The curd-forming properties of milk as affected by the action of plasmin

Purified plasmin was added to skim milk at a range of levels (0.1-10 mg L^-1) and the samples subsequently incubated to allow hydrolysis of casein by the enzyme. Milk clotting properties (as measured by Formagraph) and curd yield properties of the milk were measured after incubation. Increasing plasmin addition, with concomitantly increased levels of hydrolysis of casein, resulted in increased curd firming time and cutting time and reduced firming rate and final curd firmness (all P < 0.001). Rennet clotting time was only increased with extensive hydrolysis by plasmin. Curd yield was reduced with increasing plasmin addition, with concomitantly increased losses of protein in whey. With low levels of enzyme addition, however, the magnitude of most effects was small. Curd moisture was unaffected by plasmin addition to milk at any level of enzyme addition. It may be concluded that extensive hydrolysis of casein by plasmin, independent of any other alterations in milk quality, has a significant influence on some milk cheesemaking properties. However, it appears that plasmin may not be responsible for changes in cheese moisture in, for example, late lactation milk.