Integrative proteomics and metabolomics profiling to understand the biochemical basis of beef muscle darkening at a slightly elevated pH.

ABSTRACT

Previous studies investigated the biochemical basis of dark-cutting conditions at elevated muscle pH (above 6), but the molecular basis at slightly above normal pH (between 5.6 and 5.8) is still unclear. The objective was to determine protein and metabolite profiles to elucidate postmortem muscle darkening at slightly elevated pH. Loins were selected based on the criteria established in our laboratory before sample collections, such as pH less than 5.8, L* values (muscle lightness) less than 38, and not discounted by the grader (high-pH beef with dark color are discounted and not sold in retail stores). Six bright red loins (longissimus lumborum) at normal-pH (average pH = 5.57) and six dark-colored strip loins at slightly elevated pH (average pH = 5.70) from A maturity carcasses were obtained within 72-h postmortem from a commercial beef purveyor. Surface color, oxygen consumption, metmyoglobin reducing activity, protein, and metabolite profiles were determined on normal-pH and dark-colored steaks at slightly elevated pH. Enzymes related to glycogen metabolism and glycolytic pathways were more differently abundant than metabolites associated with these pathways. The results indicated that oxygen consumption and metmyoglobin reducing activity were greater (P < 0.05) in darker steaks than normal-pH steaks. Enzymes involved with glycogen catabolic pathways and glycogen storage disease showed lower abundance in dark beef. The tricarboxylic acid metabolite, aconitic acid, was overabundant in darker-colored beef than normal-pH beef, but glucose derivative metabolites were less abundant. The majority of glycogenolytic proteins and metabolites reported as overabundant in the previous dark-cutting studies at high pH (>6.4) also did not show significant differences in the current study. Therefore, our data suggest enzymes involved in glycogen metabolism, in part, create a threshold for muscle darkening than metabolites.

A bright cherry-red color beef is ideal during meat retail and carcass grading. Any deviation from a bright red color, such as dark red color, at the interface of the 12th and 13th rib-eye area leads to carcass discounts. Various studies have determined protein, metabolite, and mitochondrial profiles to understand the biochemical basis of dark-cutting beef (muscle pH greater than 6); however, limited knowledge is currently available on muscle darkening at a slightly elevated pH. Bright red loins at normal muscle pH and darker color loins at slightly elevated pH (not discounted by a grader) were collected 72-h postmortem from a commercial beef purveyor. Surface color, oxygen consumption, metmyoglobin reducing activity, protein, and metabolite profiles were determined on normal-pH and dark-colored steaks at slightly elevated pH. The results indicated that oxygen consumption and metmyoglobin reducing activity were greater in darker steaks than normal-pH steaks. Furthermore, the protein abundance profiles of enzymes related to glycogen metabolism and glycolytic pathways were more differently abundant than metabolites associated with these pathways. Understanding the factors involved in the occurrence of dark color steaks help to minimize losses due to discount carcasses.