EFFECT OF COAGULANTS, SOMATIC-CELL ENZYMES, AND EXTRACELLULAR BACTERIAL ENZYMES ON PLASMINOGEN ACTIVATION

Any factor that converts plasminogen (inactive form of the proteolytic enzyme plasmin) to plasmin in milk may have a negative impact on milk protein functionality. The effect of milk coagulants (calf rennet and proteases from Mucor miehei, Endothia parasitica, Mucor pusillus), leukocytes from bovine blood, somatic cells from high SCC bovine milk, and extracellular bacterial enzymes from psychrotrophic bacteria and mastitis pathogens on the conversion of plasminogen to plasmin was determined. Constituents of somatic cells isolated from high SCC bovine milk were able to convert plasminogen to plasmin. This plasminogen activator was inactivated by heating milk to 63-degrees-C for 30 min. Leukocytes from bovine blood, milk coagulants, and extracellular bacterial enzymes were not able to convert plasminogen to plasmin. Therefore, a source of plasminogen activator in milk is somatic cells. Plasminogen present in milk could be converted to plasmin in the udder before milking or during milk storage after milking, resulting in increased proteolysis of casein. Proteolytic damage to casein influences cheese yield, milk protein functionality, and dairy product quality.