While the outside of a ripe kiwi is a soft shade of camel brown, the bright green flesh can also turn brown when exposed to oxygen. Once fruit that contains an enzyme called polyphenoloxidase -- such as kiwi, bananas and apples -- exposes its naked flesh to oxygen, oxidation sets in. Oxidation is a chemical reaction that produces compounds that give some fruits a brownish color after their cellular membranes have been damaged due to cutting or peeling.
An Ounce of Prevention
Some acids, such as citric and ascorbic, can help prevent fruit from turning brown by grabbing oxygen molecules before the fruit can be fully exposed to them. This delays the oxygen from reacting with the enzymes in the kiwi, apple or other polyphenoloxidase-containing fruit. Brush or coat freshly cut fruit with lemon or lime juice, or submerge the fruit in water to delay browning.