Effect of β-casein, κ-casein and β-lactoglobulin genotypes on concentration of milk protein variants

Individual mid-lactation milk samples were collected from 116 cows of the Swedish Red and Swedish Holstein breeds with known genotypes of β-casein, κ-casein and β-lactoglobulin. Detailed milk protein composition and allele-specific expression of β-casein, κ-casein and β-lactoglobulin proteins in milk were analysed using reversed-phase high-performance liquid chromatography. Aggregate β-/κ-casein genotype was significantly associated with the κ-casein concentration in milk. The lowest κ-casein concentration was found in milk of cows with genotypes including κ-casein E (A¹A²/AE, A¹A¹/AE) and also A²A²/AA milk, whereas highest levels were associated with genotypes including κ-casein B. Casein number was positively and concentration of β-lactoglobulin negatively associated with the β-lactoglobulin BB genotype. In heterozygote cows, β-casein A¹ and β-lactoglobulin A proteins were found at higher concentrations in milk compared with the protein variant encoded by the alternative allele at these loci, whereas κ-casein A and B variants were found at similar concentrations in heterozygote AB cows.