Coagulation Properties of Milk Association with Milk Protein Composition and Genetic Polymorphism

Concentrations of the different proteins in milk are important for the outcome of the coagulation processes which yield our dairy products, whereas total milk protein content is a poor indicator of coagulation properties of milk. In order to design the milk protein composition to meet dairy processing requirements, selection for genetic variants of milk proteins have been proposed. This work aimed to study genetic milk protein polymorphism and its association with the detailed milk protein composition, and effects on milk coagulation. Both chymosin-induced (cheese) and acid-induced (fermented milk products) coagulation was considered. An association of the κ-casein B allele with improved chymosin-induced milk coagulation properties, and a corresponding unfavourable effect of the E allele, was found, probably an effect of the κ-casein concentration. In a cheese-making model it was shown that concentration of κ-casein in milk was important during the initial stages of cheese making, and also to reduce casein losses into whey. After pressing of the curd, a high ratio of casein to total protein and total casein content of milk were important for casein retention in curd and fresh curd yield. Increased casein retention in curd was also associated with the β-lactoglobulin BB genotype, possibly due to the association of this genotype with higher proportion of casein to total protein. A low κ-casein concentration in milk was associated with an increased risk of non-coagulation, a phenomenon that has recently been highlighted. The concentration of β-lactoglobulin in milk showed a positive influence on curd firmness in acid-induced gels. As the AA and AB genotypes of β-lactoglobulin were associated with increased β-lactoglobulin concentration in milk, milk from cows carrying these genotypes resulted in firmer acid gels compared to BB. There was an effect of β-lactoglobulin genotype also at equal β-lactoglobulin concentrations, possibly due to an improved structure of acid gels of milk from βlactoglobulin BB cows.