Variability in true protein, casein, nonprotein nitrogen, and proteolysis in high and low somatic cell milks

Monthly variation in milk protein (total nitrogen X 6.38) and milk fat was determined for 115 farms over 2 yr. Yearly average milk protein and fat tests in each year were 3.16 and 3.62%, respectively. The mean regression coefficient for milk protein with respect to milk fat was .47 for the entire period. Twenty-four farms were selected and grouped high or low based on their previous 2-yr somatic cell history. Monthly milk samples for each farm were tested for direct microscopic somatic cell count, total nitrogen, noncasein nitrogen, and nonprotein nitrogen. No differences in monthly nonprotein nitrogen, true protein, and casein were found between groups. Casein as a percent of total nitrogen was significantly higher for the low somatic group for seven of the 12 mo studied but was significantly higher for 9 mo when expressed as a percent of true protein. The average increase in tyrosine value for incubated preserved milk was significantly higher for the high somatic cell milk, indicating higher proteolytic activity in high somatic cell milk. Electrophoretic analysis of high and low somatic cell milk indicated that there was substantial proteolytic breakdown of alpha S-casein and beta-casein by proteases associated with elevated somatic cell counts.