Electrical measurement for detecting early postmortem changes in porcine muscle

Use of electrical measurements to detect quality defects in porcine muscle in the early postmortem period was evaluated. Justification for use of a tetrapolar, constant current electrode configuration instead of bipolar electrodes was provided for measurements at low frequencies. Interrelationships among electrical properties, pH values, ATP decline, temperature, time postmortem, and final water-holding capacity (WHC) of porcine muscle were quantified using 25 hogs. Immediately after exsanguination, a section of the left longissimus muscle (LM) was excised to obtain rigor shortening patterns and complex impedance measurements over a 10-h period at 37 degrees C. Complex impedance measurements were taken using a tetrapolar electrode configuration at 1 kHz and .156 mA. At 15, 45, and 90 min postmortem, pH, ATP/IMP absorbance (R), and conductivity measured by the Tecpro Pork Quality Meter (PQM) were measured on the right side LM. At 24 h postmortem, WHC, pH, R, PQM, Hunter Color Lab values, and subjective quality scores were evaluated on the left LM. The WHC measurements were used to group carcasses into normal (n = 17) and abnormal (n = 8) categories. Mean pH and R at 45 and 90 min were different (P < .05) but pH at 24 h was not different between the normal and abnormal groups. Onset and completion of rigor were more rapid in carcasses with low WHC (P < .05). The PQM values were greater (P < .05) in the abnormal group at 90 min and 24 h postmortem. Excised muscle measurements of relative impedance (Z*) and phase (theta*) showed Z* and theta* increased more rapidly within the first 15 min postmortem (P < .1) for samples with abnormal WHC. However, one PSE carcass showed an immediate rapid decrease in Z* and theta*. Results suggest measurement of rate of change of impedance and phase angle before 90 min postmortem would be a better prediction of ultimate quality than absolute magnitude of impedance.