Plasmin and plasminogen variation in bovine raw milk : impact of season, breed and milking system

Sammanfattning: Plasmin (PL) is a native proteinase in milk, which combines a complex system consisting of its zymogen called plasminogen (PG), inhibitors and activators, which activity is known to make considerable changes in the dairy products, which represent in irregulating of casein hydrolysis and poor clotting properties, that may slow the cheese ripening process and consequently increase the processing expense. The objective of this study was to investigate if on-farm factors (i.e., seasons breed and milking system) significantly affect the PL and PG-derived activity of raw milk. As well as, to investigate the relationship and/or possible correlations between PL and PG-derived activity and milk composition (i.e., pH, protein, somatic cell count and total bacteria count). Milk was collected from 18 different farms northern Sweden in November 2017, February and September 2018. PL and PG analyses were conducted on milk serum fraction in duplicates obtained by ultracentrifugation of skimmed milk samples. PL and PG-derived activities were analyzed by the spectrophotometric method using multi-mode microplate reader at 37°C. Urokinase (49.5 plough units) was used as PG activator to measure the total proteolytic activity of PL and PG. The production season had no effect (P > 0.05) on the average PL and PG-derived activities. The raw milk PL and PG-derived activities were significantly affected by breed. Jersey had higher (P < 0.05) PL (6,54 U/mL) and PG-derived (139,67 U/mL) activities compared to all other breeds. SRB had higher (P < 0.05) PL activity (16.2%, 31% and 39.8%) comparing to MB, SLB/SRB and SLB, respectively. MB had higher (P < 0.05) PL activity (17.13%) than SLB. With regard to SRB, SLB/SRB and MB had higher (P < 0.05) PG-derived activity (13.5%, 31%, 13.1% and 12.4%, respectively) comparing to SLB. Farms with a conventional milking system (CMS) had higher (P < 0.05) PG-derived activity compared to farms containing automatic milking system (AMS). The mean PG derived activity showed to be higher in CMS (102.66 U/mL) compared to (91.84 U/mL) in AMS. Total protein content and somatic cell count (SCC) were correlated with PL and PG-derived activities. Milk pH and total bacteria count (TBC) were not correlated with PL and PG activity.