CONTRIBUTION OF RIGOR SHORTENING AND COLD SHORTENING TO VARIABILITY IN THE TEXTURE OF PECTORALIS MAJOR MUSCLE FROM - COMMERCIALLY-PROCESSED BROILERS

1. Three experiments were conducted to establish the degree to which cold shortening and rigor shortening contribute to variability in the texture of Pectoralis major (PM) muscles of commercially processed broiler carcases chilled at different rates. 2. In the first experiment, free range and standard broiler carcases were air-chilled under normal commercial conditions at 0 degrees C. Strong negative correlations between pH values 15 min post-mortem (pH(15) (min)) and sarcomere length indicated that some cold shortening had occurred, while evidence supporting the occurrence of rip or shortening was much weaker. Regardless, of the cause of muscle shortening, weak negative correlations between shear force and sarcomere length indicated that shorter sarcomeres were associated with tougher meat. In addition, strong negative correlations between pH values 24h post-mortem (pH(24h)) and cooking losses suggested that increased juiciness is associated with higher ultimate pH values. 3. In the second experiment, carcases were either chilled rapidly in water at 0 degrees C (23 h) or in water at 10 degrees C (10 h or 23 h) to identify the individual contributions of cold and rigor shortening to textural variability more precisely. In carcases chilled rapidly in water at 0 degrees C, textural variability was low and toughness was absent, suggesting an absence of both cold and rigor shortening. However, few of these carcases had PH15 min values sufficently high (greater than or equal to 6.70) to promote a cold shortening effect. In contrast, carcases chilled in water at 10 degrees C, which had a similar deep muscle cooling rate as air-chilling at 0 degrees C, showed evidence of rigor shortening, because they had a wider range of sarcomere lengths and higher shear force values than carcases chilled in water at 0 degrees C. 4. In the final experiment, carcases were either chilled in air at - 12 degrees C, a cooling rate similar to that ofwater-chilling at 0 degrees C, or chilled in air at 0 degrees C. Cold shortening and increased toughness was evident with both chilling regimens in those carcases with pH(15) (min) values greater than or equal to 6.70. In contrast, in carcases with PH15 (min) values <670, both chilling regimens reduced sarcomere shortening and improved tenderness. However, the mean shear value of the carcases chilled in air at - 12 degrees C was almost 1.00 kg cm(-2) lower than those chilled in air at 0 degrees C. 5. In conclusion, both cold shortening and rigor shortening can occur during the commercial air-chilling of broilers at 0 degrees C and thereby contribute considerably to textural variability and incidences of toughness. Faster chilling, either in water at 0 degrees C or in air at - 12 degrees C, has been shown to eliminate the risk of adverse rigor shortening and toughness.