OXIDATION

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".
 

HOW APPLE CELLS GET EXPOSED TO OXYGEN

There are millions of tiny cells inside each apple.  One way apple cells get exposed to oxygen is by cutting the apple open.  This damages the wall that protects each cell and exposes its contents to oxygen in the air.

Bruised apples also exhibit oxidation.  When an apple gets damaged--as might happen if it were dropped--many of the cell walls inside the apple get broken.  This enables the contents of those cells to flow freely inside the apple, where they react with air that is also inside the apple (apples are 20% air--that's why they float).
 

WAYS TO KEEP YOUR APPLES FROM GOING BROWN

• Use anti-oxidants such as lemon juice, which are high in citric acid.  In commercial processing, sulphur dioxide is used.
• Remove the air from your apples.  This is why vacuum-packed canned foods stay fresh so long.
• Remove water from your apples (called "dehydration").  Polyphenol oxidase--and just about everything else that can spoil food--needs water to work.
• Use clean, high-quality cooking utensils.  Iron salts found on steel cooking utensils corroded by organic acids can act as potent catalysts to oxidation by producing per-oxides and super-oxides.
• Cook your apples.  Heat "denatures" (changes & weakens) polyphenol oxidase.
• Eat your apples quickly...before they have time to go brown!