PLASMIN AND PLASMINOGEN ACTIVITIES AND CASEIN DEGRADATION DURING RIPENING OF SWISS-TYPE CHEESE

Plasmin and plasminogen activities and casein degradation during ripening of Swiss-type cheese made from raw milk were studied. The average contents of plasmin of raw milk and green cheese were 18 nmol pNA/ml and 436 nmol pNA/g, respectively. The level of plasmin activity in cheese decreased gradually over a period from 1 to 20 days during ripening and then maintained the same level until the end of ripening (90 days). Thus there was a significant decrease (P<0.05) in plasmin activity between day 1 and 20, but there were no significant differences (P<0.05) in plasmin activity among 20, 40, 60 and 90 day samples of the cheese. The range in plasminogen activity for green cheese was from 708 to 880 nmol pNA/g, with an average of 819 nmol pNA/g. The course of the curve representing change of plasminogen activity during cheese ripening showed similar results with that of plasmin activity. Electrophoretograms of the proteins in the cheese indicated that as the ripening age of the cheese increased, major caseins were broken down, i. e., alpha(s1)-casein into alpha(s1)-I-casein and also beta-casein into gamma1-, gamma2- and gamma3-caseins. The alpha(s1)-casein in ripening of the cheese was degraded faster than the beta-casein so that in 90 days old cheese only 22% and 46% respectively of these proteins remained intact.