Electron Paramagnetic Resonance Study of Metmyoglobin and Nonheme Iron Formation in Frozen Tuna Meats

Two types of bigeye tuna meats, fatty muscle (chu-toro) and lean muscle (akami), were stored at-5,-10, and-15° and the effects of temperature and fat content on metmyoglobin (metMb) and nonheme iron (Fe) formation were determined using Electron Paramagnetic Resonance (EPR). For the sample meats, two high spin iron peaks on the EPR spectra were detected. The first peak appeared in the range of 90-130 mT and was identified as a signal of high-spin metMb, while the second peak, which appeared in the range of 140-170 mT, was determined to be a signal of highspin nonheme iron. Peak intensity and half-width were used to determine metMb and nonheme iron content. The rates of metMb and nonheme iron formation in chu-toro and akami were slower as the storage temperature was lower. Chu-toro exhibited a quick initial increase in metMb, however, the metMb content turned decreasing after reaching a maximum, while nonheme iron increased monotonically. Meanwhile, for akami, the metMb exhibited a slower initial increase before reaching a maximum value, and remained constant during the storage period investigated, while the nonheme iron exhibited a small increase. These results suggest that once metMb is formed during frozen storage, iron ion (Fe), i.e., nonheme iron, is subsequently released from the porphyrin rings of a proportion of the metMb. That is, a type of compensatory reaction is occurring. Therefore, metMb content during frozen storage would exhibit time course curves with peak values. Notably, in case of high fat tuna meat, this phenomenon might be of significance. © 2010, Japan Society for Food Engineering. All rights reserved.