What's going on? When apples are cut, an enzyme (polyphenol oxidase), that reacts with oxygen, is released-- causing the apple to turn brown. Lemon juice is full of ascorbic acid, which will react with oxygen before the enzymes in the apple. Lemon juice also is very acidic, about 2.0 pH, and the enzyme in the apple oxidizes in environments between 5.0-7.0 pH. Can you find other acidic ingredients that could prevent apples from browning?