The objective of this study was to assess the morphological integrity and functional potential of mitochondria from postmortem bovine cardiac muscle and evaluate mitochondrial interactions with myoglobin (Mb) in vitro. Electron microscopy revealed that mitochondria maintained structural integrity at 2 h postmortem; prolonged storage resulted in swelling and breakage. At 2 h, 96 h, and 60 days postmortem, the mitochondrial state III oxygen consumption rate (OCR) and respiratory control ratio decreased with time at pH 7.2 and 5.6 (p < 0.05). Mitochondria isolated at 60 days did not exhibit ADP-induced transitions from state IV to state III oxygen consumption. Tissue oxygen consumption also decreased with time postmortem (p < 0.05). Mitochondrial oxygen consumption was inhibited by decreased pH in vitro (p < 0.05). In a closed system, mitochondrial respiration resulted in decreased oxygen partial pressure (pO(2)) and enhanced conversion of oxymyoglobin (OxyMb) to deoxymyoglobin (DeoMb) or metmyoglobin (MetMb). Greater mitochondrial densities caused rapid decreases in pO(2) and favored DeoMb formation at pH 7.2 in closed systems (p < 0.05); there was no effect on MetMb formation (p > 0.05). MetMb formation was inversely proportional to mitochondrial density at pH 5.6 in closed systems. Mitochondrial respiration in open systems resulted in greater MetMb and DeoMb formation at pH 5.6 and pH 7.2, respectively, vs controls (p < 0.05). The greatest MetMb formation was observed with a mitochondrial density of 0.5 mg/mL at both pH values in open systems. Mitochondrial respiration facilitated a shift in Mb form from OxyMb to DeoMb or MetMb, and this was dependent on pH, oxygen availability, and mitochondrial density.

In two experiments, the relationship between metmyoglobin (MMb) reduction and lactate to pyruvate conversion with concomitant production of reduced nicotinamide adenine dinucleotide (NADH) via lactic dehydrogenase (LDH) was investigated. In experiment 1, nonenzymatic reduction of horse MMb occurred in a lactate-LDH-NAD system. Exclusion of NAD+, L-lactic acid, or LDH resulted in minimal MMb reduction. Increasing NAD+ and L-lactic acid concentrations increased reduction. In experiment 2, beef strip loins (longissimus lumborum muscle) were injected with combinations of potassium lactate, sodium tripolyphosphate, sodium chloride, and/or sodium acetate. Steaks were packaged in high-oxygen (80% oxygen/20% carbon dioxide) modified-atmosphere packaging and stored for 2-9 days and then placed in a fluorescent-lighted, open-top display case for 5 days at 1 degrees C. Enhancing loins with 2.5% potassium lactate significantly increased LDH activity, NADH concentration, MMb-reducing activity, and subsequent color stability during display. These research results support the hypothesis that enhancing beef with lactate replenishes NADH via increased LDH activity, ultimately resulting in greater meat color stability.

No abstract

No abstract