WHY THE INSIDE OF APPLES TURN BROWN WHEN EXPOSED TO AIR

The insides of apples turn brown exposed to air because of the built in defense mechanism against bacteria and fungus. The trigger for this is damage to the cells, such as when you cut the apple, which then results in certain enzymes within the cells being exposed to oxygen. When this happens, the enzymes react to the oxygen creating an oxidized layer that provides some protection against foreign bodies.

Technically, an enzyme called polyphenol oxidase (tyrosinase), which is composed to monophenol oxidase and catechol oxidase enzymes, when exposed to oxygen will result in phenolic compounds in the apple turning into ortho-quinones or "o-quinones." O-quinones are what is providing the protection from bacteria and fungi as they form a natural antiseptic. The o-quinones themselves have no colour, but they further react with amino acids and oxygen to produce melanin, which is how we get the brown colour on a cut apple.

Placing the apple in the refrigerator if you want to keep a cut apple from going brown quickly. This will drastically slow the chemical reactions and thus reduce the oxidation process. You can also, of course, limit the apple's exposure to air by sealing it in an airtight bag.

If you don't mind lemon flavour, spray the exposed part in lemon or pineapple juice. This will stop the poltphenol oxidase from reacting to oxygen due to acidity of the juices which denature the browning enzymes.