Effect of Serine Proteolytic Enzymes (Trypsin and Plasmin), Trypsin Inhibitor, and Plasminogen Activator Addition to Ultra-High Temperature Processed Milk

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Abstract
Proteolysis and gelation were investigated in single strength, 2% fat, UHT-processed milk following aseptic addition of combinations of plasmin, plasminogen, trypsin, trypsin inhibitor (Kunitz), and urokinase (plasminogen activator). Individual 250-ml milk containers processed by direct or indirect methods were examined for the following attributes over 10 mo: growth on slants, appearance, pH, apparent viscosity, gel formation, enzymatic activity, and casein breakdown. Control milk samples in the study did not gel. Addition of trypsin at 1.5 or 7.5 mg protein/L of milk or addition of plasmin at .3 or 1.5 mg protein/L did not result in gelation. However, containers with plasminogen at .3 mg protein/L began forming a gel at 5.5 mo. Enzyme activity in plasminogen-treated samples was not detected spectrophotometrically using an L-lysine-p-nitroanilide substrate, but extensive casein breakdown was apparent by SDSPAGE. The evidence suggests plasminogen-derived activity promotes UHT milk gelation.