Milk SCC and PMN as indicators of milk processability and subsequent cheese quality

This study investigated the effect of milk somatic cell count (SCC) and polymorphonuclear leucocyte (PMN, or neutrophil) content on the processability of milks from individual udder quarters. Two milk samples of high and low SCC (6x10 and 920x10/ml, respectively) and two of high and low PMN content (<1x10 and 700x10/ml, respectively) were selected. The milk samples were mixed in appropriate proportions such as to produce four artificial mixed milks of SCC or PMN content ~200x10/ml and ~400x10/ml. All milks were analysed for gross composition, renneting properties and N-fractions. Miniature Cheddar-type cheeses were manufactured and analysed for composition after 30 d and for proteolysis after 30 d and 90 d of ripening. Fat and protein contents of the milks generally decreased with increasing SCC and PMN and there was no effect on N-fractions in the milks or on total solids, moisture, NaCl content or pH of the cheeses. Increasing SCC and PMN had a negative effect on rennet coagulation of milk. Urea-PAGE electophoretograms indicated that the patterns of proteolysis changed quantitatively and qualitatively with increasing milk SCC and PMN during cheese ripening. High SCC and PMN milk resulted in reduced levels of residual intact αs1and β-caseins, indicating cell-associated proteinase activity. In conclusion, milks with high SCC and PMN had inferior composition and processing characteristics.