Rate of Heat Penetration of Microwave-Cooked Beef Semitendinosus as Related to Histological Characteristics and Tenderness

Three-inch pieces of beef semitendinosus muscles were cooked by microwaves for zero-, three-, four-, five-, six- and seven-minute intervals to internal temperatures of 1, 16-25, 26-35, 36-45, 46-55 and 56-65°C, respectively. An index to tenderness was obtained by the Warner-Bratzler shear and histological characteristics were studied. Increase in internal temperatures of the muscle pieces was accompanied by a decrease in shear values, the greatest decrease occurring at the seven-minute time interval. Scores for muscle disintegration (none to excessive), changes in fat cells (full to empty) and degradation of collagenous connective tissue (fibrous to granular) increased as internal temperature increased. Muscle fiber diameter, as estimated by the number of fibers per field, decreased slightly as cooking time and internal temperatures of the muscle pieces increased. Low shear values were associated with decreases in muscle fiber diameter, but not muscle fiber disintegration. As shear values decreased empty fat cells became more prevalent than full fat cells. Shear values and degradation of collagenous connective tissue were unrelated in this study