A review of the relationships of pH with physical aspects of pork quality.

The pH(1)-index, defined as the percent of carcasses with pH < 6·0 at 45 min post-mortem (p-m), gives some indication of the likely incidence of pale, soft, exudative (PSE) pork at 24 h or more p-m. However, there is considerable variation between nations in the subjective definition of PSE. Thus, pH data show that subjectively-defined PSE pork from one country might be regarded as normal pork in another country, and vice versa. The paleness of pork is inversely proportional to pH. The data from conventional monochromatic reflectance photometers are almost linearly related to pH whereas the data from the Goefo meter and fibre-optic (FO) probes follow curvilinear relationships which may be logarithmic. In the visible spectrum, correlations of FO reflectance with pH and muscle structure are strongest towards 700 nm. The relationship of pH with exudate (bag-drip) is biphasic with fluid losses increasing from pH 7 to about 6·1, then staying constant at lower pHs. This biphasic pattern is also seen in some, but not all, methods in which exudate is measured by applied pressure or in which myofibrillar water-holding capacity is measured by centrifugation. Electrical impedance and its primary determinants (resistance and capacitance in parallel) are directly proportional to pH but follow curvilinear relationships. There is at least some theoretical understanding of the relationship of pH with exudate, but much remains to be learnt about the causal relationships of pH with paleness or softness. Many of the older published studies in the field are marred by a failure to separate genetic causes of low-pH (porcine stress syndrome) from environmental causes (preslaughter handling, slaughter and refrigeration).