why cut surface of fruits and vegetable become brown

When fresh fruits and vegetables are peeled or cut open, the enzyme called polyphenol oxidase (also called tyrosinase) contained in the cells is exposed to and reacts with the oxygen in the air. The reaction that occurs, which is called oxidation, is what turns the fruits and vegetables brown. The bruising seen on some fruit, such as apples and pears, is caused by the same type of reaction. If an apple is damaged by being hit or dropped, the apple's cells in that area are damaged and the enzymes are exposed to the air inside the apple, causing them to turn brown. When preparing fruits and vegetables for use, an anti-browning solution should be used to prevent them from turning brown.