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Grass fed or DFD?
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 ten grain-fed and ten grass-fed beef cattle were collected 24 hr 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.
Keywords: Dark cutting, grass fed, metabolic profile