The aim is to achieve a state known as equilibrium to produce a so-called equilibrium modified atmosphere, or EMA. Here, oxygen and carbon dioxide can pass between the inside and outside of the package in such a way that as oxygen is consumed within the pack it is replaced by oxygen from outside, and similarly a constant level of carbon dioxide is maintained. Another advantage of a modified atmosphere is that it reduces the natural production by the fruit or vegetable of a gas called ethylene, which speeds up the ripening process.
For a packet of mixed salad, for example, a typical EMA might consist of 5 per cent oxygen, 15 per cent carbon dioxide and 80 per cent nitrogen. This could increase the shelf life of the product to as long as eight days, whereas under air it would be lucky to remain fresh for four or five days.
A modified atmosphere can also have a dramatic effect on the browning of cut fruit and vegetables such as potatoes and apples, which is a result of an oxidation reaction brought about by enzymes that are released when the flesh is damaged. In these cases, having a mixture of carbon dioxide and nitrogen in the packaging, with no oxygen, can prevent browning for several days, as opposed to just a few minutes or hours in air.