Plasmin activity and proteolysis in milk protein ingredients

Despite the widespread use of milk protein ingredients and the well-known detrimental effects of plasmin-induced casein hydrolysis on product flavour and stability, little research has been carried out on the occurrence and activity of plasmin in milk protein ingredients. In this study, 19 sodium caseinate (NaCas), 13 calcium caseinate (CaCas), 2 micellar casein isolate (MCI) and 14 milk protein concentrate (MPC) samples were studied for plasmin and plasminogen-derived activity and proteolysis after reconstitution. Results indicated a higher occurrence and activity of plasmin in MPC and MCI than in NaCas and CaCas. During storage, activation of plasminogen to plasmin and autolysis of plasmin were observed. Furthermore, extensive plasmin-induced casein hydrolysis was observed. The specificity of casein hydrolysis was similar in all samples and proteolysis per unit plasmin activity was most extensive in MPC. These results indicate that residual plasmin activity should be a factor of consideration in milk protein ingredient selection.