Introduction

Pale, soft and exudative (PSE) meat is caused by the denaturation of muscle proteins that takes place when a carcass experiences a low pH and a high temperature. Such conditions can occur, especially in the deep musculature, if the carcass of any meat species is cooled too slowely, with the result that, although the pH fall at a near-normal rate, the temperature remains high for a long time. The more commonly discussesed cause occures when the chilling regime is normal but post-mortem glycolysis is excessively rapid, so that the carcass reaches pH values near the final pH while still warm. This phenomenon occurs principally in pigs among meat animals because in this species post-mortemglycolysis is normally fast and this rate is further enhanced if the animal is stressed shortly before slaughter. In genetically stress-susceptible animals, the stress accompanying slaughter, even with careful handling, is sufficient to trigger a high rate of post-mortem glycolysis. In genetically more resistant animals, this is triggered only if the animals are badly handled. Since glycolysis generates heat, the temperature in a PSE carcass soon after slaughter is higher than before and this will exacerbate the protein denaturation. Another cause of the denaturating conditions responsible for PSE meat occurs if the extent, rater than the rate, of glycolysis is abnormally large. In this case the ultimate pH is low and the muscle proteins are exposed to pHs lower than they normally experience post mortem.

There is good evidence that myoglobin is particulary susceptible to the denaturation occuring in PSE carcasses. The so called salt-solubility of the myofibrillar protein and the myofibrillar ATPase activity are substantially reduced in PSE meat.The amount of myosin denaturation measured in this way increases with the severity of the PSE state as defined by the rate of glycolysis (alternative link).

Further evidence of myosin denaturation in the PSE state has been obtained by using differential scanning colorimetry. In the thermogram of myofibrils prepared from PSE muscle, the area of the first major peak, which is attributed to myosin, decreased to about a half. This suggest that about a half of the myosin had already undergone that denaturation event in the carcass. In addition to this, the thermogram of impact PSE muscle is sufficient altered from that of normal muscle that the first peak due to myosin can not be distinguished from the second peak (due to sarcoplasmic proteins), although when the amount of myosin denatured is only 20%.

There is growing understanding of how myosin denaturation causes the high rate and extent of drip loss characteristic of the PSE state. Myofibrils from PSE meat hold less water than normal and it has been shown that in PSE pork the filament lattice has shrunk substantially more than in normal meat. The filament lattice spacing in PSE meat has been found to be substantially smaller than in DFD meat. Such extra shrinkage would cause more fluid to be expelled between fibres and between fibre bundles causing an increased rate and extent of drip formation. There has been recent progress in understanding what causes the additional filament lattice shrinkage in PSE meat. The heads of the native myosin molecule are 19 nm long, but when myosin is heated under conditions resembling that experienced in a PSE carcass, the heads shrink to 17 nm.This small head shrinkage would be sufficient to draw the thick and thin filaments together more closely at rigor than is normal, so more water is expelled. sarcoplasmic proteins are also denatured in PSE carcasses. While the denaturation of these proteins might contribute to the highlight scattering of PSE meat, it is difficult to see how it could be responsible for either the softness or the enhanced drip of PSE meat, and we consider the denaturation of myosin to be the decisive event in determining these two quality characteristics.

The kinetics of the denaturation of myosin heads has been studied mainly by examination of the inactivity of the myosin ATPase. We have assumed that the loss of ATPase and head shrinkage involve the same event. The inactivation of the ATPase obeys first order kinetics and the dependence of the rate of inactivation on pH and temperature in known. A fall of 1 pH unit causes the rate of inactivation to increase 20-fold and a rise of 10oC increases the rate by about 12-fold. ATP reduces the rate of inactivation to about 9% of there values and its presence in pre-rigor muscle would slow down the rate of inactivation accordingly. Actin confers an even higher degree of stabilisation; on combination with actin the rate of inactivation falls to less than 1% of myosin alone. Hence when a carcass enters the rigor state, although the protection by ATP is lost, this is more than compensated for by the combination of the myosin heads with actin, and hence it is the conditions that the musculature experiences in the pre-rigor period that principally determine the extent to which it becomes PSE.

Although the importance of pH and temperature (Q10-rule) conditions to the severity of the PSE state has been much discussed, insufficient emphasis is often given to the importance of the time for which the carcass experiences these adverse conditions.

This, in a carcass experiencing a realtively moderate pH and temperature for a long time, the amount of denaturation might be greater than in one experiencing severer conditions but for a brief time.

In this course we have used the known dependence on pH and temperature of the rate of denaturation of myosin in vitro to predict in both beef and pig carcasses the time-course of myosin denaturation in the pre-rigor period. We have also considered how the fraction of myosin denaturated at rigor might be expected to depend on the rate and extent of glycolysis, and on the chilling regime.


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