Composition Of Muscle Tissue

Proteins are the important constituents that allow for the physiological functioning of meat animals. Meat provides a major source of high-quality proteins for the human diet. The proteins in muscle are categorized as sarcoplasmic, myofibrillar, and connective tissue proteins. Numerous other proteins are found in other parts of the animal, such as in the organs, bones, and skin.

Sarcoplasmic proteins, at a concentration of about 55 mg/mL, are found in the sarcoplasm or fluidlike substance that surrounds and bathes the contractile apparatus and other organelles of the muscle cell. The sarcoplasmic proteins are primarily metabolic enzymes, and the protein responsible for the color of meat is known as myoglobin. The function of myoglobin is to bind oxygen; it has a molecular weight of 17,500 and occurs in muscle at a concentration of 0.1 to 20 mg/g muscle, depending on function of the muscle and specie and age of the animal. The heme portion of myoglobin binds the oxygen, which is transported to the muscle cell via the circulatory system and bound in blood to a similar protein, hemoglobin. Other sarcoplasmic proteins include nucleoproteins, which are a complex of proteins and nucleic acids and function in protein synthesis, and lyosomal proteins, which are hydrolytic enzymes.

The contractile apparatus of muscle consists of thick and thin filaments that interdigitate and slide past each other as muscle shortens; these filaments are composed of myofibrillar proteins. The major protein of the thick filaments is myosin, and for thin filaments it is actin. Myosin has a molecular weight of 480,000 and consists of two heavy subunits and four light subunits. Two fragments are formed when myosin is subjected to digestion with trypsin. The larger fragment has a molecular weight of350,000 and is called heavy meromyosin. It is soluble at low ionic strength and contains both actin binding and ATPase activity. The smaller fragment has a molecular weight of 150,000 and is called light meromyosin. Heavy meromyosin can be further digested into S-l and S-2 fragments. The monomelic form of actin is called G actin and has a molecular weight of 42,000. G actin polymerizes to form the filament form of F actin. Tropomyosin and troponin are other muscle proteins that bind to actin and are responsible for regulation of contraction.

Connective tissue consists of a variety of cell types and proteins filaments that are secreted by them. It is the latter that are of interest in meat, since they not only hold the cells together and in place but are responsible for toughness. Muscle has three layers of connective tissue— one surrounding the entire muscle, known as the epimys-ium; another layer that segments the muscle into bundles of fibers, known as the perimysium; and a layer surrounding each individual cell or myofiber, known as the endo-mysium. Collagen and elastin are the two connective tissue proteins. Collagen is an abundant protein in muscle and is unique because it has a high complement of the amino acids hydroxyproline and hydroxylysine. It is converted to soluble gelatin at the usual cooking temperatures of meat. Elastin is, in fact, a minor component of connective tissue, contains the unique amino acids des-

mosine and isodesmosne, and does not solubilize upon heating, nor does it swell by acid or alkaline treatment.

Fats comprise 10 to 30% of the carcass weight of meat animals and are primarily triglyceride. The majority of fat is located in fat cells, but lipid also occurs in cellular membranes. The fat cells within muscle are located in the perimysial connective tissue, and their accumulation depends on genetics and nutrition. Also, species is important, as, for example, when beef or pork is compared with chicken or fish, with the latter having a much lower fat content. The composition of fat varies likewise. Softer or more unsaturated fat is more prone to autooxidation and the associated "off" or rancid odors or flavors. Other lipid components, although occurring in small amounts, are phospholipids, sterols, and free fatty acids.

Glycogen is the major carbohydrate in muscle and is normally present at a concentration of 0.5 to 1.0%. Ash is normally 1% or less and results from minerals such as iron, zinc, and calcium.

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