How apple turns to brown on being cut?

The enzyme polyphenol oxidase makes this change. On being cut only, the enzyme is activated by contact with air. The enzyme reacts with carbohydratee in the fruit and form brown color on the cut surface. If cut apple is immersed in an ascorbic acid, this color change will not happen. Because the ascorbic acid inhibits the activity of enzyme. Another one reason: Iron is present in apple in the form of ferrous ions. When cut apple is exposed to air ferrous ion is oxidised to ferric ion which is brown in color.