Myoglobin is a sarcoplasmic protein primarily responsible for meat color and can exist in three different forms namely deoxy-, oxy-, or metmyoglobin. Oxymyoglobin gives characteristic bright red color, however, oxidation of oxy- or deoxymyoglobin leads to discoloration of meat or metmyoglobin formation. Metmyoglobin reducing capacity (MRC) is an important intrinsic property that can influence color stability of meat. More specifically, increased MRC is often associated with decreased metmyoglobin formation. Metmyoglobin reduction refers to the ability of iron-heme to accept an electron. Our objective was to investigate the fundamental mechanism of oxymyoglobin dissociation and metmyoglobin reduction at different pH by electrochemical methods. Cyclic voltammetry was used to characterize the interrelationship among 3 pH conditions (pH 7.4 - physiological muscle; pH 6.4 - stress-out animal muscle, and pH 5.6 - typical postmortem muscle) on myoglobin forms inter-conversion. A 3-electrode cell system consisting of myoglobin coated high purity graphite as a working electrode, Ag/AgCl as a reference electrode, and a platinum-wire as a counter electrode were employed to study the electrochemical kinetics of myoglobin. The electrodes were immersed in nitrogen purged phosphate buffer solution in a sealed electrochemical cell. The kinetics of oxymyogobin formation and its dissociation were examined. We observed that in physiological pH condition, the oxymyoglobin was more stable, whereas, acidic pH conditions caused diminished stability for the oxymyoglobin state (order of stability of oxymyoglobin: pH 7.4 > 5.6 > 6.4). The observed heterogeneous electron transfer rate constants (ks, s-1) for the direct reduction-oxidation processes of myoglobin is greater at pH 7.4 than pH 5.6. The determined order of electron transfer rate constant is pH 7.4 > pH 6.4 > pH 5.6. We conclude that greater catalytic oxymyoglobin reduction efficiency, and slower metmyoglobin reduction rate occurs at low acidic pH in comparison to higher neutral and physiological pH conditions.

Electrochemistry of pH dependent meat color

247th National Meeting (2014)

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