Myoglobin oxygenation occurs in approximately the upper 5 mm of the meat cut. The color at the surface depends on the chemical state of this pigment and the balance between the availability of surface O2 and tissue respiration (Jose, Pethick, Jacob, & Gardner, 2009). High O2 levels favor oxygenation of the meat pigment and thus prolong the color of the meat before metmyoglobin becomes visible on the surface (Mancini & Hunt, 2005). In the present study, the 100% CO2 packaging system resulted in greater discoloration and a gray appearance (Fig. 6). The presence of low levels of residual O2 (above 0.1%) is known to favor myoglobin oxidation, resulting in the formation of metmyoglobin and the consequent darkening of the meat during storage