Muscle tissue proper is made up of minute threadlike cells that lie side by side and are so small that the individual cells may be seen by the microscope only. In the living animal the contents of the muscle cells or the contractile tissues are soft and tender, but after death a change takes place in the muscle substance which causes it to become stiff and rigid. This change is called rigor mortis, and consists in a coagulation of the substance of the muscle cells. A similar change can be produced in the muscle of a freshly killed animal by heating it. This condition is known as heat rigor (rigor caloris). In the cooking of freshly killed animals or game, this stiffness of the meat as it begins to cook is easily observed.

The meat from young fat animals is tender, while the meat from old and lean animals is tough. The reason for this may be found partly in the condition of the connective tissue and partly in that of the muscle cells. As an animal advances in age, the connective tissue becomes progressively more and more dense and tough. When fat is deposited within the meshes of the connective tissue, such fat deposit tends to make the connective tissue tender and less resistant and tough. Heavy muscular exercise naturally makes the muscle tissue more dense and firm and tough, and also makes a similar change in the connective tissue. For this reason old work oxen do not make tender beef, nor do old milch cows, age and muscular work both influencing the tenderness of the beef in the same direction. For that reason the choicest animals for food are naturally younger animals, which have not experienced severe muscular work and have been from the first kept plump and fat with good feeding.

In the care of domestic animals, their conditions can easily be controlled so as to produce these desirable attributes, and thus enable the grower to market only the choicest animals whose meat possesses the qualities above given. Naturally, meats possessing these qualities are the choicest and sell in the market for the highest prices, while meats not of this grade are sold at lower prices and in markets where the purchasers do not insist upon such qualities. In wild animals and game birds, one frequently finds the meat quite tough. This is very likely due to the fact that the animal was subjected to difficult conditions of life, required to take a great deal of strenuous muscular exercise, and frequently left with short rations. While such wild meat and game is highly flavored, it requires careful cooking to make it tender and satisfactory.

The cooking of meat is an important phase of dietetics. A choice piece of meat may be made tough and difficult of digestion by improper cooking, while a tough piece of meat may be made tender and easily digestible by proper cooking. The two essential factors in cooking meat are heat and moisture. The heat develops the flavor, and moisture rightly applied insures tenderness. A piece of tenderloin steak of the highest grade put into a frying pan and fried in grease would be made almost inedible, while a piece of brisket put into a kettle and simmered for hours can be made into a tender, easily digestible, and not ill-flavored dish. In the roasting or broiling of meat several hundred degrees of oven heat rapidly coagulates and sears the outer layer of the meat, thus holding within the meat its juices. If this heat is maintained for a considerable time, differing with the size of the piece of meat being cooked, this retained moisture makes the connective tissue of the meat tender, changing it into gelatin. A considerable time is required to gelatinize connective tissues, therefore meats which contain much connective tissue are not adapted for broiling. They may, however, be roasted, especially if ample provision is made for a frequent and copious basting of the roast. Basting provides the moisture necessary to gelatinize the connective tissue. Where there is a large proportion of connective tissue this process takes considerable time. By devoting sufficient time and attention to some of the cheaper cuts of meat, using this process, the meat may be made appetizing, easily digestible, and wholesome. The boiling of meat produces a similar effect in the connective tissue if the process is longer continued.

Besides developing its flavor and making the meat tender and digestible, sufficient cooking kills any parasites and microorganisms that might accidentally be present in the meat.