Browning of cut apples is due to

Browning of cut apples is due to enzymes.
This unappetizing phenomenon is actually due to a chain of biochemical reactions known as “enzymatic browning.” When an apple is cut (or bruised), oxygen is introduced into the injured plant tissue.
When oxygen is present in cells, polyphenol oxidase (PPO) enzymes in the chloroplasts rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones, colorless precursors to brown-colored secondary products.