Previous research reported the role of nonenzymatic metmyoglobin (MetMb) and methemoglobin (MetHb) reduction in meat color; however, limited studies have characterized the cofactors involved in nonenzymatic reduction. The objective of this study was to characterize electron donors and carriers in nonenzymatic MetMb and MetHb reduction at various temperatures and postmortem muscle pHs in vitro. Methylene blue and cytochrome c (cyt-c) were evaluated as electron carriers and nicotinamide adenine dinucleotide, reduced form (NADH) and ascorbate were considered as electron donors. All combinations of electron donors and carriers were evaluated in the following order: NADH plus methylene blue, ascorbate plus methylene blue, NADH plus cyt-c, and ascorbate plus cyt-c. Spectrophotometry was utilized to monitor the rates of reduction. The results indicated that methylene blue was an effective electron carrier than cyt-c in the presence of NADH. Temperature and pH had cofactor-specific effects on nonenzymatic MetMb and MetHb reduction. Lower temperature resulted in an increased nonenzymatic MetMb reduction for methylene blue regardless of electron donor (ascorbate, P = 0.03, NADH, P = 0.04). As pH increased, MetHb reduction was enhanced in the presence of ascorbate plus cyt-c. Nonenzymatic MetHb reduction was numerically lower than nonenzymatic MetMb reduction in the presence of NADH plus methylene blue. In summary, in addition to NADH, the current in vitro research demonstrated that ascorbate plus cyt-c could contribute to nonenzymatic MetMb and MetHb reduction at meat-pH and storage temperature.

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Theoverall goal was to evaluate the effects of repackaging nitrite-embeddeddark-cutting steaks in polyvinyl chloride (PVC) film on surface color.Dark-cutting beef strip loins (n = 8; pH = 6.39) and USDA Low Choice beef striploins (normal-pH, n = 6) were selected at a commercial packing plant.Dark-cutting loins were bisected and randomly assigned to nonenhanceddark-cutting and enhanced dark-cutting with glucono delta-lactone and rosemarytreatments. Normal-pH and nonenhanced dark-cutting steaks were vacuum packaged (VP)and served as controls, while enhanced dark-cutting steaks were packed innitrite-embedded packaging (NP). Steaks from nonenhanced normal-pH in VP, non-enhanceddark-cutting-VP, and enhanced dark-cutting-NP loins were randomly assigned to3, 6, or 9 d of dark storage. Following dark storage, steaks were repackaged inPVC and displayed for 6 d. The instrumental color, visual color, and aerobicplate count were evaluated for all steaks.  Enhanced dark-cutting steaks in nitritepackaging increased (P < 0.05) a* values and nitric oxide myoglobincontent during 24 h of dark storage. Enhanced dark-cutting steaks packaged inNP had greater a* and L* values (P < 0.05) than nonenhanced dark-cutting-VP steaks during darkstorage. Upon repackaging the enhanced dark-cutting steaks from nitritepackaging, nitric oxide myoglobin decreased (P < 0.05) during the first 12 h of display. Loss of nitric oxidemyoglobin corresponds with a darker red appearance, increased surfacediscoloration, and decreased a*values. There were no differences (P> 0.05) in aerobic plate count between enhanced dark-cutting-NP and non-enhanceddark cutting VP steaks during 9 days of dark storage. In conclusion, NPimproved surface redness; however, repackaging enhanced dark-cutting from NP steaksin PVC decreased color stability and redness of dark-cutting beef. 

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