Early postmortem muscle proteome and metabolome of beef longissimus thoracis muscle classified by pH at 6 hours postmortem
Graphical abstract
Introduction
Fresh beef tenderness is one of the most important purchasing motivators for consumers [1]. Consumers are willing to pay a premium for beef products with consistent, excellent eating quality [2]. However, tenderness determination is one of the most complex facets of meat quality and is influenced by a variety of factors, including the rate and extent of pH decline [[3], [4], [5]], sarcomere length [6], postmortem protein degradation [[5], [6], [7]], and the collagen content [8] of beef products.
No rapid, reliable, and non-destructive methods exist to predict beef tenderness. Utilizing rapid, non-destructive methodologies to predict the eating quality of beef is one of the forefront goals of the beef industry. Predicting beef tenderness would improve the ability to provide a premium market for beef products and provide a better eating experience for consumers. The ability to predict product that is tender earlier would allow earlier sorting into premium markets. A robust application to predict beef tenderness has not been identified and must be established through molecular profiles (metabolites and proteins). Recent meta-analyses have characterized differences in muscle protein profiles in meat that differed greatly in tenderness. Specifically, abundance and post-translational modification of some proteins involved in muscle contraction, muscle structure, energy metabolism, heat stress, and oxidative stress have potential to explain variations in meat tenderness [9,10]. Therefore, understanding how postmortem metabolism establishes the conditions that impact the rate and extent of tenderness development is essential to predicting tenderness differences in beef products.
Postmortem pH decline of beef influences beef tenderness by affecting protein solubility and protein degradation [3,5,11]. Beef with a more rapid rate of pH decline compared to a slower rate of pH decline tends to result in more tender beef at 1 d postmortem [3,5,12]. Marsh et al. [3] showed that the most tender aged (≥14 d aging) beef longissimus muscle had intermediate (6.0–6.3 pH at 3 h) pH values at 3 h postmortem. Glycolytic enzymes and other metabolic proteins have been identified as potential biomarkers of beef tenderness [9]. The rate of postmortem glycolysis through abundances of glycolytic enzymes can impact pH decline, thus impacting tenderness development [10,13]. Understanding how energy metabolism and the resulting pH decline set the stage for meat quality development is integral to predicting tenderness variations in beef.
The objective of this study was to test the hypothesis that the metabolome and the sarcoplasmic muscle proteome are related to the rate of early postmortem metabolism. To achieve this objective, muscle pH at 6 h postmortem was used as a metric to categorize the rate of metabolism. Because we wanted to have as much variation in early postmortem pH as possible, we used longissimus dorsi samples that were collected in a previous study [5]. These samples varied widely in their pH values at 6 h postmortem making them the ideal population to test the hypothesis proposed in the current study. This study provides valuable information that aids in identifying novel chemical phenotypes to inform the pursuit of biomarkers of postmortem metabolism impacting tenderness development.
Section snippets
Sample selection
Samples were collected from carcasses of cattle that were fed as previously described [5]. Samples were selected for inclusion in the current study on the basis of pH value at 6 h postmortem. Samples were classified as low pH (LpH) at 6 h or high pH (LpH) at 6 h postmortem (LpH; n = 9; pH at 6 h postmortem <5.55 and HpH; n = 8; pH at 6 h postmortem >5.84) Temperature, pH (1, 3, 6, and 24 h postmortem), and Warner-Bratzler shear force (WBSF; 1, 3, 7, and 14 d postmortem) were measured as
Results and discussion
The study objective was to categorize a subset of cattle {as described by Schulte et al. [5]} by metabolic rate and chemical phenotypes in an effort to identify biomarkers of early postmortem metabolism in these samples. Therefore, the results are aimed at identifying phenotypes of rapid metabolism.
Conclusions
Classification by pH values at 6 h postmortem resulted in significant differences in quality characteristics. The LpH classification had lower WBSF values at 1 d postmortem, which was explained by greater calpain-1 autolysis, desmin degradation, and troponin-T degradation at 1 d postmortem. The LpH classification had greater abundances of energy production enzymes and metabolites at 1 h identified in glycogenolysis, glycolysis, and the tricarboxylic acid cycle. Greater energy metabolites and
Acknowledgements
This work was supported by the Agriculture and Food Research Initiative (AFRI) grant no. 2018-67015-27546/project accession no. 1015001 from the USDA National Institute of Food and Agriculture.
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