Apple flesh contains an organic substance called ellagic acid. It will come into contact with the knife used to peel the apple and produce black iron ellagic acid to cover the surface. Apples also contain an oxidase enzyme, when the air meets the pulp, catalyzed by the oxidase enzyme, the organic substances in the pulp are oxidized. But the discolored apple flesh does not affect consumption. Other nutrients do not oxidize in the air, can still be used by the body.