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A physiological perspective: Electrical stimulation of post-mortem muscle

Fuller, Paula (2016) A physiological perspective: Electrical stimulation of post-mortem muscle. PhD thesis, Murdoch University.

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Electrical stimulation of carcasses is a mainstay processing technique within the sheep meat industry. A primary use is to accelerate tenderisation of meat and to reduce the variation observed in its end products. Technological advances have resulted in a new generation of medium voltage electrical stimulation (MVES) units, which offer superior outcomes to previous high- and low-voltage systems. These new units allow precisely controlled electrical inputs to individual carcasses, thus providing processors with greater control over end product outcomes. Yet even with these new systems, significant variation in meat quality still remains. This variation likely results from an unidentified interaction between the electrical input and the carcass, as previously used electrical parameters were empirically defined, based on processor experience rather than an integrated understanding of the applied parameters and the induced response in the post-mortem muscle. Thus a scientific understanding of how the electrical input interacts with post-mortem muscle to improve meat quality is required. Given the use of electrical stimulation stems from its ability to engage muscle activity, this thesis sought to examine the effect of MVES on post-mortem muscle function using contractile characteristics as a means to identify how this type of system imparts its beneficial effects.

The first experimental chapter (Chapter 2) examined the effect of various MVES parameters on dressed sheep carcasses (under commercial processing conditions), using the ΔpH of the M. longissimus thoracis et lumborum as a means by which to define carcass response and determine which electrical parameters were most effective. We observed that stimulation parameters incorporating longer pulse widths (i.e. 5ms) or those with a modulated (increasing) frequency across the electrodes produced the largest carcass response (i.e. largest ΔpH). Interestingly, the effect of this electrical stimulation on pH was influenced by hot carcass weight, with lighter carcasses (<23Kg) more responsive to all stimulation parameters tested, in contrast to heavier carcasses that did not distinguish between the different electrical inputs. As part of this study, we also determined that pre-stimulation pH is an adequate marker of carcass responsiveness to MVES stimulation, offering a way in which to determine how carcasses will respond to the applied current. In a concurrent study, we took advantage of the positioning of the stimulation system in order to compare the effect of MVES on muscle pH within the immediate stimulation period (i.e. within five minutes) compared to a time period more reflective of that in the literature, documenting a significant difference. This, in conjunction with showing a significant difference between pH measuring techniques, highlights the requirement for consideration of both the timing and methodology when comparing pH responses to MVES.

Having determined the parameters that produced the biggest effect on carcass pH, the second experimental chapter (Chapter 3) undertook a closer examination as to how these parameters affected the contractile properties of post-mortem muscle. This was achieved by establishing an isolated nerve-muscle electrophysiology rig to examine their influence on muscle in a controlled environment. The electrical parameters identified in the previous chapter were examined, with a particular interest in the effect of longer pulse widths and modulated frequency on muscle contraction given their positive effect on ΔpH. The contractile characteristics of two different types of sheep muscle [M. semitendinosus (ST) and M. semimembranosus (SM)], were examined. This study revealed that electrical stimulation with longer pulse widths produced a greater contractile response in terms of both peak tension and the overall amount of contraction. Muscle response to modulated frequency stimulation was similar to that elicited by longer pulse widths, but likely arose from enhanced activity of different contractile elements. Comparison of the different muscle responses to simulated MVES showed that overall, the oxidative-glycolytic SM muscle produced a larger contractile response than the glycolytic ST, but did not distinguish between the electrical parameters. This study also revealed a difference in the way in which the electrical impulse was transmitted through the muscle bundle, i.e., direct stimulation of muscle vs. nerve-mediated (stimulation of residual nerve activity). It appeared that more of each parameter was transmitted via the nerves in SM, but more directly transmitted in ST.

The last observations from Chapter 3 were particularly interesting, as it suggested that electrical stimulation transmitted via the nervous system may a viable option for use within the MVES set up. This possibility would provide another avenue for whole carcass stimulation, as electrical transmission via the nervous system generates a consistent and more homogenous decline of pH in all carcass muscles. Thus the last experimental chapter (Chapter 4) examined if, and to what extent, post-mortem muscle could respond to nerve-mediated electrical stimulation. This study showed that under a defined post-mortem environment (i.e., anoxic conditions), post-mortem sheep muscle does retain contractile activity in response to nerve-mediated electrical stimulation, but in a muscle-specific manner - both in the magnitude of contraction and the ability to distinguish between stimuli frequencies. We also investigated the role of several neuromodulators in muscle contraction under these conditions, specifically those that are known to mediate muscle contraction arising from nerve stimulation (ATP, NO and CGRP). The neuromodulators ATP and NO appear to affect muscle response to nerve stimulation at the level of neurotransmission in ST, in contrast to E-C coupling processes in SM. These results suggest that post-mortem SM muscle is better able to respond to nerve-mediated parameters, and this response is likely directed by the physiological properties that dictate normal muscle function (i.e., the type of innervation). Overall this study suggests that nerve-mediated electrical parameters may be a viable option for incorporation into commercial processing environment if optimised in a muscle-specific manner, with the most advantage likely gained for the processing of muscle groups containing oxidative-glycolytic fibre types.

In summary, this thesis has provided a number of insights into the mechanisms by which various MVES inputs influence post-mortem muscle function. Identification of the biological changes that occur within stimulated nerve and muscle in post-mortem tissue may be used to refine electrical stimulation parameters for specific carcass applications to enhance overall meat quality. Furthermore, we identified a potential role for nerve-mediated responses as part of the stimulation strategy. It is hoped that these findings contribute to a more sophisticated understanding of the use of electrical stimulation, and may provide the commercial sector further opportunities for refining this technology for the benefit of both the producer and consumer.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Phillips, Jacqueline and Pethick, David
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