Lamb pH and meat quality: studies on biochemical changes in high pH meat associated with pre-slaughter farmyard stress : A dissertation submitted in partial fulfilment of the requirements for the Degree of Master of Science in Food Innovation at Lincoln University

Abstract

Producing meat of the highest quality is of the utmost importance to the red meat industry. This study looked at how energy metabolic changes and protein profiles in sheep were affected by stress before post-mortem, this gave critical insights into lamb meat quality. A study of biochemical shifts related to high-pH lamb meat with pre-slaughter stress was conducted. This study was conducted using a sample of 20, six-month-old ram lambs of the Coopworth breed (two individuals were of mixed Coopworth/Hampshire breeds) that were grazed on pasture until culling. Two treatments of control and stress were applied. The control treatment was minimal stress applied before slaughter, sheep were brought straight through the yards to the slaughterhouse. In the stress treatment, stressful conditions were applied via the use of heading dogs moving the sheep around in the yards for 10 minutes, at 30-minute intervals for 3 hours before slaughter. This caused an elevated metabolic rate in these individuals pre-slaughter. Animals were harvested immediately and tissue samples were snap frozen at -80 °C. Energy metabolic changes of the two intermediate twitch muscles Gracilis (G) and Semimembranosus (SM) were studied through the use of a D-Glucose and L-Lactic acid assay. A non- significant difference was seen in the muscles G and SM for their total glycogen levels between the stress and control treatment. In the lactic acid assay, a non-significant difference was seen in the total lactic acid levels between the two muscle fibres between the stress and control treatment. For the protein profile analysis, it was decided to study four muscles, the previous two mentioned, as well as the fast-twitch muscle Longissimus lumborum (LL) and the slow-twitch muscle Supraspinatus (SS). The protein profiles of these muscles were studied through the use of a Bradford protein determination assay and gel electrophoresis imaging. The Bradford assay revealed that there was no relationship between the total soluble protein concentrations and the stress treatment in all four of the muscle fibres. The gel electrophoresis images when analysed showed a significant difference in the relative frequency of two protein bands at around 100 and 16 kDa in the two muscles SM (P-values 0.05 and 0.01) and LL (P-values 0.03 and 0.05). These results show the potential effect of stress on protein profiles, however further protein sequencing is needed before further conclusions can be made. An analysis of post-mortem pH values was also conducted from measurements after 90 minutes and 24 hours post-mortem (ultimate pH). A non-significant difference was seen after 90 minutes but, a significant difference was seen after 24 hours between all four muscle fibres and stress. Stressed carcasses had significantly higher pH values. Notably the slow twitch muscle SS exhibited higher ultimate pH values while the fast twitch LL muscle exhibited lower ultimate pH values. This indicated a fibre-type-specific response to the stress treatment in the muscle samples studied. Overall this study provided valuable insights into biochemical shifts occurring in sheep muscles under stress conditions. Further understanding of these processes is vital for the meat industry especially for the correct grading of meat before export/ sale. Future studies should look at sequencing the specific proteins that affect the stress treatment to see if they are related to meat quality parameters such as colouring, tenderness and the water-holding capacity (WHC) of meat.