Why does cutting an apple turn it brown?

Slice an apple, leave it on the counter for a little while, and you will notice the pale flesh slowly turning an unappetizing brown. This happens to many fruits and vegetables, including pears, bananas, and potatoes. It can be frustrating when you are preparing food, but the browning is not a sign that the apple has suddenly rotted. It is actually a fast chemical reaction, and understanding it reveals a hidden bit of chemistry happening right on your cutting board.

An apple's flesh naturally contains a special substance called an enzyme, specifically one known as polyphenol oxidase. It also contains other natural compounds called polyphenols. While the apple is whole and undamaged, the enzyme and these compounds are kept in separate parts of the apple's cells, so they do not interact. As long as everything stays sealed inside intact cells, the apple keeps its fresh, pale color and nothing happens.

The moment you slice or bite an apple, you break open its cells. This does two things at once: it mixes the enzyme together with the polyphenols, and it exposes the inside of the apple to the oxygen in the air. With all three ingredients now in contact, a reaction begins. The enzyme uses the oxygen to transform the polyphenols into new compounds. These new compounds then join together to form brown-colored pigments called melanins. This whole process is known as enzymatic browning, and it is the same browning seen in many cut fruits.

Because the browning depends on a specific reaction, there are simple ways to slow it down. One common trick is to coat cut apple slices in lemon juice. Lemon juice is acidic, and the enzyme works poorly in acidic conditions, so it slows the reaction. Lemon juice also contains antioxidants that interfere with the browning. Keeping cut apples cold also helps, because cold temperatures slow the reaction down. Reducing the apple's contact with air, such as wrapping the slices tightly, limits the oxygen available. The browning itself is harmless, but these tricks keep cut apples looking fresh and appetizing for longer.

This article was written using information from Wikipedia.