What does lemon juice on top of sliced apples help prevent?

Lemon juice helps prevent sliced apples from browning. Browning in apples, otherwise known as enzymatic browning, is a process caused by the presence of enzymes called polyphenol oxidases. These enzymes react with the compounds in apples once the fruit is cut and exposed to oxygen. The reaction forms brown pigments, which leads to the browning color.

Squeezing lemon juice over the apple slices can help prevent the enzymatic browning by lowering the pH level of the surface. The acidic environment created by the lemon juice hinders the activity of polyphenol oxidases, allowing sliced apples to maintain their original color for a longer period.