Figure 5. Postmortem pH fall in muscles after electrical stimulation. During stimulation the muscle pH falls (/IpH). After stimulation the rate of pH fall (dpH/dt) is increased. At 35°C, unstimulated beef muscle dpH/dt = 0.18 pH units/h. With 120-s stimulation duration, the dpH/dt = 0.3 pH units/h.

shortening and get muscles into rigor so that aging commences while muscles are still warm, others have used electrical stimulation to produce color changes that can influence grading and even have used it for carcass immobilization. Electrical stimulation tends not to be used for pigs because early work (45) suggested that acceleration of rigor mortis through electrical stimulation increased the incidence of pale, soft, and exudative pork. However, no effect was found in the shoulders, but there was an increased paleness in the legs, unless rapid chilling was used (46), it is possible to use electrical stimulation if chilling is rapid enough (15).

Although remote from the actual changes produced by electrical stimulation, muscle pH has been used extensively as an indicator of progress into rigor. Muscle pH falls after stimulation provides a reasonable guide in normal muscles but is misleading in abnormal situations such as in the muscles from stressed animals. This is one case where muscle pH does not give a true indication of the stage of rigor development, because although the pH remains high, the ATP level may be zero and the muscle in rigor. The time to reach pH 6.0, or time to full rigor mortis, will be influenced by the magnitude of the JpH and the dpH/dt (Fig. 5) and provides a useful measure with which to evaluate the effectiveness of electrical stimulation systems in inducing early rigor. It does not provide an exact relationship with tenderness changes. Temperature will exert a substantial influence on both zlpH and the dpH/dt.

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