Cooking - Tough cuts generally need a long, slow, moist method of cooking, such as casseroling. On heating, muscle fibre begins to shrink. At around 77 C, shrinkage ceases and there are colour changes and loss of liquid. This liquid is a mixture of meat extracts, water and fat. The main pigment is called myoglobin.
Muscles contain varying proportions of pigments.
Muscles which are used for physical activity contain large quantities and are darker in colour. As red meat is cooked the myoglobin changes from a purple red to a greyish brown in colour. The legs and wings, which do the most work, tend to be darker and tougher due to the myoglobin present. Other parts of the bird, eg breast, have less myoglobin and therefore become white during cooking. Game is generally tougher than poultry, beef, pork and lamb.