When an animal is slaughtered the carcass stiffens which is caused by the muscles contracting. This is called rigor mortis, the residual glycogen in the muscles produces lactic acid as a result of the process of anaerobic glycolysis, which takes place in the absence of oxygen. The lactic acid causes the PH to fall and muscle contraction occurs.
After a few hours rigor mortis disappears and the carcass starts to soften. This is called the conditioning period and is very important in producing good quality meat. During this process enzymes break down large molecules, particularly proteins and lipids causing the release of amino acids and free fatty acids. These are essential in the development of the typical meat flavour.