Effect of freezing and microbial growth on myoglobin derivatives of beef

The effect of freezing and bacterial growth on the discoloration of beef wa s 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 degrees C and analyzed after 0, 3, 6, 9, an d 12 days of storage. MB, MBO2, and METMB concentrations were measured spec trophotometrically. Frozen-thawed beef samples experienced less "blooming'' (conversion of MB to MBO2) and more rapid discoloration than fresh cuts du ring 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 r ates 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/c m(2).cm(2) area) samples from day 0 through day 6 of storage. For storage p eriods of less than a week, bacterial growth is not a major cause of meat d iscoloration. After day 6, the high bacterial growth rate resulted in a rap id increase in METMB formation. Possible mechanisms for MB oxidation in fro zen-thawed beef are suggested.