Why does the increase of plasmin worsen the coagulation properties of milk in dairy sheep?

The concentrations of whole casein and fat are the factors which play the most important role in determining the cheese-making qualities of milk. Plasmin (PL) is the main proteolytic enzyme in milk and has been found to be associated with enhanced casein hydrolysation. ß-casein is the most susceptible to breakdown by this enzyme. Most of the enzyme is in the form of the inactive zymogene plasminogen (PG), which is converted to PL by the action of PG activators. PL activity is also associated with alteration of mammary epithelium permeability and a subsequent increase in paracellular flow. These proteolytic activities worsen the milk quality, coagulation properties and cheese yield. The level of PL in milk increases towards the end of lactation, with mastitis, and with the increasing age of the animals. At present the relationship between PL, SCC and casein are not clearly defined. Some authors found positive relationships between PL and SCC with CN (Baldiet al., 1996; Bianchiet al., 2004), others negative (Fantuzet al., 2001), and some have found no relationship (Albenzioet al., 2004). The aim of this work was to study why milk with higher PL content has worse coagulation properties.