Changes in milk protein fraction as affected by subclinical mastitis.

From cows that had both healthy quarters and quarters with subclinical mastitis [somatic cell count (SCC) 84,000 vs. 293,000/ml in bucket milk], foremilk, bucket milk, and stripping and residual milks were collected. Young milk was obtained 1.5 h later following a repeated oxytocin injection. Compared with milk from healthy quarters, milk from quarters with subclinical mastitis showed elevated SCC, plasminogen, and protein and had increased activity of n-Acetyl-beta-D-glucosaminidase (NAGase) and plasmin, as well as elevated portions of whey proteins and gamma-casein in the total protein. The SCC and the other mentioned parameters were also higher in the foremilk and the stripping and residual milks compared with bucket milk, independent of the udder health status; however, decreased values were found for total protein. Young milk showed an increase of SCC and n-Acetyl-beta-D-glucosaminidase activity compared with bucket milk. Because of lower levels of total plasmin and gamma-casein, we concluded that this young milk was newly synthesized milk containing some casein degradation products and that proteolysis of casein continued in the udder until the next milking. The n-Acetyl-beta-D-glucosaminidase activity was shown to be a better indicator for subclinical mastitis and correlated better with protein degradation than did SCC.