Why do apples turn brown?

Answer 1

Fruits such as apples and Pears will rot if exposed to certain fungi. These fungi are relatively weak but can attack damaged or overly ripened fruit. When fruits such as apples ripen, they give off a gas called ethylene. Ethylene gas accelerates the ripening process, which will lead to over ripening.

The easiest way to prevent browning is to put the sliced apples in water. You can also squeeze like lemon juice on and helps it keep from browning. Most of the time apples brown when cut them. Apples can turn brown even when they are not cut though, but it takes considerably longer to brown.

Basically because the apple contains Iron. The iron inside the apple rusts when it comes in contact with air and that's why it turns to that brownish color.

Answer 2

Apples, like other fruits, contain an enzyme called catecholase which facilitates the formation of catechol on the surface of bruised or cut apples. The catechol reacts with the oxygen in the air, producing the brown color (polyphenol).