Why a cut apple left in open turns brown?

When an apple is cut or even bruised on a piece, oxygen is introduced into the injured plant tissue. When oxygen is present in cells, polyphenol oxidase or 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.

Because when oxygen touches it injures its plant tissue, and the produces too many enzymes and then turns to a dark brown/tan color. can i get the branliest answer