Why do apples turn brown?

When an apple is sliced or cut into pieces, oxygen is introduced into the damaged tissue. These physical effects also trigger an enzyme in the chloroplasts known as polyphenol oxidase (PPO) to oxidize the phenolic compounds in the apple. The chain of biochemical reactions is called enzymatic browning, which is a kind of self-protection mechanism. The damaged or crumbled apple tries to protect itself against microorganisms and fungal attacks, so it forms an increasingly darkening layer that does not attract the attention of living organisms. Enzymatic browning is not just an event that occurs on apples. 

When the pear, banana, and eggplant are cut, it turns brown very quickly. Enzymatic browning is also responsible for the desired dark colors of plum, coffee, black tea, and cocoa. To reduce browning in apples, you can reduce the contact with oxygen by putting the slices in water or put them in lemon water that shows an acidic character. Acids naturally present in lemon slow down the reaction by reducing enzyme activity. If you don't mind the added sugar, you can cover the slices with honey, caramel, or sugar syrup to prevent exposure to oxygen. Scientists are working on genetically modified apples that don't provide the PPO enzyme, so maybe brown apples are a thing of the past.