Xix

Contraction Ca2+ pump Na + /K+ pump

Fig. 1 Diagram showing an overview of muscle metabolism. The dotted arrows are pathways that become nonfunctional in postmortem muscle. (From Ref. 2. Reprinted courtesy of Marcel Dekker, Inc.) (View this art in color at www. dekker.com.)

releases a large amount (as much as 25% by weight) of its fluid (called drip).

Cold Shortening

The typical dependence of postmortem metabolism on temperature is invalid under certain conditions. Muscles from beef and lamb have a higher rate of ATP breakdown and pH decline at 4°C than at 100C. Muscles from these species, when excised from the carcass, undergo a slow contraction called cold shortening.1-6-1 The muscles shorten by as much as 50% of their length. This shortening also reduces meat tenderness. Cold shortening can occur on the carcass as well, particularly under conditions with high efficiency and rapid cooling.

Pale, Soft, Exudative (PSE) Condition

Pigs that have the ryanodine receptor mutation[7] have an unusually rapid rate of postmortem glycolysis. The muscle pH may drop below 5.5 within the first 15 30 minutes postmortem instead of the normal 4 to 6 hours. The rapid pH decline while the muscle temperature is still high results in myosin denaturation and loss of water-binding activity. Stress and high ambient temperatures at the time of harvest increase the severity of the PSE condition.

Dark Cutter (Beef) and Dark, Firm, Dry (Pigs)

Both of these conditions occur when the muscle glycogen has been largely depleted before the animal dies. In bovine animals, this occurs quite often with bulls that have been socially regrouped.[8] The incidence is 2 5% among steers and heifers, but may approach 15% in bulls. Stress and fights lead to the glycogen depletion. With pigs, the dark, firm, dry meat results from the same ryanodine receptor mutation that causes PSE, but in the former case, the glycogen has also been depleted before harvest. In both cases, the ultimate pH is between 6.2 and 6.8. The high pH results in higher water-binding activity and a darker surface color.

INTERVENTIONS THAT ALTER POSTMORTEM METABOLISM

Extremely rapid postmortem chilling has been adopted to reduce bacterial growth and improve food safety. A modest improvement in pig meat quality can be achieved by rapid chilling, but no economically feasible cooling system has been devised to prevent the most severe PSE meat. Injection of muscle early postmortem with sodium bicarbonate can prevent the PSE condition, apparently by decreasing the rate and extent of pH decline.1-9-1

Rapid chilling may result in an undesirable decline in meat tenderness, especially in beef and lamb. An alternative method to speed postmortem glycolysis is early postmortem electrical stimulation.1-10-1 Electrical stimulation of the carcass (within the first 30 minutes after death) results in vigorous muscle contraction and rapid glycolysis. In beef carcasses, the pH may drop to around 6.3 after a couple minutes of stimulation. Unfortunately, a wide variety of stimulation voltages and stimulation equipment types has been adopted, so comparing results from different studies has been difficult. Electrical stimulation in most cases provides a modest increase in meat tenderness.[11]

CONCLUSION

The metabolic and proteolytic activities of muscle tissue do not cease at the time of death. Postmortem metabolism should be slowed in pig muscle, but accelerated in bovine

Fig. 2 Chemical and physical changes that occur in muscle postmortem. The time course corresponds to that occurring in normal pig muscle. Abbreviations: ATP adenosine triphos phate; CP creatine phosphate; LA lactic acid; Ext extensi bility. (From Ref. 2. Reprinted courtesy of Marcel Dekker, Inc.) (View this art in color at www.dekker.com.)

Fig. 2 Chemical and physical changes that occur in muscle postmortem. The time course corresponds to that occurring in normal pig muscle. Abbreviations: ATP adenosine triphos phate; CP creatine phosphate; LA lactic acid; Ext extensi bility. (From Ref. 2. Reprinted courtesy of Marcel Dekker, Inc.) (View this art in color at www.dekker.com.)

and ovine muscle for optimum meat quality. It remains a challenge to control and/or manipulate the various enzymatic activities postmortem to ensure uniform meat products with desirable eating quality.

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