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Regarded nutritionally, meat is a very good source of essential amino acids, and to a lesser extent, of certain minerals. Although vitamins and essential fatty acids are also present, meat is not usually relied upon for these components in a well-balanced diet. On the other hand, an organ meat, such as liver, is a valuable source of vitamins A, B1 and nicotinic acid. Even in respect of its accepted nutrient role, however, little is yet known about possible differences in the value of meat from different species, breeds and muscles. Although the role of muscular tissue is the same wherever it occurs, and consists predominantly of contractile proteins, the amino acid composition of which is said no to varygrossly between species (Crawford.1968), the accessories of the contractile process are certainly no identical even between the muscles of a given species (cf.5.3). There are differences in the contents of ancillary protein, of free amino acids, of fatty acids and of various other substances, and in their character. These can be presumed not to be without nutritional significance, albeit subtle.

It is well know of course that a muscle containing much connective tissue will provide meat which is relatively resistant to digestion and absorption, and that this will be worsened by faulty cooking; but how important this may be in relation to the absorption of the nutrients of the meat has been little studied. Moreover, since connective tissue proteins have a lower content of essential amino acids than those of contractile tissue, meat having a high percentage of collagen or elastin will also have relatively lower intrinsic nutritive value. There is less of the essential sulphur-containing amino acids in connective tissue(Bender and Zia,1976), and an inverse relationship in meat samples between hydroxyproline (as an index of connective tissue) and truptophan (Dahl, 1965). Nevertheless, kofranyi and jekat (1969)demonstrated that, for human consumers, connective tissue may not be nutritionally disadvantageous until the ratio of connective tissue nitrogen to muscular tissue nitrogen is greater than 1. indeed a mixture of 84per cent muscle nitrogen and 16per cent gelatin nitrogen was shown to have a biological value of 99, whereas that of 100per cent beef muscle was 92(in relation to whole egg protein as 100). Incidentally,although the replacement of meat by beef tendon in emulsion-style sausages lowers the overall acceptability of the product, preheating the tendon at as low as 60도씨 permits a higher level of incorporation before adverse effects are noted(Sadler and young,1993).

 The increasing development of the mechanical deboning of meat, and the inclusion of mechanically recovered meat in food products, has made an assessment of its nutritional value important. When whole animal carcasses are mechanically recovered, the overall composition is close to that of hand-deboned meat(Field,1974), but the content of calcium, ash and iron is higher in the former (Newman, 1980-81). It is likely to contain bone collagen. Clearly, the nutritional value of mechanically recovered meat will vary with the bone source and the levels of connective tissue and calcium present.

 Insofar as connective tissue may be added to meat products, however, in amounts greater than those there is clearly a need, in the interests of consumers, for methodology to identify added collagen. In this regard, artificially prepared collagen(rind) can be differentiated histochemically from natural collagen by its birefringent colour under polarized light (Flint and Firth, 1983)

11.1.1. Amino Acids

The amino acid composition of the proteins of the principal types of meat is shown in Table 11.1. In respect of the essential amino acids, beef would appear to have a somewhat higher content of leucine, lysine and valine than pork of lamb, and a lower content of threonine. Despite the minor nature of these species differences, however, it should be pointed out that the meat represented in Table 11.1 is of random origin. It is certainly feasible that more significant differences may exist between specific muscle locations, or that breed, and animal age, have important effects. It has been reported, for example, that the contents of arginine, valine, methionin, isoleucine and phenylalanine increase (relative to the concentrations of other amino acids) with increasing animal age(Gruhn, 1965).

Futher, there is evidence that the content of certain essential amino acids may differ at different parts of the carcass. Some data on tryptophan and lysine in certain pork muscles are given in Table 11.2.

 The amino acid content may be affected by processing (e.g. heat, ionizing radiation :7.2.2 and 9.1.1 above); but, unless processing conditions are both severe and prolonged, such destruction is minimal. Rather more important is the possibility that certain amino acids may become unavailable (Bender, 1966). Thus Dvorak and Vognarova(1965) found that after heating beef for 3hr at a series of temperatures, 90per cent of the available lysine was retained at 70도씨 and only 50per cent at 16도씨. There is a linear relationship between loss of available lysine in canned beef and the severity of the process (Ziemba and Malkki, 1969). A 20per cent fall on available tryptophan and methionine was obseverd in canned pork after only 40min at temperatures above 70도씨 (Hibbert, 1973). Bender and Husaini (1976) found no loss in availablemethionine when beef was autoclaved for I hr at 115도씨, but when it was processed in the presence of other food constituents, such as wheat flour and glucose, there was a loss in net protein utilization which could be related to a fall in available methionine. Amino acids can also become unavailable during the prolonged storage made possible by canning. Bender(1966) found that veal, canned in 1823, had a

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