Factors affecting the rate of metmyoglobin accumulation in pre-packaged beef

Factors affecting the rate of discoloration of pre-packaged beef have been compared quantitatively in a number of experimental animals. Inter-muscular variability is the most important single factor and ranges from the stable M. longissimus dorsi to the unstable M. psoas major, the degree of discoloration of the latter muscle being almost eight times greater after 96 h storage at 0°C. Among other intrinsic factors studied, age post mortem has a slight accelerating effect while pH, within the narrow range exhibited by experimental animals, has no effect. Inter-animal variability also has an effect, but only a slight one. Of the extrinsic factors studied, temperature is most important from a practical standpoint, the degree of discoloration after 96 h storage at 10°C ranging from two to five times that at 0°C, depending on the muscle. Light produces only a minor accelerating effect but UV produces serious discoloration in all muscles within hours, even at 0°C. Slight differences exist between packaging films but these are not commercially significant.