PLASMIN ACTIVITY AND MILK COAGULATION

Monthly milk samples were collected from 19 Holstein and 19 Jersey cows in separate herds during 10-mo lactations (380 total samples). Effects of breed-herd, lactation number, season, and stage of lactation and the interactions of these parameters on plasmin, plasminogen, and milk-clotting measurements were determined. Lactation number had the greatest influence on plasmin activity, which increased during lactation but was not influenced by breed-herd, pH, protein, or fat. Stage of lactation and season were the only factors that influenced plasminogen. Percentage of plasminogen activated was highest during late lactation, in milk from fourth and later lactation cows, and during fall and winter. Plasmin activity did not influence milk-clotting parameters. Clotting time decreased as pH decreased and as protein and fat increased. Increased protein and fat improved firming rate of curd. Milk samples collected in the fall had the highest firming rates, and stage of lactation did not influence firming rate. Lower pH, higher protein, and higher fat decreased cutting time. Increased protein and fat improved curd firmness. Jersey herd milk had firmer curd than Holstein herd milk. Lactation number and stage of lactation did not influence curd firmness.