It is quite surprising how many ways there are to keep apple slices from turning brown, all of them are quite simple, but only one way requires no effort at all! First let’s look at what causes apple slices to brown and then at the various things that can keep apples from turning:
Pick a nice fresh apple off a tree and it will be weeks before it starts to turn soft and rot, but freshly cut apples turn brown within minutes of being sliced because iron-containing chemicals inside the cells of apples react with the oxygen in the air. This happens every day when iron rusts or a scab on a cut turns brown. The chemical reaction that occurs is called oxidation, and the enzyme that regulates oxidation in apples is called "polyphenol oxidase" (PPO), also known as "tyrosinase". There are millions of tiny cells inside each apple. One way apple cells get exposed to oxygen is by cutting the apple open. This damages the wall that protects each cell and exposes its contents to the oxygen in the air. Bruised apples also show signs of oxidation. When an apple gets damaged--if you drop one for example--many of the cell walls inside the apple get broken, which enables the contents of those cells to flow freely inside the apple, where they react with air that is also ins ide the apple (apples are 20% air, that's why they float!).
Now that we know what causes browning, how can we stop it? One common way is to put lemon on sliced apples, but this changes the taste substantially. The mixture of lemon juice and water is called acidulated water, lime can also be used. This also works for peaches, pears, and avocados.
One can also use orange juice, pineapple juice or apple juice to prevent browning. Other suggestions are Sprite or similar lemon-lime sodas, ginger ale, salted water, cinnamon, water and crushed vitamin C tablets, even dry white wine or flat Champagne! There is also a commercial product called Fruit Fresh, but like everything mentioned so far is has a distinct effect on the taste and the flavor of the sliced fruit.
Another alternative is to invest in a shrink wrap machine. Whilst effective this can be a costly investment and it constantly requires purchasing more shrink wrap.