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.
|Number of pages||4|
|Publication status||Published - Feb 1 1997|
ASJC Scopus subject areas
- Food Science