Plasmin-plasminogen system and milk coagulation properties of two Greek dairy sheep breeds

The enzymatic activities of plasmin (PL), plasminogen-derived (PG) and plasminogen activator (PA) and their correlation with the milk coagulation properties and major milk components were studied in two Greek sheep breeds during their whole lactation. Individual milk samples were collected at 3-week intervals from 14 Karagouniko and 14 Chios sheep breed. The enzymatic activities of PL, PG and PA were measured by colorimetric assay. The milk coagulation properties (rCT rennet clotting time, K20 curd firming time, A30 final curd consistency) were determined with a Formagraph. The results showed that PL, PG and PA activities and rennet clotting time (rCT) did not differ between the two breeds. However, K20 and A30 differed significantly between Karagouniko and Chios milk. The stage of lactation (early, mid or late) significantly affected PL, PG and PA activities. PL and PA activity levels were lower during late lactation (after the 150 days of the lactation period) compared to the corresponding levels during early and mid lactation, while a significant reduction of PG activity was observed from mid to late lactation stage. Lactation stage also significantly affected milk coagulation traits: A decrease in rCT occurred at mid lactation (from 100 to 150 days) while K20 increased from the early to the mid stage of lactation. The activities of PL and PA were significantly affected by the health status of the udder; milk with >300,000 SCC had 51% higher activity of PL and 86% higher activity of PA than milk containing <300,000 SCC. PG activity was 12% lower in milk with low SCC in comparison with milk with high SCC, but the results were not statistically significant. The coagulation properties of milk were not affected by SCC. A weak significant correlation exists between plasmin, PL + PG and K20.