We investigated factors that accelerate dimethylamine formation in gadoid dark muscle. The degradation rate of trimethylamine-N-oxide into dimethylamine and formaldehyde during frozen storage was compared between ordinary muscle and dark muscle from walleye pollock, southern blue whiting, and hoki. Dimethylamine was generated faster in dark muscle than in ordinary muscle in each species, and it was produced most abundantly in hoki dark muscle compared with the other two species. We investigated the mechanism that caused dimethylamine to be generated more abundantly in dark muscle during frozen storage, and found that the amount of dark muscle nonheme iron, which catalyzes trimethylamine-N-oxide degradation, was higher than that in ordinary muscle in each species, and hoki dark muscle in particular contained the highest levels of nonheme iron among these three species. Moreover, dark muscle in all three fish species had a higher taurine content (known to accelerate dimethylamine formation) than ordinary muscle. These data suggested two candidate factors, namely nonheme iron and taurine, that may accelerate dimethylamine generation during frozen storage. In addition, gel filtration results suggested that walleye pollock dark muscle contains as yet unidentified low-molecular-weight agents that stably accelerate dimethylamine generation.

Trimethylamine-N-oxide (TMAO) degrades to dimethylamine (DMA) and formaldehyde, which cause a decline in fish quality. Suppression of DMA formation in frozen surimi was investigated using croaker, lizardfish, and walleye pollock. The leaching process in surimi processing was shown to reduce not only the TMAO, iron, and taurine content, but also to reduce unidentified factors that accelerate DMA formation in lizardfish muscle; in contrast, unidentified factors that suppress DMA formation were reduced in croaker and pollock muscle. Sucrose, used as a cryoprotectant, was shown to decrease DMA formation in vitro, likely due to the reduction in freezing-induced concentration of solutes. The effects of pH on DMA formation were different in minced frozen meat among the three species. DMA formation was not observed in croaker when the pH varied between 6 and 8. On the other hand, DMA was elevated in lizardfish under acidic conditions, and DMA formation in pollock was maximal when the pH of the meat was neutral. Thus, the suppression of TMAO degradation by surimi processing results from the removal of TMAO, iron, and reductants from fish meat; sucrose also reduces DMA formation. Furthermore, unidentified factors in croaker, lizardfish, and pollock meat substantially affect DMA formation.

:Myoglobin is known to be present exclusively in cardiac and red skeletal muscles, but not in white skeletal muscles. Thus, to date, only a few studies on myoglobin from fish species with white flesh have been reported. For comparative examination, we directly isolated myoglobin from cardiac muscle of hoki (Macruronus magellanicus), one of the most important commercial fish species with white muscle. The ferrous myoglobin was separated from its ferric met -form by anion exchange column chromatography. The absorption spectra of hoki myoglobin were similar to those of bigeye tuna skeletal myoglobin, in both oxy -and met -forms. However, hoki oxymyoglobin was found to be susceptible to autoxidation in 0.1 M buffer (pH 7.2) at 25°C, with its rate being more than 3 times higher than that of bigeye tuna oxymyoglobin.

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The suppression of trimethylamine-N-oxide decomposition to dimethylamine (DMA) and formaldehyde is required to ensure the stability of frozen minced meat from gadoid species. The effects of a mixing treatment on the stability of frozen minced meat were investigated. Minced meat samples from pollock, southern blue whiting (SBW), and hoki were mixed for 1 − 10 min with 8.3% sucrose using a silent cutter and stored at −23℃ for 6 months. The salt solubility of minced pollock and SBW, but not hoki meat, was stabilized by the 10-min mixing treatment, and DMA formation was reduced with increased mixing time, which may have been due to oxidation of the minced meat by the O 2 in the air. Minced SBW meat mixed for 10 min had similar properties to surimi even though water-and oil-holding capacities were slightly different.