Changes of Myofibrillar and Connective Tissue Proteins Due to Temperature

The following description (2) of the processes occurring during cooking emphasizes the relationship between the myofibrillar and connective tissue proteins (neglecting aging effects during cooking). As the temperature of a piece of meat is raised, the myofibrillar and connective tissue proteins denature at different rates, and it is the properties of the denatured proteins that determine the texture of the meat. At temperatures between 40 and 65°C there is an increase in toughness (Fig. 7) (76) as determined by shear value, which is caused by aggregation of the denatured myofibrillar proteins. At this stage, there is a loss of fluid and shrinkage of the muscle fibers within the endomysial sheath. The collagen in this sheath is not affected at this temperature. However, because the endomysial sheath was under tension in the raw meat, the shrinking of the myofibrillar proteins releases the tension, which forces fluid out of the meat. As the temperature continues to rise from 63 to 80°C, the shear values increase further due to additional shrinkage as the collagen in the endomysium and perimysium denatures and the water is squeezed out. Complete shrinkage of the sheath is prevented by the presence of the myofibrillar proteins. With further increases in temperature above 80°C, there is a reduction in shear values (increase in tenderness), possibly due to peptide bond cleavage and/or cross-link rupture of denatured collagen and breakdown of myofibrillar proteins. The increased tenderness on prolonged heating is almost certainly primarily due to degradation of the denatured collagen.

During cooking, meat collagen is denatured and, because of its partially crystalline nature, it shrinks at about 65°C (depending on animal age and species) to form insoluble gelatin. The thermally stable cross-links modify the shrinkage characteristics so that when some residual tension is maintained after denaturation, there is considerable remaining strength. The tension generated during cooking varies considerably depending on the heat stability of the perimysium, which in turn is determined by the nature and extent of the cross-linking. Thus, the older the animal, the higher the proportion of heat-stable cross-links and the greater the tension generated on shrinkage. The total amount of collagen in itself is no indicator of strength or texture in the cooked meat. Instead, the solubility of collagen is important, as that changes with animal age and thus affects the consequent cross-linking in a complex way. It is the heat stability of the cross-links that affects the strength/texture components of tenderness. The changes in collagen can cause noticeable effects during normal cooking, as they are responsible for the extensive curling of chops or steaks that occurs during grilling and the amount of cook loss.

The end point of cooking is of interest to the consumer. A rare steak is very pink and has an internal temperature less than 60°C, and a well-done steak is grey to brown at internal temperatures greater than 75°C. Dissatisfaction often arises if the color and "degree of doneness" do not match (77). Meat proteins, especially myoglobin, are more stable to temperature as the ultimate pH rises, and the degree of doneness at the same internal temperature often appears to be insufficient (ie, redder). For institutional cooking, variable ultimate pH values give a variable appearance to the steaks.

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