Your veterinarian is correct. Cows with mastitis (i.e., high somatic cell count) synthesize less casein which is needed to produce cheese. The mastitis infection reduces cellular production of casein. Total milk protein generally remains constant as more whey proteins and non-protein nitrogen enter from the blood (cells are more permeable), but these forms of nitrogen will not yield cheese. The cow also secretes increased levels of immunoglobulins (antibodies) to fight the infection. Thus, cheese plants often pay large bonuses for low somatic cell count milk as cheese yield is increased. The shelf life of all dairy products is also shortened due to the secretion of enzymes associated with the high somatic cell counts.
Mike Hutjens, Extension Dairy Specialist, University of Illinois - Illini DairyNET
