Background Research
The reason that apples brown, is that there is a certain enzyme called polyphenol oxidase (PPO) which they produce which reacts with the oxygen when the flesh is exposed from being cut. When oxygen is present in the cells the PPO enzymes in the chloroplasts oxidize the phenolic compounds, which are naturally present in the apple tissue, to o-quinoes. The o-quinoes are what produces the brown colour when it reacts; forming compounds with amino acids or proteins. This reaction between the PPO enzyme and the oxygen is basically an oxidation reaction, when the cells are damaged allowing oxygen to react with the enzymes to form the browning on the surface of the apple.
All apples brown, however different apples have a different amount of the enzyme in itself, which can vary the speeds at which different types of apples brown. This is because of the different sugar levels or amount of PPO produced.
There are many ways in which the reaction time between the enzyme and the oxygen can be slowed or prevented. Examples of ways to slow or prevent it are inactivating the enzyme by cooking (heat), reducing the pH on the surface (adding acids), reducing the amount of available oxygen (putting sliced apple under water or sealing it), or by adding preservatives.
There are many ways in which the reaction time between the enzyme and the oxygen can be slowed or prevented. Examples of ways to slow or prevent it are inactivating the enzyme by cooking (heat), reducing the pH on the surface (adding acids), reducing the amount of available oxygen (putting sliced apple under water or sealing it), or by adding preservatives.