How long does it take for an apple to turn brown?

Depending on the type of apple, it can turn brown almost instantly after being cut or take up to several hours. The oxidation process causes the apple to turn brown; oxygen present in the air reacts with the enzymes on the flesh of the apple.

Cooking apples, adding preservatives, minimizing the oxygen present and reducing the pH on the surface of the apples are all viable ways to stop or greatly reduce the speed of the oxidation process. Squeezing lemon juice onto freshly cut apples is an easy way to lower the pH and preserve the color of apples for up to 24 hours.