Post-slaughter influences on the formation of metyyoglobin in beef muscles.

By the use of electrical stimulation and/or hot-boning and/or water baths adjusted to temperatures in the range 1-41°C, beef muscles (Semimembranosus, Longissimus dorsi and Psoas major) were exposed to different pH/temperature/time regimes following slaughter and the formation of metmyoglobin (metMb) at their surfaces was monitored during subsequent aerobic storage at 1±1°C. The time course of formation was complex but, in general, consisted of an initial, rapid phase, lasting a few days, followed by a slower, or equilibrium, phase which ultimately yielded to a second rapid phase. Exposure to high temperature and low pH led to increased rates of metMb formation in the Longissimus dorsi and Semimembranosus muscles sliced 48 h after death but not in Psoas major muscles sliced 23 or 48 h post slaughter. However, ageing the Psoas major muscles, in vacuo, at 1°C caused the high temperature treated samples to be less colour stable than those held at 1°C. When the air-stored samples were transferred to an anaerobic environment at 23·5°C reduction of the metmyoglobin occurred, the rate of reduction being inversely proportional to the concentration of metMb developed during aerobic storage. It is suggested that the rate of metmyoglobin formation at the surface of beef muscle slices is dependent on at least two factors-the oxygen consumption rate (OCR) and the activity of an enzymic reducing system. In most practical circumstances the activity of the reducing system is the most important factor and this can be affected by the time, temperature and pH history of the muscle.