ANALYSIS OF MILK PROTEIN COMPOSITION IN CROSSBRED AND PUREBRED DAIRY COWS

Milk proteins have been widely studied in the last fifty years because of their great importance and strong relationship with milk quality and milk coagulation properties (MCP). The general interest in dairy science gave the possibility to achieve many important scientific results, such as polymorphism identification concerning this kind of molecules and differences among species and breeds. Studies which considered effects of milk protein polymorphisms on milk characteristics and milk properties started since 1970 in such countries like Italy, where more than 70% of milk is used in the cheese-making industry and transformed in typical products, which play a key role for the economic valorization, like the Protected Designation of Origin Products. The importance of this kind of topics and the possibility to improve milk and the cheese-making processes brought to a strong change concerning breeding value aims and selection strategies employed in dairy science. In the last decades many farms focused on high milk production animals, but this sort of management took to a worsening of health and functional characteristics of the animals, such as fitness, robustness, longevity and fertility, as well as genetic variability. Today situation get different, because an acceptable compromise was found among farm productivity and milk quality; greater economic value was given to typical products and local breeds were protected, because strong linked to the environmental preservation, local tradition, and historical tradition conservation; moreover crossbreeding schemes are employed, because they carry out very appreciable animal performances, but breed merits are kept and at the same time some very important effect, like heterosis, are exploited. Considering such options and lack of studies in literature, it would be desirable to gain further knowledge on the effects of some sources of variation on milk protein fraction and milk protein component. Aims of the study were to find and develop a new reverse phase HPLC method able to quantify and identify the most common milk protein fractions; to investigate the sources of variation on milk protein fractions and composition of individual milk samples collected in dairy herds which employed crossbreeding schemes; to investigate the sources of variation on milk protein fractions and composition of individual milk samples collected in multi-breed dairy herds located in mountainous areas. A new reverse phase HPLC method for the identification and quantification of the most common protein fractions and genetic variants of bovine milk was developed, including some minor components, like lactoferrin. The method was validated by testing linearity and repeatability. For genetic variants identification, milk of DNA-genotyped animals was used to find peaks and to associate them with the known genotype of animals. Calibrations with a coefficient of determination higher than 0.99 were obtained for every single genetic protein variant, nevertheless commercial standards for every variant were not available. Method was after employed for individual milk samples collected in crossbred dairy herds located in the north of Italy and to compare pure Holstein with different bull semen of Alpine European and Nordic breeds, like Montbeliarde, Brown Swiss and Swedish Red. It came out that milk protein fractions, are influenced by the stage of lactation and parity; breed effect was significant especially for κ-CN, α-LA and β-Lg, which are linked. Among crossbred combinations, Brown Swiss and Montbeliarde crosses showed high significance, because of the selection strategy or the natural presence of the B variant of the κ-CN, respectively; B variant is very important in the cheese making industry because animals carrying AB or BB genotype show a higher milk casein content and also a higher milk quality. Method was after employed for individual milk samples collected in herds where two breeds were attended at least; six breeds were employed, three high-production and three dual-purpose: Holstein-Friesian, Brown Swiss, Jersey and Gray Alpine, Simmental and Rendena, respectively. Four types of dairy herd were considered in this study: modern herd type, traditional herd type with silage use, traditional herd type without grazing use and traditional original herd type. Effect of stage of lactation and parity influenced milk protein fractions; among breeds, Jersey showed a higher milk protein content than the other breeds; in Gray Alpine a new genetic κ-CN variant was detected using our reverse phase HPLC method, but not identified, so further investigation is required to confirm this aspect, employing other techniques maybe coupled with HPLC. Traditional dairy herd type with silage use was the best strategy with the highest milk protein content, although it showed the highest somatic cell content; modern herd type gave unexpectedly inferior performances and traditional herd without grazing was instead the worse concerning milk protein content