Progressive changes in Warner-Bratzler shear values of meat cores and in dimensions and integrity of muscle fibers from beef semitendinosus cores heated in tubes in a water bath to simulate oven roasting of top round roasts at 93 and 149°C to endpoint temperatures of 40, 50, 60 and 70°C were evaluated. Slower heating and higher endpoint temperatures resulted in greater cooking losses. Warner-Bratzler shear values and muscle fiber diameters decreased from 40 to 50 to 60°C. Sarcomere length of muscle fibers decreased from 60 to 70°C. Muscle fibers disintegrated as internal temperature increased. Fiber disintegration was greater at the faster rate of heating for cores heated to 60°C than at the slower rate. A faster rate of heat penetration might result in greater degradation of muscle fibers at a lower temperature. An increase in fiber disintegration with heating of cores from 60 to 70°C suggested an increase in tenderness, but changes in shear values were not apparent in this temperature range. It appears that other factors oppose the tenderizing effect of increased disintegration of muscle fibers at higher temperatures.

Progressive structural changes in heated bovine semitendinosus (ST) muscle fibers were studied using phase contrast microscopy and scanning electron micrcscopy. ST cores were heated at two rates which simulated the oven roasting of top round roasts at 93 and 149°C to endpoint temperatures of 40, 50, 60 and 70°C. Increased heating of muscle fibers resulted in the disintegration of Z-line structure; increases in intermyofibrillar spaces; shortening of sarcomeres; cracks and breaks in the myofibrils at the Z-line; and fragmentation or granulation of the myofibrils. The slower rate of heating resulted in extensive granulation and fragmentation of the muscle fibers, whereas the faster rate resulted in more cracks and breaks in fibers. The effect of heating rate on the disintegration of muscle fibers suggested that the rate of heat penetration might influence the type and extent of disintegration of muscle fibers. Heating to 70°C resulted in extensive fragmentation of muscle fibers. Heat-induced changes in the structure of muscle fibers are discussed in relation to possible influence on the tenderness of heated meat.