Slicing (or biting into) an apple allows the phenols and enzymes that are normally kept separate by the plant’s cell structure to commingle. As soon as this phenol-enzyme mixture is exposed to oxygen (usually the second you cut the apple open…unless you’re slicing an apple in the vacuum of space…if so, kudos!), the browning process begins. The longer it sits, the more brown pigments are formed, until all the available phenols have been used up and the fruit is as brown as it’s going to get.