ELECTRICAL STIUMULATION

Electrical stimulation (ES) of carcass muscles soon after slaughter accelerates their normal decline in pH and may enhance tenderization during conditioning.
Although there was some early work on ES in the US, commercial possibilities did not become apparent until it was shown in New Zealand that stimulation prevented COLD SHORTENINGin lamb.
ES now is widely used for BEEF and LAMB. There are some who consider it can be used for pork, although care must be taken to prevent PSE.
As well as protecting against cold shortening, electrical stimulation may improve

meat tenderness,
color and appearance, and
subjective scores for youthfulness.

increased

Apart from guarding against cold shortening, the beneficial effects of ES on meat tenderness could also involve muscle fiber fracture. In other words, when the muscle contracts violently after lsughter it may rip up the muscle fibers making them more tender.
ES is ineffective on dark-cutting beef and post mortem muscle stimulation is of no value without an accelerated decline in pH. In other words, dark-cutters have little or no glycogen after slaughter, so they cannot make much lactic acid, so they cannot benefit from advanced glycolysis
ES may enhance the activation of lysosomal cathepsins while the carcass is still warm. In other words, ES may turn on some of the enzymes that make meat more tender during the conditioning period.
Different voltages have been used: from 32 to 1600 V.
Different amperages have been used: from 0.5 to 6 A.
Types of electrode have included

aluminum foil
probe or pin types
rectal probes
multipoint electrodes
hooks and shackles

Electrode positions usually have spanned great lengths of carcass from neck to hindlimb, often with electrodes in muscles but sometimes in the spinal cord. Contralateral differences in the effectiveness of ES may be detected when one hindlimb is shackled and the other side moves freely.
The time delay between exsanguination and stimulation has been extended up to 60 minutes, during which time carcasses have received variable treatments such as evisceration and splitting. Stimulation during exsanguination is not effective.
Impulse frequency and duration of application range from 3 to 400 Hz. Muscles differ in the optimum frequency required for their maximum stimulation.
Wave forms of stimulatory impulses have included

square waves of interrupted direct current
unchanged or partly modified sinusoidal waves of alternating current.

In summary: higher voltages are required the longer the delay between exsanguination and ES.

ES and the Nervous System.

Even in a simplified laboratory model of carcass stimulation, with a muscle strip and a pair of stimulatory electrodes, the response of necrobiotic muscle may be quite complex. Although stimulation may accelerate post mortem metabolism, muscle with an already accelerated rate of metabolism may lose its excitability at a faster than normal rate. Thus, animals with intrinsically fast glycolytic rates may be detected by their reduced electrical excitability.

Unless special precautions are taken to the contrary, muscle strips contain severed intramuscular nerves and neuromuscular junctions among the muscle fibers. Immediately post mortem, all three components may be excitable with their own particular activation thresholds and, as these change post mortem, it is difficult to identify the point between the axon and the muscle fiber that responds first to ES.

The complete final common pathway from the spinal cord to the muscles survives for many minutes in pork carcass and probably longer out in the carcass.

The excitability of muscle strips decreases progressively post mortem so that either a higher voltage and/or a longer duration stimulus is needed to obtain a constant response, and it is likely that the initial loss of excitability is caused by fatigue in the excitation-contraction pathway. If neuromuscular junctions are pharmacologically blocked in samples taken shortly after animal exsanguination, excitability is decreased. This suggests that the high excitability of muscle strips at this time is caused by intramuscular motor axons and/or their neuromuscular junctions.

ES and Rate of Stimulation

Living muscles or strips taken immediately after animal exsanguination respond to a progressive increase in stimulus frequency by twitching at a correspondingly faster rate, until the twitches merge into a sustained contraction or tetanus, as shown below.

As the time between animal exsanguination and muscle stimulation is increased, muscle strips become progressively less able to maintain tetanus, as shown below.

Immediately after the excitation of axons and muscle fibers, there follows an absolute refractory period of complete inexcitability to a second stimulus since the response to the first stimulus is still in progress. Next comes a relative refractory period when, if the second stimulus is of sufficient magnitude, excitation may be elicited. The duration of the relative refractory period probably increases progressively post mortem. Many of the stimuli delivered at a high frequency may, therefore, arrive during a relative refractory period and elicit no response. For this reason, the stimulus frequency for ES of carcasses is usually kept low.

As shown in this example,

impulses arriving at 5 impulses per second produced a stronger effect (line goes up, indicating muscle contracted) than impulses arriving at 10 impulses per second.

The pH decline in meat post mortem may render meat less excitable to ES via the nervous system since a low pH reduces the amount of acetylcholine released at the neuromuscular junction. The accumulation of calcium ions by transverse tubules might also be involved in the decreased excitability of muscles after ES.

ES and Red & White Muscle

White muscles have a greater response to ES than red muscles, because anaerobic fibers respond more readily.

The higher rate of glycogenolsysis in fast-twitch muscles is caused by a high content of phosphorylase, greater activation of phosphorylase and a higher content of creatine phosphate.

The effects of ES on meat are not limited to the actual period of stimulation, but persist afterwards, perhaps because of changes in the sarcoplasmic reticulum. Electrical stimulation causes swelling of the sarcoplasmic reticulum, transverse tubules and mitochondria, together with autolytic ultrastructural changes. Increased binding of glycolytic enzymes to actin filaments also may be involved.

Red and white muscles also differ in the way in which temperature affects the activity of the sarcoplasmic reticulum.

ES and Current Pathway

Current pathway is difficult to assess in whole carcasses. In homogeneous conductors, resistance is proprtional to the distance between electrodes, and is inversely proportional to conductor cross sectional area. In homogenous conductors, resistivity (resistance specific to conductor material) normally is measured between opposite faces of a one centimeter cube. However, not only are carcasses interrupted by tracts of fat with a high resistance and by bones with a variable resistance, but muscles themselves are electrically anisotropic. Resistivity is inversely proportional to meat temperature, it tends to be greater across rather than along muscle fibers, and it may show a transient increase post mortem followed by a progressive decline.

Resistance of a whole carcass is modified by factors such as,

the time lapse between animal exsanguination and muscle stimulation,
the distance between electrodes,
electrode surface area in contact with meat or connective tissue and not blocked by fat,
whether or not the carcass is whole, eviscerated or split, and
electrode location.

Continuous DC currents only elicit strong stimulation when they are first applied, but interrupted DC currents of square wave impulses may be used to prolong this initial response. However, DC currents of any type soon cause Polarization at the electrode-tissue interface. Polarization increases resistance and decreases responses. A muscle that has almost ceased responding to unidirectional square waves may respond with original intensity if polarity is reversed, as shown below.

. To avoid polarization, square waves of alternating polarity may be used but apparatus to produce these at high voltage may be expensive. Thus, there are some advantages to using apparatus which supplies modified sine wave impulses derived from the regular 50 or 60 Hz commercial power supply.