Energy metabolism plays a critical role in determining meat quality attributes. Deviations from normal pH and temperature declines generally result in meat quality defects. In our previous work, fat-like, rancid, heated oil, chemical, and musty/earthy/hummus flavors increased, whereas metallic, sour, and salty flavors decreased as ultimate pH increased (Grayson et al., 2016). In that study, the greatest incidence of tough longissimus steaks occurred in pH classes that were higher than normal, but not high enough to be removed from commodity trade. In a companion study, increased ultimate pH resulted in very dark lean color and less stable lean color compared to muscles with normal ultimate pH (McKeith et al., 2016).
Moreover, relationships that existed between biochemical traits and phenotypes for flavor attributes, tenderness, and color traits were very different in carcasses with normal muscle pH than the relationships that existed in carcasses with high muscle pH. (King et al., unpublished data). By design a large range of intermediate muscle pH values were not represented in these studies. Thus, the proposed experiment was conducted to establish these relationships between metabolic traits and palatability and consumer appeal in carcasses representing the range of pH values typically seen in the U.S. beef trade.
Metabolic factors influencing ultimate muscle pH are not well understood. Extensive variation in ultimate muscle pH occurs even in cattle that have been managed and harvested together. We have documented increased mitochondrial abundance coupled with inefficient mitochondrial function in carcasses classified as dark cutters (McKeith et al., 2016).
Muscles with greater abundance of mitochondria utilize aerobic metabolism for energy production to a greater extent than muscles with fewer mitochondria. Increased abundance of glycolytic enzymes and intermediates have been associated with improved tenderness (Carlson et al., 2017a, King et al., 2019, Santos et al., 2016) and color stability (Canto et al., 2015, Joseph et al., 2012). We (King et al., 2019) have suggested that glucose, glucose-6-phoshate, and malic acid could be used as a “metabolic fingerprint” to explain differences in beef longissimus tenderness.
The objectives of this study were: 1) determine the nature of relationships between metabolic factors and palatability and consumer appeal across the range of pH represented in the U.S. beef trade, 2) evaluate the contribution of mitochondrial abundance and mitochondrial efficiency to muscle pH and variation in beef palatability and color traits in carcasses with intermediate ultimate pH values, and 3) validate metabolic biomarkers for beef palatability and consumer appeal on an independent population and provide insight into the mechanisms by which they influence beef flavor, tenderness, and color.
Carcasses were selected at grading from a large commercial beef processing facility to have longissimus muscle pH of >5.6, 5.6 to 5.74, 5.75 to 5.9, and greater than 5.9. The strip loin and top sirloin butt were retrieved from the left side of each selected carcass and transported to the US Meat Animal Research Center. A small sample was obtained from each muscle to confirm muscle pH and the subprimals were vacuum packaged and aged until 14 days postmortem, before they were cut into 2.54-cm-thick steaks for further analysis.
Trained sensory panel analysis was conducted to evaluate beef flavor profiles, tenderness, and juiciness. Slice shear force was also used to assess tenderness. Simulated retail display was used to quantify initial color variation as well as resistance to discoloration during display. Muscle metabolism was evaluated by determining mitochondrial abundance, oxidation of the mitochondrial, myofibrillar, and sarcoplasmic fractions of the muscle, lipid oxidation, levels of glycogen, glucose, glucose-6-phosphate, lactic acid, and malic acid. Moreover, concentrations of perioxiredoxin-2 and heat shock protein 70 were quantified. Sarcomere length and postmortem protein degradation were determined.
As expected, muscle pH had a strong inverse relationship to the concentration of glycolytic intermediates. Muscle pH was also influenced by the abundance of mitochondria as well as the extent of oxidation resulting from defects in the electron transport chain. This is likely due to the depletion of glycogen by inefficient mitochondrial function in response to stimuli. Muscle pH had a profound effect on the meat quality attributes of tenderness, lean color, and flavor. Muscle pH had a curvilinear relationship to tenderness with steaks with intermediate pH being less tender than those with higher or lower pH values. Increased protein carbonyls resulting from inefficient mitochondrial function were associated with less tender beef. Moreover, increased levels of oxidative damage resulting from mitochondrial inefficiency was associated with darker, less red beef steaks than those with less oxidative damage.
High muscle pH was associated with increased sensory panel ratings for heated-oil, fat-like, and spoiled putrid flavor attributes. Increased levels of protein carbonyls were associated with higher ratings for beef flavor identity, brown-roasted, and umami flavor attributes. Clearly, these metabolic factors contribute to variation in tenderness, flavor, and lean color of beef longissimus lumborum and gluteus medius steaks. The relationships among the metabolic traits are different for each muscle. Moreover, the associations of these metabolic traits with meat quality attributes differ across muscles.
Despite understanding the etiology of dark-cutting beef for over 100 years, little is understood as to why a small percentage of animals in a production lot are afflicted with the condition. The present research provides insight into role of mitochondrial function determining muscle pH. Moreover, the relationships between metabolic factors and quality attributes are complex. These results provide clarity in some of these relationships, more completely explaining variation in tenderness and lean color. Moreover, these results provide evidence regarding animal-to-animal differences in beef flavor attributes. These results will allow management strategies to further manage and enhance beef quality attributes.