Stabilität von Oxymyoglobin und Nitrosomyoglobin beim Gefriervorgang

The stability of myoglobin (Mb) derivatives during freezing was assessed and compared with a model system containing oxy-Mb(MbO₂) and nitroso-Mb(NOMb). The met-Mb(MetMb) production ratio (%) in a discoloured commercial frozen beef (vacuum-packed) sirloin was based on reflectance spectra and the buffer extraction method. Sliced fresh beef (round) was vacuum packed and frozen (-25 -30 °C) and then examined for colour changes during 1 month of storage. The discoloured sample showed 43 % MetMb production. No discoloration of the freshly sliced rounds was noted, even when repeatedly frozen and thawed. Sirloin that had been 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 production of MbO₂ as much as possible. 0.1 % NaNO, was added to the reaction mixture to convert MetMb to NOMb. The solution was diluted 1:10 with the same medium (0.1M acetate buffer, pH 5.5) to assess colour changes and reducing ability (RA). Frozen storage was under anaerobic and aerobic conditions respectively for MbO₂ and NOMb. The initial MbO₂ production ratio was 86 % and was followed by a drop to 53 % after 1 week of freezing and then to 32% after refreezing for 1 month. RA decreased with the duration and number of times of refreezing. Up to 100% NOMb was produced under aerobic conditions at 20 °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 solution. It is evident from the results obtained that MbO, is fairly stable in meat cuts but unstable in a model solution as compared with NOMb. Its stability changes however under varying freezing conditions, e.g. storage period.