Coating apple slices with highly acidic juice, such as pineapple or lemon juice, helps deter browning. Apples brown when they come into contact with air and oxidize. Acidic juices slow down the oxidation process because they contain antioxidants.
Apples brown because they contain polyphenol oxidase enzymes. The concentrations of these enzymes vary based on the variety of apple. The age of an apple and the conditions under which it was grown also impact PPO levels.
Apples can be boiled to keep them from browning. However, doing so impacts their texture. Coating apples with sugar or syrup can also keep them from browning.