Apples turn brown due to a process called oxidation. When an apple is cut or bruised, the flesh is exposed to oxygen, which causes the enzymes in the apple to react and produce a brown pigment called melanin. This process is similar to the way that a banana turns brown when it is peeled or cut.

To prevent apples from turning brown, you can:

- Store them in a cool, dry place.

- Wrap them individually in plastic wrap or foil.

- Place them in a container of water with a little bit of lemon juice or ascorbic acid (vitamin C).

- Treat them with a commercial anti-browning agent.

You can also buy apples that have been treated with 1-methylcyclopropene (1-MCP), a chemical that inhibits the production of ethylene, the hormone that causes fruit to ripen and turn brown. Apples that have been treated with 1-MCP will stay fresh for longer than untreated apples.