Tenderness and Structural Changes in Beef Semitendinosus Muscles Heated at Two Rates to Four End Point Temperatures

Progressive changes in tenderness of meat cores and in structure of muscle fibers from beef semitendinosus cores heated in test tubes in a water bath to simulate oven roasting of top round roasts at 93 and 149°C to end point temperatures of 40, 50, 60, and 70°C were evaluated. Slower heating and higher end point temperatures resulted in greater cooking losses. Warner-Bratzler shear value of cores and diameter of muscle fibers decreased from 40 to 50 to 60°C and sarcomere length of muscle fibers decreased from 60 to 70°C. Cores heated to 40 and 50°C at both rates exhibited considerable variation in shear values. Disintegration of muscle fibers steadily increased with increases in internal temperature. The slower rate of heating resulted in extensive granulation within muscle fibers, whereas the faster rate resulted in more cracks and breaks in the fibers. Fiber disintegration was greater at the faster rate for cores heated to 60°C than at the slower rate. 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 and the resulting tenderness of the meat. Scanning electron photomicrographs showed extensive fragmentation of muscle fibers at the Z line, shortening of sarcomeres, and the disintegration of Z-line structures in muscle fibers heated to 70°C. Extensive fragmentation of muscle fibers heated to 70°C suggested an increase in tenderness, but a decrease in shear value was not apparent with heating from 60 to 70°C. It appears that other factors oppose the tenderizing effect of increased disintegration of muscle fibers at higher temperatures.