Effects of freezing and thawing on the autoxidation of bluefin tuna myoglobin.

Myoglobin (Mb) was isolated from the dark muscle of bluefin tuna Thunnus thynnus by Sephadex G-75 gel filtration and CM-Sephadex C-50 ion-exchange chromatography. This Mb was dissolved in 50mM phosphate buffers of several pH values, and examined for the rate of autoxidation (metMb formation) under various conditions of freezing and thawing.When frozen quickly and kept at -80°C for 2h and then thawed, the Mb solution at pH 6.5 gave rise to a metMb formation rate of 5-15%, whereas the solutions at lower or higher pH values gave much higher rates of up to 70-80%. In contrast, metMb formation exceeded 80% regardless of pH values when frozen slowly at -20°C and then thawed after 20h.Thawing temperatures (0-20°C) affected the autoxidation rate of Mb to a less extent, though the higher the temperature, the slower the metMb formation.