Collagen. Table 24 shows that the white connective tissue contains nearly 32 per cent of collagen and about 7 per cent of elastin. Collagen in the presence of moisture and at high temperatures yields gelatin. Bogue states that collagen is changed to gelatin more rapidly at the boiling temperature of water, or at temperatures above boiling obtained by pressure, and more slowly at temperatures below boiling. The gelatin is dissolved in the water or broth. If the meat is cooked long enough, the larger part of the connective tissue is dissolved, so that the muscle fibers may fall entirely apart. Sometimes in cooked meat only the outer portion of the fibers is separated and the inner portion is still connected. The surface of the meat reaches a high temperature before the interior of the meat, thus the collagen of the connective tissue near the surface may be changed to gelatin first.

Elastin. Vandergrift and Gies have reported the yellow connective tissue as containing about 32 per cent of elastin and about 2 per cent of collagen. The elastin is a very resistant, firm protein and is not changed or affected by heat and moisture. In cooking, yellow connective tissue is not softened. Consequently cuts of meat containing large amounts of elastin in the connective tissue will be tough after cooking, whereas cuts containing large amounts of collagen may have part or all of the collagen changed to gelatin.

Table 25 Connective Tissue Proteins in Meat (Mitchell, Zimmerman, and Hamilton)

Less tender cuts have more connective tissue. Data given in Table 25 are from the earlier work of Mitchell et al., so that the percentage of elastin given is too high, but the comparative amounts in different cuts are shown.

In later work Mitchell and co-workers found that the percentage of elastin in muscle is small, but in general the less tender contain a larger proportion than the tender cuts.

From the experiments, results given in Table 26, Mitchell et al. find no relation between the ordinary market grading of beef carcasses and the connective-tissue content, the eye-muscle of ribs from the carcass of a canner cow having no more collagen than that from a choice steer. The results of their "investigation lend no support to the belief that the appearance, texture, and firmness of beef lean give reliable information concerning its tenderness." This suggests that other factors than the connective-tissue content alone affect the tenderness of meat, for most persons would prefer taking a chance on the eye-muscle of ribs from a choice steer being more tender than that from a canner cow.

Pork contains little connective tissue. Mitchell et al. have found that all the different cuts of pork contained more nearly the same percentage of connective tissue than the different cuts of veal or beef.