When an apple is cut, the cells within the fruit are exposed to oxygen in the air. The reaction between the oxygen and certain compounds in the apple cells causes them to break down, which leads to the browning of the apple slices.
This browning process is called oxidation, and it’s the same thing that happens to iron when it rusts. Oxidation is a chemical reaction that causes the apple to turn brown. However, unlike rusting iron, the browning of an apple is not harmful. In fact, many people actually prefer to eat slightly browned apples because they tend to be sweeter than those that are still pale.
So, if you’re looking to keep your apples fresh and free from browning, all you need to do is keep them away from the air. Put them in a sealed container or plastic bag, and they should stay nice and white for days.