Quality Changes And Underlying Mechanisms Of Different Ultimate PH Beef During Ageing

Among the different qualities of beef, tenderness and consistency have been identified as one of the most important characteristics that determine the consumer eating satisfaction of beef. Variability in the meat tenderness is the most critical quality problem facing the beef industry. In this frame, predicting meat tenderness has been identified as one of the major goals in the ambitious agenda of the beef industry, though more than sixty years of research in the field have provided both encouraging and contradictory results.These difficulties mainly stem from the biological complexity underpinning the tender meat phenotype, not only at the genomic, transcriptomic or proteomic level, but also in relation to exogenous factors such as breeding conditions(rearing environment, feeding, physical activity and pre-slaughter stress) and post mortem handling(temperature, p H, storage duration, hanging method, delays in transfer to cold tunnel, electrical stimulation).As we know, p H has an important impact on beef water holding capacity, color and tenderness at post moterm. Thus, p Hu has been widely used as an indicator of potential meat quality. Additionally, the rate of tenderization is related with p Hu. The inconsistencies in meat quality due to variation in p Hu will continue to pose a problem to meat processors until the exact relationship between all the factors is understood.At present, little information is available concerning the effects of p Hu on Chinese beef cattle quality. In the present study we performed proteomics analysis of Longissimus dorsi different p Hu meat samples from China beef cattle in order to understand whether the p Hu can affect beef quality during ageing period.In this study, the main results are as follows:1. Distribution of beef p Hu in ChinaThe distribution frequency of p Hu ≤5.80 beef in China reached 62%, the distribution frequency of 5.80< p Hu ≤6.10 and p Hu > 6.10 respectively were 21% and 17%. As highlighted earlier, the normal beef p Hu should be below 5.8. The results showed that about 38% beef in China had higher p Hu, which may have a negative impact on beef quality.2. Quality changes of different p Hu beef during postmortem agingAccording to ultimate p H(p Hu), analysed samples were segregated into three p Hu groups named high(p H > 6.1), intermediate(5.8 < p Hu ≤ 6.10) and low(p H < 5.8) p Hu beef groups and aged up to 9 days post mortem at 4°C. There are significan differences in L*, drip loss, cooking loss and MFI(P<0.05) due to meat p H were observed until after 9 d post mortem. High p Hu samples have the lowest WBSF at 1 day post mortem and significantly more tender than low and intermediate p Hu meat at all ageing timepoints(P <0.05). No significan difference between low and intermediate p Hu beef were observed until after 5 d post mortem. From then on, intermediate p Hu beef had the slowest tenderization rate than others.3. Protein degradation of different p Hu beef during postmortem agingBy SDS-PAGE, we found major myofibrillar protein(α-actin,desmin,actin,troponin-T,myosin light chain1,troponin-I,troponin-C,myosin light chain2) degradation were the least extensive in intermediate p Hu beef among all groups during postmortem aging.By Western Blot, desmin and troponin-T degradation was also less extensive in intermediate p Hu samples compared to the other two p H categories.4. μ-calpain degradation of different p Hu beef during postmortem agingBy Western Blot, it was found that rapid μ-calpain autolysis in high p Hu meat was concurrent with the faster degradation of myofibrillar protein, desmin and troponin-T in high p Hu group. Autolysis of μ-calpain in the high p Hu samples had already occurred at 30 min postmortemas indicated by the presence of the 78 k Da subunit. μ-calpain progressively autolysed with time such that intact μ-calpain was completely degraded to 76 k Da at 9 days post mortem. μ-calpain autolysis in low p Hu meat was initially detected at 1 day post mortem and by 9 days, only the 76 k Da subunit was detected. In comparison, μ-calpain autolysis in intermediate p Hu meat was more delayed than other p Hu groups such that the 78 k Da subunit was still detectable at 9 days ageing.So it may be hypothesised that meat tenderness is p Hu compartmentalised by the variable degradation rates of myofibrillar proteins, which are in turn regulated by endogenous proteases.5. Proteomic analysis of different p Hu beefTo identify proteins contributing to meat quality, a proteomic profiling of cattle longissimus dorsi muscles was conducted by i TRAQ in high, intermediate and low p Hu groups based on 24 h post-mortem p H. The results of proteomics showed a total of 927 proteins were identified. When the value of differential expression fold was above 1.5 and the value of P was below 0.05, 61 differentially expressed proteins were identified between the high/low p Hu group beef and the intermediate/low p Hu group beef. The differentially expressed proteins were classified metabolic enzymes, cellular defense/stress proteins, and structural proteins according to their predicted function. The majority of the metabolic enzymes(creatine kinase M chain, glycerol-3-phosphate dehydrogenase, L-lactate dehydrogenase A chain, Triosephosphate isomerase, phosphoglycerate kinase, phosphoglucomutase-1 isoform X1) identified were involved in the energy metabolism of the cell and affected meat quality changes. The cellular defense/stress proteins(heat shock protein beta-1 isoform X1,peroxiredoxin-6) can be related to regulation and stabilization of the myofibrillar proteins. Voltage-dependent anion-selective channel protein 1 isoform X4 affected meat tenderization by apoptosis pathway.