Apples, along with other types of fruit, begin to die the moment they are picked from the tree because they stop receiving vital nutrients, water and oxygen from the tree. The cells in apples break down and eventually die.
Apples have strong cells and cell walls that help them stay fresh longer than animals cells, which is why an apple will last longer than a piece of meat. However, once picked, oxygen in the air begins breaking down the apple's cells and eventually the cells react, turning the fruit soft, brown and rotten. Rotting is a natural chemical reaction that occurs to break down the apple, allowing its seeds to break free and receive the nutrition they need to grow again.