The muscle fibers contain several constituents, and these are grouped as follows by Howell: "(1) Inorganic salts. (2) Ferments. (3) Pigments. (4) Non-nitrogenous extractives (lactic acid, etc.). (5) Nitrogenous extractives (creatine, urea, etc.). (6) Carbohydrates and fats. (7) Proteins."

Inorganic salts. In meat cookery the inorganic salt most noticeable from the standpoint of flavor is sodium chloride or common salt. The juice from meat that has had no salt added before or during cooking tastes salty. This is due to the sodium chloride of the body fluids, which contain about 0.85 per cent. Care should be taken in adding salt to meat, or the salt added in cooking plus that of the juices will result in an excess of salt in the drippings and in gravy made from them. The muscle fibers do not taste as salty as the juices, particularly if a large portion of the fluid has been lost in cooking. The tissues do not contain as high a percentage of sodium chloride as the fluids of the body.

The inorganic salts are found in a complex salt equilibrium in meat.

The ferments. Several different kinds of enzymes are found in muscle tissue. The proteolytic, amylolytic, and lipolytic enzymes act on proteins, carbohydrates, and fats, respectively.

Pigments and hemoglobin. When animals are slaughtered for food the blood is drained from the body. However, some blood clings to walls of the capillaries in the tissues. This blood together with the muscle pigments gives to meat its pink or red color.

The hemoglobin of the red corpuscles is the pigment giving blood its purplish-red color. The combination of hemoglobin with oxygen forms oxyhemoglobin, which is a bright scarlet. Meat is a darker bluish-red when first cut, but if the cut surface remains exposed to the air it becomes a brighter red color. When hemoglobin and oxyhemoglobin are acted upon by enzymes, acids, alkalies, or heat, various decomposition products are formed. Hematin is one of the decomposition products formed by heat. The coagulating temperature of oxyhemoglobin is around 64°C. Thus when meat is cooked to temperatures higher than this the production of hematin brings about the brown or gray color of well-done meats. Dilute acids or alkalies may also produce hematin from hemoglobin.

Muscle pigments. The red pigments of muscles resemble the hemoglobin of the blood. These pigments are sometimes designated as chromatin sub-stances. Whipple and Robscheit-Robbins call them muscle hemoglobin, and they have reported that the properties of the muscle hemoglobin are practically the same as or identical with those of blood hemoglobin. Whipple has found the content of muscle hemoglobin higher in active hunting dogs than in inactive house dogs. He states that under different conditions the muscles may contain from 10 to 80 grams of muscle hemoglobin for each 100 grams of blood hemoglobin.

Skeletal muscles vary in color in the same animal and in different species. Sometimes the pigmentation varies in the same species but with different types or breeds. Dairy cattle have darker-colored muscles than the beef types. The color of the muscle of the young animal is often lighter than that of the same muscle from an older animal. Veal is lighter in color than beef, and the leg muscles of broilers are lighter than those of mature fowls. Different muscles of the rabbit vary in color from a very light pink to a deep red. The different muscles of pork also vary in color. In general the muscles of the lower animals are not pigmented, with the exception of the heart muscle. The muscles of fish are white or light in color, but the muscles of nearly all mammals are red. Needham states that "red pigmentation seems to occur in muscles from which the most persistent and prolonged activity is required." Earlier investigators have also suggested that in general pigmentation occurs in muscles used most frequently.

Effect of ripening upon meat pigments. Meat that has been ripened 40 to 60 days is usually a deeper gray in color, when cooked, than unripened meat from the same animal, provided the cooking conditions are the same. The darker color may be due to the action of enzymes and acid upon the hemoglobin, so that more hemoglobin is broken down to produce hematin at lower cooking temperatures, or it may be due to other factors.

Non-nitrogenous extractives. Lactic acid is always present in muscle tissue. The amount is small while the muscle is at rest and increases during and after exercise. After death the percentage of lactic acid in the muscle increases.

Nitrogenous extractives. Muscle tissue contains end products of protein metabolism. These are removed as they accumulate during life from the tissues by the body fluids and are excreted through the kidneys. Creatine and creatinine are found in the muscles and to a lesser extent in the blood. Uric acid and other nitrogenous extractives are also found in the muscles. They are of interest in meat cookery, because they are the source of part of the distinctive flavor of meat, and stimulate the flow of gastric juice.

Carbohydrates and fats. The carbohydrate found in the muscle tissue is glycogen.

Burns states "the fat content of the cell is unique. Every cell has a fairly constant content of lipide, although when stained by the usual methods to demonstrate fat, no evidence is given of such a content. This masked fat is only made visible when the cell is diseased or disintegrated."

The proteins of the muscle fibers. The proteins of the muscles are composed largely of two types: (1) the structural proteins, which consist largely of collagen and elastin and (2) the protoplasmic proteins, variously called myosinogen, myogen, myosin, and myoglobulin. The two types of proteins behave differently when heated, the effect of heat on the structural or connective tissue proteins having been considered. The proteins of the plasma are soluble in certain concentrations of salt solutions, such as sodium chloride, sodium sulfate, magnesium sulfate, ammonium sulfate, sodium phosphate, and other salts. Hence, they are often referred to as the soluble proteins.

The entity of the meat proteins is in the same status as that of flour and other food proteins. (See proteins of flour, Chapter XL) They may or may not be a mixture of proteins having similar properties.

The soluble muscle proteins are most commonly designated as myosin and myogen. They are classified as globulins, the myosin being completely precipitated and the myogen partially precipitated by saturated magnesium sulfate. However, complete information about these proteins is lacking. Moran quotes Muralt as suggesting that the fibrils are composed of a firmer material (myosin) surrounded by the more liquid or gel-like myogen (sarcoplasm). The evidence for this was based on X-ray studies, but is not generally accepted. Since myogen is doubly refractive, this is almost conclusive proof that it occupies the light bands in the muscle fibers.

Characteristics of proteins. One of the outstanding characteristics is the extent to which solubility is affected by small changes in salt concentrations. Smith has reported that exhaustive extraction with any given salt yields only a fraction of the total protein. The highest yield, about 90 per cent, was obtained with 1.87 M NH4CL, followed by 1.65 M LiCl, and a still lower percentage with the other salts tried. Howe found that the total globulins (myosin and myogen) compose about one-third of the total protein of the muscle, and that the insoluble protein composes approximately one-half of the total protein of the muscles of the cow, calf, and rabbit. In addition to variation with concentration, with the number of extractions, and with different salts, the solubility varies with pH. In general the proteins are less soluble at the isoelectric point.

The plasma proteins undergo denaturation by heat, acids, and surface orientation. That the lactate ion affects the extent of denaturation and the resulting density of myosin and albumen has been mentioned in Chapter I (The Relation Of Cookery To Colloid Chemistry). Still greater changes are brought about by heat when the meat is cooked. In meat not only myosin and myogen may be found, but also denatured myosin and denatured myogen. In cooked meats these proteins may be entirely denatured.

Isoelectric point of muscle proteins. Smith gives the isoelectric point of myosin as pH 5.0-5.3 and that of myogen as pH 6.3. Burns has given the isoelectric point of myosin at pH 3.9 and that for the myoproteins at pH 4.5 and 5.