Oxidation/Reduction Reactions

Why Do Apples Turn Brown?

Have you ever seen an apple that has been sliced and has started to turn brown? Would you believe that the same process that causes metal to rust is responsible for changing an apple's color? Both the metal and the apple are going through the process of oxidation.

Apples change color when exposed to air because of an enzyme present in the apple. The enzyme is known as polyphenol oxidase. As soon as this enzyme is exposed to air, the color of the apple begins to change; however, this reaction only occurs at a certain pH. If the pH of the apple was changed to below five, the reaction would not occur. How would you change the pH of an apple? How about adding a little acidic lemon juice? Lemon juice is commonly added to fruit salad or guacamole to keep fruit from turning brown.