Association between technological properties of bovine milk, cold induced depletion and genetic variants of β- and κ-casein

Abstract: Caseins (CN) represent about 80% of the total protein in bovine milk and are of major importance in dairy production. The four casein types, i.e., αs1-, β-, αs2-, and κ-casein are in turn divided into different genetic variants. The genetic variants of interest in this study were β-CN variants A1, A2 and I, and κ-CN variants A and B. The genetic variants of caseins are believed to influence many milk properties including gross composition, micelle size, coagulation properties (rennet coagulation time and curd firmness) and protein profile. The aim of the study was to investigate the relation between these properties, the three genetic variants of β-CN and to some extent also genetic variants of κ-CN. The aim was also to investigate to what extent different genetic variants of β-CN were able dissociate from the micelle into milk serum due to cooling. The results indicated relation between milk gross composition and genetic variants of β-CN, where β-CN variant I showed highest protein content (3.80 %) compared to β-CN variants A1 (3.35 %, p<0.001) and A2 (3.45 %, p<0.001). The rennet coagulation time in cold milk (milk with depleted micelles) with β-CN genotype A1A1 was up to 90 % shorter than for cold milk with β-CN genotype II (p<0.001). Meanwhile, no significant difference between the two genotypes was found in tempered milk (native micelles). However, there was no clear association between cooling and curd firmness. The casein profile and the casein number were analysed both in tempered and cold milk serum phase. When comparing the relative concentration of β-CN in milk serum phases of cold and tempered milk, respectively, the increase associated to β-CN II milk was 235 % and 72 % higher, than the increase associated to β-CN A1A1 and A2A2, respectively. Furthermore, the casein number in cold milk serum phase associated to the β-CN genotype II was 18 % and 16 % lower, respectively, than in the serum phases corresponding to the A1A1 and A2A2 genotypes; though, also here the observed differences were only numerical. This might indicate that the level of cold induced depletion might vary between different genetic variants of β-CN.