1. When the apple is cut,PPO will receive a lot of oxygen
2. Thus the PPO will rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones.
3. O-quinones then produce the well documented brown colour by reacting to form compounds with amino acids or proteins or they self-assemble to male polymers.
4. Apple contain polyphenol oxidase enzymes (PPO) in the chloplast.
