Quantitative measurement of metmyoglobin in tuna flesh via electron paramagnetic resonance

The potential of electron paramagnetic resonance (EPR) spectroscopy in the quantitative determination of metmyoglobin (metMb) in tuna flesh was examined and compared with conventional visible spectrophotometry (VIS). Both fresh and stored tuna samples were directly subjected to EPR measurement at −150 °C without pigment extraction, and their metMb concentrations ([metMbTUNA]EPR) were determined from a calibration curve. A linear calibration curve with good correlation (R2 = 0.987) was obtained by a plot between EPR intensities and the known [metMbCALI]VIS concentrations, where [metMbCALI]VIS is the concentration of metMb obtained from visible spectrophotometry for the stock metMb solution. The results show that differences between [metMbTUNA]EPR and [metMbTUNA]VIS for tuna meats are negligible at low concentrations of metMb. However, [metMbTUNA]EPR tends to be higher than [metMbTUNA]VIS at a higher concentration of metMb. This is probably due to incomplete pigment extraction from tuna samples that have been stored for a long period of time. This results in experimental inaccuracy of [metMbTUNA]VIS at higher concentrations of metMb. The overall results suggest that the EPR method is a suitable technique for quantitative measurement of metMb in tuna meat without pigment extraction. Since the EPR method operates at −150 °C, this technique could also be very useful in determining the metMb content in frozen tuna meat during low-temperature storage without thawing.