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.



Fresh cut apples turn brown when iron-containing chemicals inside apple cells react with oxygen in the air. We see this every day when iron objects rust, or when scabs on cuts turn brown.

The chemical reaction is called "oxidation", and the enzyme that regulates oxidation in apples is called "polyphenol oxidase" (PPO), also known as "tyrosinase".



Oxidation - air...... Lemon juice helps retard the apple's reaction to air, or try to remove all the air from the storage bag. Even a few minutes in contact with air will cause the fruit to start to turn brown.

This is the same thing that causes brown spots in apples when you drop them. The easiest way to prevent browning is to put the sliced apples in water so that the enzyme does not have access to oxygen. You can also heat the apples to denature the enzyme.