Relative role of catalysts and reductants in the formation of metmyoglobin in aerobically stored beef

The formation of metmyoglobin (metMb) in sterile minced and intact slices of beef has been determined during storage at 1°C. The oxidation of oxymyoglobin in minced beef is first order, the rate constants for different muscles, although being pH dependent, are similar for all the muscles studied. The rate is about four or five times faster than that reported for oxymyoglobin in solution, indicating that catalysis occurs. In intact slices the reaction profile is more complex as a pseudoequilibrium level of metmyoglobin forms after a few days at 1°C, the concentration of metMb remaining constant for several days and then being followed by further rapid oxidation to 100% metMb. Due to variations in an aerobic reducing system present in intact beef the duration of the pseudoequilibrium varies markedly from muscle to muscle; this reducing system is destroyed by mincing. The rate of metMb formation in intact muscles varies with sample shape, thin slices being more colour stable ones. Reasons for this dependence on sample shape are discussed.