Effect of freezing and microbial growth on myoglobin derivatives of beef

The effect of freezing and bacterial growth on the discoloration of beef was assessed by measuring myoglobin derivatives myoglobin (MB), oxymyoglobin (MBO₂), and metmyoglobin (METMB) on the surfaces of fresh and frozen-thawed packaged beef cuts stored at 2 °C and analyzed after 0, 3, 6, 9, and 12 days of storage. MB, MBO₂, and METMB concentrations were measured spectrophotometrically. Frozen-thawed beef samples experienced less 'blooming' (conversion of MB to MBO₂) and more rapid discoloration than fresh cuts during storage. By day 3, >20% METMB was formed in the frozen- thawed samples, whereas the fresh samples reached this value after day 6 of storage. The rates of MB oxidation were similar (P > 0.05) for sterile and frozen-thawed inoculated (Pseudomonas fluorescens at a rate of 1.5 colony forming units/cm² · cm² area) samples from day 0 through day 6 of storage. For storage periods of less than a week, bacterial growth is not a major cause of meat discoloration. After day 6, the high bacterial growth rate resulted in a rapid increase in METMB formation. Possible mechanisms for MB oxidation in frozen-thawed beef are suggested.