Meat proteins actin

Important constituents of muscle fibres are two proteins, actin and myosin, which are involved in muscle contraction (Chapter II.5). Although the composition of meat is very variable, a representative average composition is listed in Table 12.21. 

Lawrie (1985) classified meat proteins into three types salt-soluble (myofibrillar) proteins, water-soluble (sarcoplasmic) proteins, and insoluble connective tissue proteins. The myofibrillar proteins actin, myosin and actomyosin are the major proteins responsible for determining the heat stability of comminuted meat emulsions. 

The minor amino acid typically present in the meat myofibrillar protein actin (also in some myosin isoforms and in dipeptide anserine, see Section 2.3.3.1.3) is L-3-methylhistidine (2-8), which is formed by methylation of histidine bound at the 73rd position of the protein chain. The functional significance of this modification of actin is not known it is probably related to the metabolism of phosphates, with which the side chain of methylhistidine interacts. Methylhistidine does not occur in other protein-rich foods such as milk, eggs and soybeans. Its contents might therefore serve as a criterion for determining the quality ingredients in meat products. 

Muscle proteins have traditionally been grouped into three categories based on their solubility the sarcoplasmic proteins (consisting mainly of enzymes), myofibrillar proteins (actin, myosin, tropomyosin, troponin, etc.) and connective tissue (mainly collagen). The mixture of myosin, actin, tropomyosin, troponins and other myofibrillar proteins is called actomyosin (AM). Myofibrillar proteins contribute significantly to the technological properties of fish and meat [1-2,5-8]. 

In contrast to milk, where samples are primarily derived from cows, meat analysis has to be performed in samples of a widely different animal origin including cattle, lamb, swine, poultry, and fish. Muscle is a complex matrix with a pH of 5.7, composed of muscle fibers, various types of connective tissue, adipose tissue, cartilage, and bones. Sarcoplasmic proteins such as myoglobin, and glycolytic enzymes are soluble in water while the myofibrillar proteins such as myosin and actin are soluble in concentrated salt solutions (14). The connective tissue proteins, collagen and elastin, are insoluble in both solvents. 

That the radiolysis of meats containing similar proteins and comparable fatty acids involves similar primary and secondary processes leading to a common set of radicals stable at -40°C is shown by the ESR spectra in Figure 12 for irradiated, enzyme-inactivated chicken, beef, ham, and pork [3, 62], These spectra reflect the commonality in radicals derived from the muscle proteins, myosin and actin, and fi-om the constituent triglycerides, which have slightly different fatty acid compositions. The minor consequences of this compositional... 

In a muscle tissue (Fig lb), the fiber cells are thin and elongated as opposed to the polygonal cells in plants. The major myofibrillar proteins, myosin and actin, form the myofilament bundles in the sarcoplasm of the fiber cell. Bundles of fiber cells form the muscle tissue. Connective tissue distributed between individual (endomysium) and bundles of fiber cells (perimysium) as well as around the whole muscle (epimysium) holds the cells and the muscle tissue together. Majority of lipids is located in the adipose tissue depots associated with the connective tissue between the bundles of fiber cells in poultry and red meat as well as fish muscle [2]. 

Galluzzo, S.J. and Regenstein, J.M. (1978) Role of chicken breast muscle proteins in meat emulsion formation - myosin, actin and synthetic actomyosin. /. Food Sci, 43,1761. 

Suiimi is a concentrate of insoluble muscle proteins (ca. 20%). It forms a solid cohesive gel with water (ca. 80%), which solidifies when warm For production, lean fish meat is ground at 5-10 °C and extracted with water until basically only myosin, actin, actomyosin and small amounts of collagen remain. The addition of paramyosin (cf. 13.1.4.2.2) intensifies the structure of the gel. In the further processing of Surimi to Kamboko, starch (ca. 5%), egg white, flavor enhancers, colorants and aroma substances are added, whereby an attempt is made to imitate crab or mussel meat. The resulting mixture is solidified by denaturation of the proteins first at 40-50 °C and then at 80-90 °C. Fibrous structures are produced by extrusion. 

For example, intense proteolyses of muscle proteins, due to the action of endogenous proteolytic enzymes, have been reported to occur during the processing of dry-cured ham. This gives rise to the formation of free amino acids and short peptides (especially from actin through the action of cathepsin D in meat and from caseines as a result of plasmin and other proteases in cheeses) that contribute directly or indirectly to the flavour characteristics of the final product. In the case of octapeptide Lys-Gly-Asp-Glu-Glu-Ser-Leu-Ala, isolated from beef broth, this reportedly showed umami taste with a threshold value of about 500 mg/1. 

---