EFFECT OF POSTMORTEM TEMPERATURE ON CHICKEN M-PECTORALIS-MAJOR - MUSCLE SHORTENING AND COOKED MEAT TENDERNESS

1. Muscle shortening, sarcomere lengths and pH values were measured in strips of chicken M. pectoralis major (PM) muscle incubated at different time (0 to 24 h) and temperature (0-degrees to 40-degrees-C) combinations immediately after slaughter; their effects on cooking loss and meat tenderness determined. 2. Maximum muscle shortening of 39% and 43% occur-red at 0-degrees-C and 40-degrees-C respectively. At 0-degrees-C, most shortening occurred within 90 min post-mortem when the pH of the muscle ranged from 7.13 to 6.52. In contrast, at 40-degrees-C, most shortening occurred during the development of rigor mortis, between 90 and 380 min post-mortem, when the muscle pH ranged from 6.16 to 5.89. In a similar manner, minimum sarcomere lengths of 1-38 mum were reached after 90 min at 0-degrees-C while more severe sarcomere shortening, to 0.96 mum and 0.86 mum at 30-degrees-C and 40-degrees-C respectively, was not complete until after 380 min post-mortem. Between 5-degrees-C and 20-degrees-C, muscle shortening ranged from 25 to 34% while minimum sarcomere lengths of 1.33 mum were recorded. 3. Cooking losses increased on average from 7 to 16% between 30 and 380 min post-mortem, with maximum losses of 19% being achieved by the end of the 24-h incubation period. 4. At 0-degrees-C, shear force values increased from 2.94 kg/cm2 to 4.34 kg/CM2 between 30 and 90 min post-mortem while the muscle pH was > 6.5. At all other temperatures, increases in shear force values were not detected until 380 min post-mortem when the muscle pH had fallen to 5.9 and rigor mortis had set in. At all times after 380 min, however, the muscle strips incubated at 0, 5 and 40-degrees-C had lower shear values (range 3.17 to 5.49 kg/cm2) than those incubated from 10-degrees-C to 30-degrees-C (range 5.06 to 7-22 kg/cm2). 5. A significant quadratic relationship was found between the degree of shortening and subsequent cooked meat tenderness, in which peak toughness occurred at 30% shortening. This would suggest that the actual extent of muscle shortening per se has an important role to play in determining the tenderness of chicken post-mortem. Consequently, with unrestrained chicken muscle, where extensive shortening occurred at 0-degrees-C and 40-degrees-C (i.e cold- and rigor shortening) the cooked meat was more tender than that subjected to intermediate post-mortem temperature regimens.