Changes in milk protein fraction as affected by subclinical mastitis

From cows that had both healthy quarters and quarters with subclinical mast itis [somatic cell count (SCC) 84,000 vs. 293,000/ml in bucket milk], forem ilk, bucket milk, and stripping and residual milks were collected. Young mi lk was obtained 1.5 h later following a repeated oxytocin injection. Compar ed with milk from healthy quarters, milk from quarters with subclinical mas titis showed elevated SCC, plasminogen, and protein and had increased activ ity of n-Acetyl-beta-D-glucosaminidase (NAGase) and plasmin, as well as ele vated 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 pr otein. Young milk showed an increase of SCC and n-Acetyl-beta-D-glucosamini dase activity compared with bucket milk. Because of lower levels of total p lasmin and gamma-casein, we concluded that this young milk was newly synthe sized 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 subclini cal mastitis and correlated better with protein degradation than did SCC.