Salt-soluble proteins, myofibrillar protein

A few workers have attempted to separate lean meat into its individual protein fractions. Turner et al. (1979) fractionated meat into salt-soluble protein (SSP), insoluble myofibrillar protein (IMP), and connective tissue protein (CTP) by centrifugation. Fat was also separated. Using this technique to fractionate minced pork shoulder, Knight (1988) studied the influence of the SSP, IMP, and CTP fractions, and the effect of fat on cooking losses in model systems. The interactions between the three fractions were also examined. The composition of the individual fractions prepared is shown in Table 2.6. 

SSP = salt-soluble protein IMP = insoluble myofibrillar protein CTP = connective tissue protein. 

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. 

Before selecting a method to measure a specific aspect of protein functionality, one must decide on the complexity of the testing matrix. Researchers have used a single purified protein, a crude extract of proteins, a prototype food product, or an actual product to study protein functionality. For meat studies, formulated meat systems, ground muscle, myofibrillar proteins, salt-soluble proteins, actomyosin,... 

Actomyosin. At high salt concentrations ( . . 0.6 M KC1), actin and myosin combine to form actomyosin filaments giving a highly viscous solution. Actomyosin retains the ATPase activity of myosin and demonstrates "super-precipitation" on the addition of ATP (24,34). As expected, there are differences between actomyosins of rabbit and fish with respect to solubility (10,22,35,36), viscosity (46) and ultracentrifugal behavior (477. Since actomyosin is the most readily available form of myofibrillar proteins from fish muscle, its behavior relative to deterioration during frozen storage has been most frequently studied. 

Tappel (99) reported on the hydrolytic activity of various enzymes. They found that the relative hydrolysis of muscle proteins increased in the following order pepsin, Rhozyme A-4, ficin, papain, bromelain, protease 15, Rhozyme P-11, and trypsin. Wang et al. (100) observed that papain was twice as active as ficin towards elastin, a minor component of connective tissue. Ficin and bromelain had equal enzyme activity towards collagen, a major component of connective tissue. Kang and Rice (101) studied the effects of various tenderizing enzymes on water-soluble sarcoplasmic proteins, salt-soluble myofibrillar proteins, and the insoluble stromal proteins. Table VII tabulates the results of some of these studies. 

Muscle proteins are an important component of meat and can be classified according to solubility as sarcoplasmic (water soluble), myofibrillar (salt soluble), or stromal (insoluble) proteins. The application of CE to the analysis of meat proteins has been predominantly for separation of sarcoplasmic proteins in aqueous extracts from fish, bovine, and chicken muscle. The sarcoplasmic proteins that are present are mainly metabolic enzymes and therefore their separation profiles are useful for the purpose of species identification. Some reports also exist of the simultaneous separation of sarcoplasmic and myofibrillar meat proteins using SDS-CGE. 

The increased gel strength of beef surimi compared with beef MSM was thought to be due to the increased protein content. In addition, since the centrifugation stage also removed water-soluble proteins, the remaining protein is made up predominantly of the myofibrillar proteins actin and myosin. These salt-soluble proteins form a strong, highly elastic gel when heated. 

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. 

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