When muscles of freshly slaughtered animals are electrically stimulated, they contract. There is a concomitant increase in biochemical reactions in the muscle cells leading to an accumulation of lactate and resulting in an immediate drop in the muscle pH (zlpH). After stimulation, the rate of pH fall (dpH/dt) as the muscles go into rigor is increased (Fig. 5) (40). If a muscle is free, it will contract during stimulation. When stimulation ceases, the muscle will partially relax but unless the muscle is subjected to some force, either restrained or tensioned, it will remain in a shortened state. Stimulation of suspended bodies or sides with a balanced arrangement of opposing muscle systems ensures that there is sufficient physical force to pull most muscles back to rest length. If muscles have been freed from their natural attachments soon after stimulation, however, as occurs with hot boning, there may be no restraining force to return stimulated muscles to their natural length. The same situation applies to stimulation of isolated muscles. Electrical stimulation of muscle influences the binding of many enzymes associated with the glycolytic pathway, and this is exhibited in the accelerated rate of pH fall and of ATP disappearance and the extremely rapid loss of creatine phosphate (41) after stimulation.
Although there are various theories as to the mechanism of how electrical stimulation promotes meat tenderness, most meat scientists tend to favor the view that stimulation accelerates glycolysis so that the time when muscles will contract on exposure to cold is reduced, thereby avoiding cold shortening.
There is also strong support for stimulation increasing meat tenderness by accelerating postmortem aging, because rigor is being achieved while temperatures are still high. It has been claimed (42) that much of the effect of electrical stimulation, under conditions where cold shortening is unlikely, results from mechanical damage and is not due to rapid aging at high temperatures. Interestingly, it has been found (43) that supercontracture bands occur in both stimulated and nonstimulated muscles.
Cold shortening occurs when muscles of freshly slaughtered beef and sheep are subjected to temperatures below 8°C while there is sufficient energy supply for contraction to occur. Cold shortening will occur in pigs and poultry, but the critical temperatures are lower. In normal beef and sheep muscles, this equates to a tissue pH of about 6.0. A rule of thumb in prevention of cold shortening is to maintain temperature above 10°C until muscle pH falls below 6.0 (44). For a discussion on cold shortening, see the article Meat science. Electrical stimulation, therefore, can have an immediate benefit under commercial conditions by causing the pH to fall and thereby shortening the time before carcasses can be chilled without causing toughness. Although many meat processors use electrical stimulation to reduce the time when muscles are susceptible to cold
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