A story on NPR:
If Genetically Modified Apples Dont Brown, Can You Tell If Theyre Rotten?
"No matter how you slice it, every apple turns brown eventually. 'When their flesh is cut, the oxygen in the air interacts with chemicals in the flesh of the apple,' says Susan Brown, a plant scientist at Cornell University. An enzyme called polyphenol oxidase, or PPO, makes melanin, an iron-containing compound that gives apple cells a brown tinge. The same type of 'oxidative' browning happens in the browning of tea, coffee or mushrooms, explains Brown."