The mechanisms involved in accelerated discoloration of frozen tuna meat after thawing were investigated. Bluefin tuna myoglobin (Mb) was dissolved in 50 mM phosphate buffer (pH 5.5-7.4) with or without 0.2 M NaCl and quickly frozen in dry ice-acetone, followed by a 2-h storage at -80 °C and subsequent thawing at 20 °C. The “frozen/thawed” Mb solution thus prepared, along with unfrozen (intact) Mb, was examined for the free energy for unfolding, helical content, and autoxidation rate. In 50 mM phosphate buffers, the free energy for unfolding (AGD) was generally lower in frozen/thawed Mb than in unfrozen Mb, with the highest value (7.3 kcal/mol) at pH 6.5. Similar tendency was observed in 50 mM phosphate buffers containing 0.2 M NaCl, although ΔGD H2° values were generally lower. Helical contents of frozen/thawed Mb were practically the same as those of unfrozen Mb, regardless of pH. Frozen/thawed Mb showed a higher autoxidation rate than unfrozen Mb. It was concluded that, during freezing and thawing, Mb suffered some conformational changes in the nonhelical region, resulting in a higher susceptibility to both unfolding and autoxidation.
ASJC Scopus subject areas
- Agricultural and Biological Sciences(all)