enzyme that will react with the phenols and oxygen also contained in the apple. The reaction between the two forms what is essentially rust on the fruit. When this fruit is bruised or cut, the cells are damaged and oxygen reacts with the apple’s enzyme and turn brown.
There are many ways to slow this process, such as cooking the fruit, adding lemon juice to reduce the pH on the apple’s surface, or adding preservative substances such as sulfur dioxide. Another way to stop the fruit from browning is to reduce the amount of oxygen, by vacuum sealing it or submerging it in water. However, the rate of browning will be increased if you use cutlery that has some amount of corrosion, as found with some types of steel knives, as there are more iron salts to speed up the reaction.
Also, it is safe to eat the brown part.