Use of exogenous urokinase to accelerate proteolysis in Cheddar cheese during ripening

Urokinase was added to milk for Cheddar cheese manufacture to examine the possibility of accelerating proteolysis during ripening by conversion of indigenous plasminogen to plasmin. Cheddar cheeses were manufactured from milk containing 0, 0.25, 0.5, 1 or 5 units urokinase ml^-1 and were analysed for composition, plasmin activity, plasminogen level and proteolysis. The addition of urokinase to the milk significantly increased plasmin activity in the cheese with concomitant decreases in plasminogen level and plasminogen:plasmin ratio. Activation of plasminogen to plasmin occurred principally during the first 24 h of manufacture. Increased plasmin activity in cheese caused increased production of γ-caseins and water-soluble N. Some differences were apparent in reversed-phase HPLC chromatograms of ethanol-soluble and -insoluble sub-fractions of water-soluble extracts from the cheeses. The increased plasmin activity caused by the addition of urokinase generally increased the liberation of free amino acids during ripening. The addition of urokinase to cheesemilk effectively accelerated primary proteolysis in Cheddar cheese during ripening.