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.

Preliminary work was to develop a method for determination of milk casein content using commercially available infrared milk testing equipment. Fresh whole milk samples were split into two portions. One portion was tested for total protein with an infrared milk analyzer. The other portion was adjusted to pH 4.6 with a specific volume and concentration of phosphoric acid. Milk casein precipitated at pH 4.6 and was removed by filtration. Next, the noncasein protein filtrate was tested for protein content using an infrared milk analyzer. The difference between the percent protein of the original milk sample and the percent protein of the noncasein protein filtrate equals the percent milk casein. The infrared casein determination for 36 different individual herd milk samples was not significantly different from results obtained by the official International Dairy Federation casein testing method. The infrared method is faster than the chemical method and could be done with infrared milk testing equipment that is commonly available in many cheese manufacturing plants. Repeatability of the infrared method was comparable to the repeatability of the chemical method.