Lemons contain a high level of acid, which breaks down the enzymes that can cause fruit to turn brown. To stop the browning agents on fruit, the enzymes must be knocked out or unable to mix with other chemicals.
Browning of fruit is caused by three separate chemicals, including phenols, enzymes and oxygen. When an apple is cut and the inside is exposed, the cells are broken open and the phenols and enzymes begin to react with one another and with oxygen. The result is a brown chemical. Lemon juice is the strongest edible acid available that can stop the action of the enzymes. Apples can also be coated with mayonnaise or salad dressing to block out the oxygen, which will also stop the browning process.