Proteolysis in samples of quarter milk with varying somatic cell counts. 1. Comparison of some indicators of endogenous proteolysis in milk.

Eighty-six samples of quarter milk from 31 cows, the mean SCC of which exceeded 400,000 cells/ml, were analyzed for SCC, chloride content, pH, clotting time, total N, soluble N, casein N, proteose-peptone content, plasmin activity, and amount of free NH2 groups, parameters that are related to the udder health condition and to the deterioration of the protein quality. Analysis of the milk from each quarter separately improved the correlations considerably compared with analyses of samples from all 4 quarters mixed together. Measurement of free NH2 groups, using the 2, 4, 6-trinitrobenzene sulfonic acid, was not appropriate for estimating the extent of protein deterioration in milk samples with different SCC. Unlike SCC, the proteolysis index, plasmin activity, and the proteose-peptone content were highly correlated themselves and with most of the parameters related to proteolysis in milk. Proteolysis occurred in milk samples with SCC as low as 250,000 cells/ml. The quality of milk protein decreased unavoidably during lactation, regardless of SCC.