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Development of meat quality attributes is prominently influenced by the rate and extent of postmortem pH decline. Abnormally low ultimate pH (pH < 5.4) adversely impacts meat color, texture, water holding capacity, and protein content. Postmortem muscle acidification is usually explained by the degradation of muscle glycogen via glycogenosis and glycolysis to yield lactate and hydrogen ions, which accumulate in the muscle and cause a drop in pH. The role of mitochondria in this process is not well-established; therefore, the purpose of this study was to determine the role of mitochondria in postmortem metabolism. Muscle samples were excised from the longissimus lumborum muscle of six market weight pigs within 5 min of exsanguination. Samples were either immediately frozen in liquid nitrogen or designated for mitochondria isolation. Frozen muscle samples were homogenized into a reaction buffer that mimics postmortem glycolysis. Either 0 or 0.5 mg/ml isolated mitochondria was incorporated into the buffer with or without mitochondrial inhibitors for complexes I, IV, and V. Ali-quots were removed at 0, 30, 120,240, and 1440 min for pH and metabolite analysis. Data were analyzed with a mixed model in SAS-JMP, and changes over time were determined by repeated measures. Mitochondria lowered (P < 0.05) pH values at 240 and 1440 min regardless of inhibitors. The reduction in pH was coupled with enhanced (P < 0.05) glycogen degradation, glucose 6-phosphate formation, and lactate accumulation. These data indicate that the effect of mitochondria was through increasing the rate of glycolytic flux and not through mitochondrial electron transport chain as indicated by the lack of inhibitors effect; however, the mechanism by which mitochondria enhanced glycolytic flux is still unknown. To study this phenomenon further, mitochondrial samples were homogenized with a polytron and then centrifuged, and the resulting supernatants were transferred to another tube while pellets were resuspended again. Mitochondrial supernatants or pellets were added to the in vitro model to test which compartment was responsible for the effect. Mitochondrial supernatants produced the same effect as preparations including total mitochondria. No effect was observed with the mitochondrial pellet. To further narrow our target of investigation, mitochondrial supernatant proteins were precipitated with perchloric acid (PCA). Supernatants were centrifuged and neutralized before retesting in the in vitro system. The effect of mitochondrial supernatant was lost after PCA treatment. These data indicate that a mitochondrial-based protein is capable of increasing glycolytic flux in postmortem muscle. Additional research is needed to identify the causative agent. [ABSTRACT FROM AUTHOR]

Fresh beef quality development is dictated by the extent of postmortem pH decline. During the conversion of muscle to meat, pH drops from 7.2 to an ultimate pH near 5.6. However, some animals exhibit insufficient postmortem muscle acidification (pH > 6.0) which lead to dark, firm, and dry (DFD) or dark cutting meat. Cattle fed a grass-based diet often produce meat with a greater ultimate pH, which can be undesirable to consumers due to its darker color. Recently, our group showed that the ultimate pH of meat in pigs is controlled by muscle metabolic profile. We hypothesized that grass-fed cattle have a more oxidative metabolic profile than grain-fed cattle. To test this notion, longissimus dorsi samples from 10 grain-fed and 10 grass-fed beef cattle were collected 24 h postmortem. Muscle samples were analyzed for ultimate pH and glycolytic potential. The abundance of lactate dehydrogenase (LDH), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and succinate dehydrogenase complex subunit A (SDH-A) was determined by immune-blotting. Greater ultimate pH (P < 0.05) was observed in the muscle of grass-fed cattle compared with grain-fed cattle, while no differences in glycolytic potential were detected. As predicted, the grass-fed beef exhibited a more oxidative metabolic profile as indicated by the greater (P < 0.05) abundance of SDH-A and the lower (P < 0.05) abundance of LDH and GAPDH. To confirm these findings and to minimize sources of variability, muscle samples from both feeding strategies each were pulverized in liquid nitrogen, then incorporated at 1:10 (wt/vol) into a reaction buffer that contained all metabolites needed for glycolysis. Using this in vitro model, we used muscle tissue as the source of glycolytic enzymes and compared anaerobic glycolysis under the same environment. Aliquots were removed at 0, 30, 120 240, and 1440 min for pH and lactate analysis. Greater (P < 0.05) pH values were observed at 240 and 1440 min when grass-fed beef muscle tissue was used. The elevated pH values 240 and 1440 min were coupled with lower (P < 0.05) lactate accumulation. The longissimus dorsi of grass-fed cattle exhibits a greater ultimate pH and a more oxidative metabolic profile than grain-fed cattle. The greater ultimate pH may contribute to the darker color of grass-fed beef. [ABSTRACT FROM AUTHOR]

Regulation of mammary protein synthesis potentially changes the relationships between AA supply and milk protein output represented in current nutrient requirement models. Glucose and AA regulate muscle protein synthesis via cellular signaling pathways involving mammalian target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK). The objective of this study was to investigate the effects of essential AA (EAA) and acetate or glucose on mTOR and AMPK signaling pathways and milk protein synthesis rates. A bovine mammary epithelial cell line, MAC-T, was subjected to different media containing 0 or 3.5 mmol/L EAA concentrations with 0 or 5 mmol/L acetate or 0 or 17.5 mmol/L glucose in 2 separate 2 x 2 factorial studies. In a separate set of experiments, lactogenic bovine mammary tissue slices were subjected to the same treatments except that the low EAA treatment contained a low level of EAA (0.18 mmol/L). Supplementation of EAA enhanced phosphorylation of mTOR (Ser2448) and eukaryotic initiation factor 4E binding protein 1 (4EBP1, Thr37/46), and reduced phosphorylation of eukaryotic elongation factor 2 (eEF2, Thr56) in MAC-T cells. Concentration of ATP and phosphorylation of AMPK increased and decreased, respectively, in the presence of EAA in MAC-T cells. Acetate, EAA, or glucose numerically reduced AMPK phosphorylation by about 16% in mammary tissue slices. Provision of EAA increased phosphorylation of mTOR and 4EBP1, intracellular total EAA concentration, and casein synthesis rates in mammary tissue slices, irrespective of the presence of acetate or glucose in the medium. Phosphorylation of mTOR had a marginally negative association with AMPK phosphorylation, which was positively related to eEF2 phosphorylation. Casein synthesis rates were positively and more strongly linked to mTOR phosphorylation than the negative link between eEF2 phosphorylation and casein synthesis rates. A 100% increase in mTOR phosphorylation was associated with an increase in the casein synthesis rate of 0.74%·h-1, whereas a 100% increase in eEF2 phosphorylation was related to a decline in the casein synthesis rate of 0.33%·h-1. Although AMPK phosphorylation was responsive to cellular energy status and had a negative effect on mTOR-mediated signals in bovine mammary epithelial cells, its effect on milk protein synthesis rates appeared to be marginal compared with the mTOR-mediated regulation of milk protein synthesis by EAA. [ABSTRACT FROM AUTHOR]

Fresh turkey meat color is determined by many factors that include muscle fiber type composition and heme protein concentrations. These factors either are affected by or influence biochemical events occurring postmortem. Deviations in the processing environment also can result in aberrant fresh meat quality and may ultimately change the quality characteristics of further processed products. Our objective was to describe the underlying cause and significance of the two-toning color defect in fresh turkey breast. In the first experiment, pectoralis major muscles were collected, classified as single- or two-toned, and analyzed using image processing to characterize fresh turkey color. Samples from the large and small lobes of the pectoralis major muscle were collected for pH, glycolytic intermediates, protein abundance, mRNA expression, and quality characteristics. In the second experiment, time from stun to exsanguination was tested as a promoter of fresh turkey color. Results from the first experiment showed that the turkey breast possesses two distinct lobes. The large lobe had greater (P < 0.05) glycolytic potential, lactate content, lactate dehydrogenase (LDH) abundance, and centrifugal drip loss, while pH, myoglobin mRNA expression, and soluble protein levels were lower (P < 0.05) compared to the small lobe. Results from the second experiment showed that reducing time from stun to exsanguination enhanced (P < 0.05) fresh turkey color by mitigating the differences between the two lobes. Our results also showed that birds exsanguinated first had greater (P < 0.05) muscle pH values and body temperatures. These results show inherent differences in breast muscle and processing conditions interact to establish variations in fresh turkey color. [ABSTRACT FROM AUTHOR]

Insufficient acidification results in dark, firm, and dry beef. While this defect is often indicative of a stress event antemortem, muscle tissue may change in response to feeding regime. Longissimus dorsi muscle samples from 10 grain-fed and 10 grass-fed market weight, angus-crossbred beef cattle were collected postmortem. Lower (P < .05) L* and a* values were recorded for steaks from grass-fed cattle. Higher (P < .05) ultimate pH values were noted in lean of grass-fed cattle compared to grain-fed cattle, yet differences in lactate, glycogen and glucose were not detected. Further, increased (P < .05) ultimate pH values and lower (P < .05) lactate accumulations were noted when samples from grass-fed cattle were subjected to an in vitro glycolysis system. Muscle from grass-fed beef possessed nearly two-fold more (P < .05) succinate dehydrogenase and (P < .001) myoglobin than that of grain-fed cattle. These data show lean from grass-fed beef has greater enzymes reflective of oxidative metabolism and suggest dark lean from grass-fed cattle may be a function of more oxidative metabolism rather than a stress-related event antemortem., (Copyright © 2019. Published by Elsevier Ltd.)

Fresh hams display significant lean color variation that persists through further processing and contributes to a less desirable cured product. In an attempt to understand the underlying cause of this color disparity, we evaluated the differences in muscle characteristics and energy metabolites across semimembranosus (SM) muscles differing in color variation. The L* (lightness) and a* (redness) values were highest and lowest (P<0.001), respectfully in the most caudal aspects of the muscle while the ultimate pH was the lowest (P<0.001). Correspondingly, this region possessed highest (P<0.01) glycolytic potential (GP) and lactate dehydrogenase (LDH) levels but did not differ in the amount of myoglobin or myosin heavy chain type I isoform. These data show that differences in muscle may contribute to ham color variation but suggest other factors may mitigate or exacerbate these variances., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Postmortem energy metabolism drives hydrogen accumulation in muscle and results in a fairly constant ultimate pH. Extended glycolysis results in adverse pork quality and may be possible with greater adenonucleotide availability postmortem. We hypothesized that slowing adenonucleotide removal by reducing AMP deaminase activity would extend glycolysis and lower the ultimate pH of muscle. Longissimus muscle samples were incorporated into an in vitro system that mimics postmortem glycolysis with or without pentostatin, an AMP deaminase inhibitor. Pentostatin lowered ultimate pH and increased lactate and glucose 6-phosphate with time. Based on these results and that AMPK γ3(R200Q) mutated pigs (RN⁻) produce low ultimate pH pork, we hypothesized AMP deaminase abundance and activity would be lower in RN⁻ muscle than wild-type. RN⁻ muscle contained lower AMP deaminase abundance and activity. These data show that altering adenonucleotide availability postmortem can extend postmortem pH decline and suggest that AMP deaminase activity may, in part, contribute to the low ultimate pH observed in RN⁻ pork., (Published by Elsevier Ltd.)

Meat quality development, or the transformation of muscle to meat, involves a myriad of biochemical pathways that are largely well-studied in living muscle tissue. However, these pathways are less predictable when homeostatic ranges are violated. In addition, there is far less known about how various management or environmental stimuli impact these pathways, either by substrate load or altered cellular environment. Likewise, it is largely accepted that oxygen plays little to no role in the conversion of muscle to meat, as anaerobic metabolism predominates in the muscle tissue. Even so, the oxygen tension within the tissues does not fall precipitously at exsanguination. Therefore, transition to an anaerobic environment may impact energy metabolism postmortem. Antemortem handling, on the other hand, clearly impacts meat quality development, yet the exact mechanisms remain a mystery. In this paper, we will attempt to review those factors known to affect postmortem energy metabolism in muscle and explore those areas where additional work may be fruitful., (Copyright © 2013 The Authors. Published by Elsevier Ltd.. All rights reserved.)