Sliced apples turn brown due to a chemical reaction called oxidation, which occurs when the cells of the apple are damaged, exposing the inner parts of the fruit to oxygen in the air.

Apples contain an enzyme called polyphenol oxidase (PPO) or tyrosinase, which is normally kept separated from substances called phenolic compounds present in the apple tissue. When you slice an apple, you break open the cells, allowing these enzymes to come into contact with the phenolic compounds.

When PPO comes into contact with the phenolic compounds, it triggers a series of reactions that convert the phenols into quinones. These quinones then react with amino acids to form compounds that appear brown. This process is essentially the same as what causes browning on other fruits and vegetables and is completely natural.

To slow down this process, you can apply acidic juices like lemon juice, which can help stabilize the color by preventing the enzyme from working. Another way to delay browning is to store sliced apples in water or in an airtight container, limiting the apple’s exposure to oxygen.