STABILITY OF OXYMYOGLOBIN AND NITROSOMYOG LOBIN DURING FREEZING

The stability of myoglobin (Mb) derivatives during freezing was assess ed and compared with a model system containing oxy-Mb(MbO(2)) and nitr oso-Mb(NOMb). The met-Mb(MetMb) production ratio (%) in a discoloured commercial frozen beef (vacuum-packed) sirloin was based on reflectanc e spectra and the buffer extraction method. Sliced fresh beef (round) was vacuum packed and frozen (-25 -30 degrees C) and then examined for colour changes during 1 month of storage. The discoloured sample show ed 43% MetMb production. No discoloration of the freshly sliced rounds was noted, even when repeatedly frozen and thawed. Sirloin that had b een minced, frozen and thawed showed 56% MetMb. The model solution - a reaction mixture (pH 5.5) consisting of MetMb (1%) and ascorbic acid (0.5%) - was prepared, evacuated and oxygenated to encourage the produ ction of MbO(2) as much as possible. 0.1% NaNO2 was added to the react ion mixture to convert MetMb to NOMb. The solution was diluted 1:10 wi th the same medium (0.1M acetate buffer, pH 5.5) to assess colour chan ges and reducing ability (RA). Frozen storage was under anaerobic and aerobic conditions respectively for MbO(2) and NOMb. The initial MbO(2 ) production ration was 86% and was followed by a drop to 53% after 1 week of refreezing for 1 month. RA decreased with the duration and num ber of times of refreezing. Up to 100% NOMb was produced under aerobic conditions at 20 degrees C within 24 hours. NOMb was oxidized to some extent by freezing. NOMb remained stable after refreezing and storage for 6 months. No residual nitrite could be detected in the NOMb solut ion. It is evident from the results obtained that MbO(2) is fairly sta ble in meat cuts but unstable in a model solution as compared with NOM b. Its stability changes however under varying freezing conditions, e. g. storage period.