Apples discolour or turned brown when peeled or bruised and exposed to air. This discolouration or browning is due to oxygen, O₂, reacting with chemicals released, breaking down the cells in the fruit. The reaction is called enzymatic oxidation as it is a process catalysed by the enzymes present in the apples. However, the enzymes are destroyed by certain chemicals (e.g. Vitamin C - Absorbic acid) or by high heat. Vitamin C, being an highly reactive anti-oxidant reacts with the O₂ in the air, preventing/slowing down the enzymatic oxidation of the apples. The diagram below shows the results of half an apple left in the air and the other half treated with lemon juice (which contains high contents of Vitamin C).

Image taken from http://www.nsapples.com/teacher/t5schem.htm

Recent researches have shown that the browning of apples can be prevented or inhibited using a combination of combinations, either natural products or their derivatives that are food additives. Various combination of enzymatic inhibitors, reducing agents and anti-microbial compounds containing Ca²⁺ were tested. They all slowed down microbial decay and changes in organic acids and sugars during storage. It was also found that irradiating gamma rays could reduce rate of browning of apple juice and increase anti-oxidant activity. This is found to have importance especially in improving the shelf-life of food. More evidently, fresh-cut fruits could now be preserved longer without the need of special packaging.

For more information please see the references below.

1. http://www.sciencedaily.com/releases/1999/01/990113074812.htm

2. http://www.nps.ars.usda.gov/publications/publications.htm?SEQ_NO_115=122938