New markers of bulk milk quality in relation to mastitis: studies on polymorphonuclear leukocytes and α-lactalbumin

All dairy processors depend on a continuous supply of high-quality bulk milk from milk producers to be able to distribute liquid milk and dairy products. Mastitis, inflammation of the bovine udder, is the most common disease in dairy cows, and leads to altered milk composition and impaired milk quality. The somatic cell count (SCC) is currently used as a marker for udder health, and indirectly, for bulk milk quality, but because it is somewhat insensitive and unspecific, there has been an interest to find alternative markers. The main objective of this thesis was to acquire more knowledge about two potential markers for bulk milk quality: polymorphonuclear leukocyte count (PMNC) and α-lactalbumin (α-LA). Another objective was to study if the combination of α-LA, haptoglobin (Hp) and serum amyloid A (SAA) in an acute phase index (API) could be useful as an alternative marker for bulk milk quality. Bulk milk samples were collected from Swedish dairy farms and analyzed for PMNC, α-LA, SCC, haptoglobin (Hp), serum amyloid A (SAA), fat, lactose, total protein and casein contents, casein number, protein composition, proteolysis and coagulating properties. An API was calculated for each sample by combining results on α-LA, Hp and SAA. Samples with high PMNC had a lower casein number than samples with low PMNC, while samples with high SCC had lower lactose and casein contents, lower casein number and more proteolysis than low SCC samples. There was no significant difference in the inflammatory markers SCC, PMNC, Hp and SAA between milk samples containing low, medium or high concentrations of α-LA. Differences between α-LA groups were, however, found in some milk quality parameters as high α-LA concentration was related to low concentrations of αs1-, αs2- and β-casein, and high concentrations of lactose and β-lactoglobulin. High API was related to low lactose content and casein number. In conclusion, PMNC and α-LA were not considered more useful markers of inflammation and milk quality in bulk milk than SCC, which is currently used.