The answer is simple: using oxygen helps to retain the appetising colour of red meat.

The colour of fresh red meat is largely determined by the protein myoglobin, which is present in the tissue. Myoglobin itself is purple, but can react with oxygen to create two other pigmented forms, oxymyoglobin, which is red, and the brown metmyoglobin.

In air, the concentration of oxygen is at a level that favours the formation of metmyoglobin, turning the meat brown, which consumers can find unattractive. However, under much higher concentrations of oxygen, the deep red oxymyoglobin is more likely to be produced. So, with the appropriate MAP mixture – typically between 60 and 80 per cent oxygen – fresh red meat will retain its appetising colour. Although myoglobin is present in both red and white meat, white meat animals are 'fast twitch', needing quick bursts of energy and they get this energy from glycogen which is stored in the muscles. This results in less myoglobin presenting in the tissue which is why oxygen is not needed in the MAP gas mix. Red meat animals are 'slow twitch' which means they have muscles designed for long periods of activity such as grazing. This results in myoglobin that stores oxygen in the muscle cells. The more myoglobin, the redder or darker the meat and the more potential there is for metmyoglobin to form, giving the meat an unappetising brown colour.

So, that's why oxygen is added to the MAP gas mix for red meats, but not poultry.