Plasmin activity in Manchega ewe milk: The effect of lactation, parity and health of the udder, and its influence on milk composition and rennet coagulation

Milk from 40 Manchega ewes was collected monthly and analysed during a complete lactation (5 months). Milk samples were classified by their origin in 3 categories, termed PR (primiparous ewes), M1 (multiparous ewes with no damage of the udder in the previous lactation) and M2 (multiparous ewes with udder damage in the previous lactation). The influence on plasmin of several factors as stage of lactation, parity, somatic cell count and udder health status was studied, as well as the effect of plasmin activity on milk composition and rennet coagulation. Plasmin activity decreased throughout lactation but was not affected by parity or somatic cell count (P > 0.05). A strong negative correlation was found between plasmin activity and protein (especially casein), presumably due to the proteolysis of ß-casein. Plasmin also worsened rennet coagulation, increasing rennet clotting time (RCT) and negatively affecting curd firmness (A₆₀), especially in multiparous ewes. However, the good health condition in the herd may have camouflaged some effects of plasmin over renneting.