Effect of microfiltration of milk on plasmin activity

The effects of microfiltration and diafiltration of milk on plasmin activity and plasminogen level and their heat resistance in retentate were studied. The retention of plasmin and plasminogen was similar to that of casein or fat during concentration of milk with an 800 kDa cutoff membrane. The retention of plasmin and plasminogen was high since they are associated with the casein micelle. After the whey proteins were removed by diafiltration, retention of plasmin was statistically higher (P < 0.01) than that of plasminogen, casein or fat. An increase in plasmin activity was observed during diafiltration. Plasmin activity was proportional to the concentration of β-lactoglobulin and this correlation was strong (R2 = 0.926, P < 0.001) and may be the reason for increased plasmin activity. Pasteurization of whey protein-free retentate at 95 °C for 15 s did not significantly affect plasmin or plasminogen-derived activities.