Calpain activities A representative nondenaturing 12.5% PAGE casein gel result is shown in Figure 6. A comparison of calpain activity is made between the RC, CC, and SC treatments based on the light intensity and size of the clear zones. During the 1st 24-h postmortem, there was not a great change in the activity of m-calpain regardless of chilling treatment. However, at 3- and 12-h postmortem, μ-calpain of the RC group remained the same (similar intensity and size of clear zones indicate the similar μ-calpain activities in the muscle), but intensity of μ-calpain activity zone was reduced in both CC and SC groups. By 24-h postmortem, the absence of clear zones indicated that μ-calpain completely disappeared in meat no matter what chilling method was used The differences in μ-calpain activity correlated with the differences in pH decline noted among the 3 chilling treatments. For example, SC group had the fastest drop in pH and lost its μ-calpain zone mostly among the 3 treatments. The increased rate of autolysis was significant because autolysis of μ-calpain reduced the Ca2+ requirement for calpain activity from approximately 3 to 50 μM (pre-autolysis) to 0.5 to 2.0 μM, bringing the Ca2+ requirement closer to the physiological range (Zimmerman and Schlaepfer 1991).