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
As well as protecting against cold shortening, electrical stimulation may
However, cooking losses may be increased by ES.
color and appearance, and
subjective scores for youthfulness.
Apart from guarding against cold shortening, the beneficial effects of
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
probe or pin types
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
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
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
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.
White muscles have a greater response to ES than red muscles, because
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,
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.
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
. 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.