Variation in Milk Protein Composition and its Importance for the Quality of Cheese Milk

Wedholm, A. 2008. Variation in milk protein composition and its importance for the quality of cheese milk. Doctoral dissertation. IS 978-91-85913-46-6, ISSN: 1652-6880 Variations within the bovine milk protein profile can have both positive and negative impacts on the quality of cheese milk. As main contributors to the cheese structure, the caseins are important for the cheese yield. During the last decades, the proportion of casein in relation to the total protein amount (the casein number) has decreased in Swedish bulk milk. This decline is unexplained and the reason for this deteriorating trend is unknown. Therefore, more knowledge in how the protein composition varies is needed. The main objective of this thesis was to increase the understanding of how the cheese yield is affected by variation in the detailed milk protein profile. Milk was sampled from a total of 134 individual cows of Swedish red and white (SRB), Swedish Holstein (SLB) and Danish Holstein (SDM). The milk was analysed for major milk components, urea, calcium, somatic cell count, pH, level of protein degradation, the detailed milk protein composition and low-molecular-weight milk peptides. Low-fat model cheeses were made from four litres of pasteurised skimmed milk and cheese yield was recorded. Variations in the milk protein profile between breeds or herds, before and after pasteurisation and the effect of protein composition on cheese yield were studied. Finally, low-molecular-weight peptides identified in the acid whey fraction from milk samples with different levels of somatic cell count were related to earlier publications on identified cleavage specificities of milk proteases. Concentrations of total protein, total casein, β-casein and κ-casein were higher in SRB milk compared to SLB milk. As expected, pasteurisation did not influence the milk protein markedly and only small changes in concentration of αS1-casein and proteose peptone component 3 were seen. The milk proteins αS1-, β-, and κ-casein and β-lactoglobulin B had significantly positive effects on the cheese yield whereas the casein number was significant for the transfer of protein from milk to cheese. Moreover, several forms or proteolytic products of αS1-, αS2-, and β-casein positively affected this trait. About 30% of the milk samples were poorly coagulating or non-coagulating with a significantly lower concentration of κ-casein. Finally, peptide cleavage sites corresponding to earlier identified specificities of elastase and cathepsing B were identified in milk with high somatic cell count.