Oxidation occurs when fruits and vegetables are cut and exposed to the air. When you cut into a fruit or vegetable, the cells are severed, causing them to release enzymes, and be exposed to the outside air. Oxidation causes discoloration–usually a browning or darkening of the flesh–of fruits and vegetables. Although fruits and vegetables that are discolored due to oxidation are usually still perfectly edible, you can stop oxidation using a few simple methods.
Treat the fruit or vegetable with ascorbic acid immediately after cutting into it to stop oxidation. Ascorbic acid is a natural property in many citrus fruits, such as oranges and lemons. Squeeze a few drops of lemon juice on the cut portions of fruit or vegetable.
Cover the fruit or vegetable tightly with plastic food wrap. Covering the fruit or vegetable will prevent oxygen from reaching the cut cells, and thus prevent oxidation.
Cook, boil or otherwise heat the fruits and vegetables right after cutting them to prevent oxidation. When you cook fruits and vegetables, the heat destroys the enzymes that cause oxidation and the resulting discoloration.
Freeze the fruits and vegetables to avoid oxidation. Steam or blanch the vegetables or fruits prior to freezing. Wrap the fruits and vegetables in plastic wrap or place them in plastic freezing containers, plastic-coated freezer paper or "can-or-freeze" glass jars.