Why do apples turn brown when cut?

Inside an apple, polyphenol oxidase (PPO) enzymes are kept separate from phenolic compounds by cell membranes. Cutting the fruit breaks the cells, mixing the two. PPO catalyses a reaction between the phenols and oxygen, producing quinones that quickly polymerise into brown melanin. This enzymatic browning is the same process that turns cut avocados and potatoes brown. Lemon juice slows it because its citric acid lowers the pH and vitamin C acts as an antioxidant, reacting with quinones before they turn brown. Some newer apple varieties have been bred or engineered with lower PPO levels so they brown much more slowly.