

The reaction can be slowed or prevented by inactivating the enzyme with heat (cooking), reducing the pH on the surface of the fruit (by adding lemon juice or another acid), reducing the amount of available oxygen (by putting cut fruit under water or vacuum packing it), or by adding certain preservative chemicals (like sulfur dioxide). On the other hand, using cutlery that has some corrosion (common with lower quality steel knives) can increase the rate and amount of the browning by making more iron salts available for the reaction.