Factors Affecting

Delay Postmortem

Postmortem delay before stimulation can have a threefold effect: muscle temperature can fall, greatly reducing the magnitude of zlpH; glycolysis can have progressed so that muscle pH has fallen, reducing the z/pH that can be achieved; and the nervous system can decay and become unresponsive so that its stimulation cannot elicit any muscle response. It has been shown that the z/pH produced by any given stimulation decreases as prestimulation pH decreases (40,47). The zfpH is 0 with a prestimulation pH of about 6.3.

The influence of temperature on the rate of pH fall has been considered in many reports (48-50) (Fig. 6). Since the advent of stimulation as a tool for meat science, the temperature dependence of the pH drop during stimulation and of the subsequent pH fall has been determined. When the muscle temperature falls, the magnitude of zlpH is reduced. For example, in beef m. sternomandibularis, zlpH ranges from 0.6 pH units at 35° to 0.018 units at 15°C. The energy of activation of /IpH in stimulated beef m. sternomandibularis is calculated to be 97 kJ/mol (51), or very similar to that for calcium-activated actomyosin ATPase (44,50,52).

Muscle Type

Muscle type can influence zfpH, for example, the fast-twitch beef m. cutaneous trunci, largely composed of white muscle fibers; and gives high values for zlpH (and dpH/dt), whereas in the slow-twitch m. masseter, composed of red fibers, there is neither a distinct zlpH nor an acceleration of dpH/dt, which is naturally rapid (0.4 pH units/h) (53). It has also been shown that zlpH is minimal in predominantly slow-twitch muscles of beef (54). The greatest zlpH

Figure 6. The various postmortem dpH/dt values for stimulated and nonstimulated muscle are plotted to show that the differences exist at all temperatures and illustrates how important it is to stimulate at high temperatures for full effectiveness. These results were obtained from beef m. sternomandibularis stimulated and held at the temperatures indicated.

The magnitude of zfpH is governed by the muscle fiber type, initial glycogen stores within the muscle, the electrical characteristics (current, frequency, pulse shape, and stimulation duration), the temperature of muscle, and the time after death that stimulation was applied.

Figure 6. The various postmortem dpH/dt values for stimulated and nonstimulated muscle are plotted to show that the differences exist at all temperatures and illustrates how important it is to stimulate at high temperatures for full effectiveness. These results were obtained from beef m. sternomandibularis stimulated and held at the temperatures indicated.

values have been shown in beef triceps brachii and the least in the m. semimembranosus (55). It was reported (56) that within the m. sternomandibularis, fast fibers with strong ATPase and weak succinic dehydrogenase (SDH) reactions showed most glycogenolysis in response to electrical stimulation, while slow fibers with weak ATPase and strong SDH reactions showed the least.

Muscles also differ in their optimum response frequency. Beef m. longissimus is more responsive to 14.28 pulses/s than to 40 pulses/s (40). The converse has been shown for the beef m. semimembranous (57). Rat muscles have a much shorter twitch time than beef muscles, with optimum responses to stimulation with a waveform of 33 pulses/s (58).

Electrical Characteristics

For any given muscle, the pH response will be governed by the electrical characteristics of voltage (current), and pulse frequency, shape, and polarity. In general, the higher the current (at a constant resistance, current increases with increased voltage), the greater will be the effect. This response will be asymptotic to some maximal value, however, so continually increasing the current will not lead to a continuing increase in effect (40,55). If the current level becomes too high, the muscle can cook due to resistive heating, this is especially true for high-frequency input. A practical consequence of this was the melting, within 45 s of the tendons of the m. gastrocnemius when used to suspend lamb carcasses during stimulation with 800 V rms at 50 Hz in some early studies.

For all of the species tested, the pulse shape does not seem to be a critical factor, with equivalent responses being achieved with 10 ms duration half-sine wave pulses and 5 ms duration square-wave pulses at the same frequency (40). The pulse frequency, however, can exert a profound effect, because of the interaction with muscle type and response frequency. If a single muscle is considered, for example, beef m. sternomandibularis, the magnitude of z)pH is maximal with a pulse rate of between 9 and 16/s. At frequencies between 10 and 20 pulses/s, zfpH values are, respectively, 40 and 75% greater than at 50 and 100 pulses/s (40). Pulse polarity has not received much attention, although it was indicated (35) that for low-voltage stimulation the polarity of pulses influenced the magnitude of zipH and the movement of the carcass during stimulation. The greatest responses were obtained when the cranial end of the animal was positive relative to the rest of the body. The physical response to low-voltage alternating polarity waveforms used for stimulation of beef cattle can be movement at half the waveform frequency, which disappears as the voltage is raised.

Many different voltages, frequencies, and pulse shapes have been used, and these are sometimes inaccurately described in scientific publications. In the United States, many groups have pulsed the 60 Hz AC waveform with 1- or 2-s periods of current flow, then a similar period of rest (19,46,59), whereas groups in Australia, the UK, Sweden, and New Zealand have used discrete pulses. An unusual, but apparently effective, combination of high- and low-voltage pulses has been used by the Dutch (60). In many instances the reasons for the chosen frequency do not appear to have a scientific basis.


Almost any stimulation can affect the dpH/dt. Rates of pH fall in curarized muscles are much slower than those in muscles of animals slaughtered without curarization (61). Vigorous muscle movement during slaughter of normal animals can induce the apparent maximal twofold increase in dpH/dt (61). Sometimes there can be unexpected electrical stimulation arising from the stunning (62) and immobilization of carcasses and even effects from current application from downward hide pulling (63).

Attempts have been made to relate increases in dpH/dt to phosphorylase a and myofibrillar ATPase stimulation (47). The muscle temperature has a major effect on dpH/ dt (Fig. 6). In nonstimulated beef m. sternomandibularis, the energy of activation is 40-45 kJ/mol, whereas that of stimulated muscle approaches 70 kJ/mol (51). This high energy of activation means that any cooling of the muscles will have a marked effect on the time for rigor to be achieved. Because the achievement of ultimate pH is asymptotic, any reduction in maximum rate can greatly prolong the period when muscle is above the ultimate pH and still has a measurable level of ATP. The rate of pH fall and ATP loss is affected not only by the temperature during stimulation, but also by the temperature history of the muscle after stimulation during the period of study (64).

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