Total proteolysis in bovine raw bulk milk of Northern Sweden

Sammanfattning: Proteases are enzymes that catalyze protein breakdown. The major indigenous protease is plasmin, that together with proteases originating from somatic cells and bacteria, accounts for most of the proteolytic activity in bovine milk. In many dairy products, the key characteristics depends on the concentration and composition of the milk proteins. It has been seen in other studies that environmental and production parameters can influence the level and activity of proteases. The objective of this study was to measure the level of total proteolysis in raw bulk milk of Northern Sweden and investigate if there are correlations between the total proteolysis and any of the parameters: plasmin, somatic cell counts, total bacteria number, milk pH, total milk protein, breed, milking system, production system (tied or loose house), number of animals or different seasons. The total proteolysis, that was measured as the level of peptides in the milk samples, was significantly correlated to the milk protein content (P<0.001) and the milk pH value (P<0.01) in a Pearson correlation. The same connection was seen when one variable at a time was compared in a regression; there was a correlation between proteolysis and milk pH (P<0.05), as well as milk protein content (P<0.001). In addition, there were significant differences between the breeds, where Jersey cows had 25.7 % higher proteolytic activity compared to the other breeds. There was also a difference between the housing systems, where milk from tied cows had 10.6 % higher proteolysis, compared to milk from loose housed cows (P=0.002). However, when all variables were included in a multivariable regression model, no variable was significantly correlated to total proteolysis. Nevertheless, some interesting trends were observed. The !-value was negative for all the breeds (SRB, SLB, Mountain Breed and farms with both SRB and SLB), when they were compared to the Jersey breed. Also, a trend towards a correlation between plasmin and proteolysis, seen in the single regression, remained in the multivariable regression. The measurement of proteolysis was complemented with an SDS PAGE electrophoretic analysis of eight milk samples, in order to study the protein profile. The images fortified the speculation on plasmin being the major protease responsible for the proteolysis in this study, since !-CN (the major plasmin substrate) appeared to be more extensively degraded in the samples with high proteolytic activity. Because of the fact that the degree of explanation for the variance in total proteolysis was low in the multivariable regression, even in the best model (R-sq. adj. 12.16 %), it was concluded that there are probably other factors, e.g. individual differences – that was not included in this study, that highly affect the proteolytic activity in bovine milk. However, according to the results in this study, investigations with focus on genetical differences might be an interesting perspective for future investigations.