Apples contain an enzyme known called polyphenol oxidase. When the apple is cut, the polyphenol oxidase is released, which then mixes with oxygen and starts breaking down the apple. Because it is not attractive and it does slowly damage the apple, cooks have developed several ways to slow apple browning.
Keep the sliced or peeled apples in water. A temporary solution is to keep the apples away from the air by keeping them submerged in water.
Dissolve ascorbic acid in water. Dip the apples in the solution to coat. Ascorbic acid can be found in most groceries stores. It comes in powdered form. As an alternative, according to the University of Illinois Extension, you can crush an unflavored vitamin C tablet and mix it into water. It contains ascorbic acid and despite making the water cloudy works just fine for keeping apples from browning.
Dissolve the juice of one lemon in 1 to 2 cups of water. Dip the apples in this solution to coat. The lemon adds a nice lemony flavour to the apple as it prevents browning.
Soak the apples in salty water or ginger ale for ten minutes. Michele R. Warmund of the University of Missouri Extension says that this method works for fruit slices in general.
Blanch cut apples in boiling water for two minutes. Remove them from the boiling water and place them in cold water to stop the cooking. Heating the apple denatures the enzyme responsible for browning.
Choose a variety of apple that doesn't brown as fast. Granny Smiths and Cortland apples brown much slower than Red Delicious.
Linda Harris of the University of California Department of Food Science recommends using apples within two hours of peeling and slicing to ensure quality and safety.