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

A semi-purified preparation of streptokinase, a plasminogen activator from Streptococcus uberis, was added to cheesemilk at levels of 0.1%, 0.25% or 0.50% (v/v) to assess its use for the activation of plasminogen and, if this occurred, to evaluate the effects of the resultant increase in plasmin activity on proteolysis in Cheddar cheese during ripening. Streptokinase successfully activated plasminogen in cheese to plasmin, as indicated by increases in plasmin activity and concomitant decreases in plasminogen levels; plasmin activity increased with increasing addition of streptokinase. Increased plasmin activity accelerated proteolysis in cheese during ripening; increased breakdown of β-casein and concomitant increases in levels of γ₁-, γ₂- and γ₃-caseins were observed, using urea polyacrylamide gel electrophoresis, in experimental cheeses compared to control cheeses. Peptide profiles of 70% ethanol-soluble fractions, analysed by reversed-phase HPLC, showed greater production of hydrophobic peptides eluting in the region of 55-60 min in cheeses made from milk with added streptokinase compared to control cheese, which was attributed to increased plasmin activity. The increased plasmin activity did not affect the levels of total free amino acids. In conclusion, addition of streptokinase to cheesemilk resulted in activation of plasminogen to plasmin, which accelerated proteolysis in Cheddar cheese during ripening.