Have you ever noticed that apple slices color change after it is being cut and kept for a while? There must be some chemical reaction taking place to change its original color to dull brown color.
Yes, you are right. When an apple is cut, the cut portion of the fruit (plant tissue) is open to oxygen. Apples are rich in iron. Oxygen from the apple reacts with the iron present in the cut cells. This results in producing iron oxides, which is similar to the rusting of any iron objects.
Apple has an enzyme called poly phenol oxidase which catalyzes the reaction and makes it faster. If you cut an already cut apple (which is already turned brown), you will see that the new cut area is still fresh and natural (not brown). Leaving it open for some time turns its color too to brown.
Though the coloring makes the apple a little unattractive, it won’t cause any health problems. You can try some tricks to avoid this browning of apple. Try this experiment on apple to learn more about this phenomenon.