Insoluble 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. 

The myofibrillar proteins make up 50-60% of the total protein of muscle cells. Insoluble at low ionic strengths, these proteins dissolve when the ionic strength exceeds -0.3 and can be extracted with salt solutions. Analysis of isolated mammalian myofibrils86 shows that nine proteins account for 96% or more of the protein myosin, which constitutes the bulk of the thick filaments, accounts for 43% and actin, the principal component of the thin filaments, 22%. 

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. 

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. 

Recent studies highlighted the film-forming properties of fish and meat myofibrillar proteins [127-134]. Films formed from an aqueous solution were found to be water-insoluble and completely transparent, with good mechanical and gas barrier properties [53]. Their mechanical strength is close to that of PE films. The thermoplastic features of myofibrillar proteins [135,136] could also be tapped for industrial-scale production of these films using techniques commonly implemented to obtain synthetic thermoplastic polymers (e.g., extrusion or thermoforming). 

Muscle proteins in typical mammalian muscle tissue constitute around 20% of the muscle weight The major proportion of muscle is made up of muscle fibre proteins (elongated, threadlike cells) called myofibrillar proteins. Smaller amounts of soluble sarcoplasmatic proteins and insoluble structural proteins from connective tissue are also present (Table 2.16). Myofibrillar and sarcoplasmatic proteins are almost complete (whole) proteins, while the nutritional value of structural proteins is very low as they are almost indigestible. Table 2.17 gives the amino acid composition of some pure animal proteins Table 2.9 presents the amino acid compositions for the main types of meat proteins. 

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. 

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