In the ordinary family the greater part of the proteid diet is probably furnished by meat, so that a knowledge of the composition and nutritive value of this article of food is important. The structure of the meat may be best seen if one with a sharp knife scrapes a small piece of meat, thus separating the muscle fibre from the white connective tissue. Under the microscope the muscle fibre is seen to consist of bundles of smaller fibres held together by delicate connective tissue in which fat cells are imbedded. These muscle fibres vary in length in different kinds of meat, and the length of fibre probably plays some part in the digestibility of the meat - the short fibre meats being the more digestible.
The toughness or tenderness of meat depends partly upon the muscle fibres and partly upon the connective tissue, though as a rule the same conditions that have made the connective tissue tough and strong will have had a similar though less effect upon the muscle fibre. In general the muscles that are most used or most exposed to wind and weather will be both tougher and richer in flavor than those not so exposed. The young animal will, of course, have more delicate tissues and less toughened fibres than the older or harder worked animal.
Fibre Of Meat. a. Fibre b. Fat c. Connecting tissue.
The composition of different pieces of meat, even from the same animal, differs greatly, the proteid of beef, for instance, varying all the way from twelve per cent to twenty-one, according to the cut of meat and to the feeding of the animal from which it is obtained.
The proteids of meat include a number of different substances, the chief of which are fibrin, myosin and albumin. After the animal is killed the myosin coagulates, thus causing the hardening of the muscle, known as rigor mortis. In this condition the meat is very tough, and the hanging of meat is practiced in order to give time for the disappearance of this rigor by the re-solution of the myosin.
The presence of albumin in the meat can be easily shown by soaking a small portion of the meat in water for a few minutes, and then heating this water. The albumin dissolves in the water and coagulates upon heating just as white of egg would do under similar conditions. The scum that forms in the water when a piece of meat is boiled, is largely this same albumin. Beside the true proteids, gelatine may be obtained from meat in varying quantities. The connective tissue upon boiling becomes gelatine, and it is due to this as well as to the gelatine obtained from the bones that water in which meat has been cooked so often sets into a jelly. The color of meat is due largely to the same substance that gives the color to blood, haemoglobin. Its flavor depends chiefly upon the nitrogenous substances called extractives, though the characteristic taste of pork and mutton is caused partly by the fats they contain. These extractives have no real food value, but act as stimulants.