The effect of farmyard stress on meat quality: A model for predictive biomarkers of meat pH

Abstract

Background/Objectivs: High pH meat is frequently associated with poor shelf-life and eating quality, and pH is a commonly used marker for meat quality. Early detection of high pH lamb is desirable and can improve meat quality control. Due to the variability in the occurrence of high pH at meat processing plants in New Zealand, there is a need for reproducible models that allow the study of abnormal pH in meat. We devised a pilot trial using standardised farmyard stress to produce high pH meat. Method: This experiment compared lambs exposed to some of the normal pre-slaughter stress factors within a New Zealand pastoral system with a control group with minimal pre-slaughter stress. It used two groups of 10 lambs, held in the pasture paddock next to the abattoir. The 10 “low stress” animals were kept quietly in pens prior to slaughter while the second group of 10 animals were exercised by moving with a dog for 5-10 minutes every hour for 4 hours prior to slaughter (“moderate exercise stress”). Rapid metabolomics (rapid evaporative ionisation mass spectrometry, REIMS) analyses were carried out on the immediate post-slaughter samples. The carcasses were aged and meat quality measurements were taken at different post mortem time points: pH (at 1.5 h, 24 h), colour (at 4 days), cooking loss and shear force (at 7/8 days). Results: The pH declined significantly over the 24 h post-slaughter in all muscles. At 1.5 h post-mortem, there was no significant difference in the average pH of muscles among non-exercised (pH 6.650, N=100) and exercised groups (pH 6.683, N=99). When the pH of two different fibre types was compared as an early sign of pH response to pre-slaughter exercise stress, the slow/oxidative muscle types had significantly higher pH than the same muscle types from non-exercised group (p=0.011). This was not seen in fast/glycolytic fibre muscles. At 24 h post-mortem muscles, the pH of the muscles from exercised group (N=99, pH 6.14) was significantly higher in comparison to the muscles from non-exercised (N=100, pH 5.85, p=0.000). Exercise stress also resulted in differences in other meat quality measurements. Colour, cooking loss and shear force of some muscles were changed after exercise stress. REIMS analysis found clear metabolite-based differences between muscles from exercised and control sheep, and between muscles. Conclusion: We have developed a model for generating high pH lamb based on farmyard pre-slaughter stress. This resulted in changes to several important meat quality markers. Samples from this trial have been used for fingerprinting mass spectrometry to identify new potential predictive markers of high ultimate pH. This simple farmyard stress model is a useful tool for research on high pH meat under controlled yet realistic conditions, and follow up validation trials has confirmed the reproducibility of this model. In future, biomarkers of high pH in the meat could be used for enhancing animal welfare and confirm the relationship between minimising pre-slaughter stress and improving the consistency of meat quality.