End of Project ReportNeutrophils (PMN cells) constitute one of the main cell types in milk. Increased PMN level is an indication of mastitis. An ELISA method has been developed to determine PMN levels in milk. This may allow (in addition to somatic cell count [SCC]) selection of infected quarters at drying off, thereby allowing antibiotic therapy to be limited to those quarters. PMN counts may also be used to select milk for processing. Little information is available on the contribution of different somatic cells in milk to cheese-making efficiency. The overall objective of this study was to establish the influence of the quality of raw milk, as determined by somatic cell level and type, on milk biochemistry and cheese quality. The work firstly included modification to a method for an enzyme immunoassay, which could enumerate milk PMN. Subsequently, the impact of somatic cell and PMN content on biochemistry of individual udder quarter milks and simulated bulk cow milks, and quality of cheese manufactured from such milks was investigated. The somatic cell and PMN content of bulk herd milks was also investigated.
The modification to the test of O’Sullivan et al (1992) allowed the accurate measurement of PMN levels in milk. The strong relationship or correlation between SCC and PMN of 92% in the individual quarter milks has confirmed previous preliminary data. This is important since PMN in conjunction with SCC may now provide a more reliable method of selecting milks for processing. The reduction in casein at elevated SCC and PMN levels may have resulted in the trend towards deteriorated milk coagulation properties. A very heterogeneous selection of proteolysis patterns was observed in the miniature cheeses. This substantial difference in proteolytic activity in milk from different quarters had not been observed previously. Enzymes associated with the cells in high SCC milk were retained in the cheese curd and thus, contributed to proteolysis during ripening. Addition of low volumes of high SCC milk had an obvious impact on proteolysis patterns and cheese ripening. However, such trends were generally less clear with increasing PMN milk than those observed for addition of high SCC milk. The poor correlation between SCC and PMN obtained in both cow and herd bulk milks, compared to the correlation in quarter milks was probably due to the mixing of high and low SCC milks from either quarters or cows. Thus, the true effect of PMN may not be observed in bulk herd milk but may still have an adverse effect on milk quality. Whether elevated bulk milk SCC and PMN level is due to milk from a smaller number of cows with extremely high SCC/PMN being included with milk from a predominantly healthy herd, or, to large numbers of cows with sub-clinical infections, probably contributes to variation in the effects of SCC/PMN on dairy products
