Effects of A1 and A2 β-casein variants on milk proteins and technological traits in Holstein cows

The genetic variants of β-casein (CSN2), especially A1 and A2 alleles, have recently risen the attention for their putative human health implications but also for their role in the cheese making process. For this reason, this study aimed at better evaluating the effects of the β-casein (CN) A1 and A2 variants both on milk protein composition and technological traits in a population of 498 Holstein cows reared in 28 herds. A validated reversed phase high performance liquid chromatography method was used to quantify six CN and two whey protein (WP) fractions which were expressed as percentage of total milk nitrogen content. In total, 11 lab-measured milk technological traits were also considered, including milk coagulation properties (MCP), cheese yields and curd nutrients recoveries. Data were analysed with a linear mixed model including the fixed effects of cows’ days in milk, parity and CSN3, CSN2 and BLG genotypes and the random effect of herd/date. β-CN genotypes had significant effects on all individual CNs and WPs except for κ-CN and on non-protein N fraction. In terms of MCP, β-CN A1A1 had a significantly (P<0.05) lower rennet coagulation time (RCT) than A2A2 (16.4 vs 20.4 min, respectively). Curd firmness at 30 min (a30) was also positively influenced by the A1A1 genotype (43.0 mm for A1A1 vs 35.2 mm for A2A2). Cheese yield and recovery of nutrients in the curd were also slightly higher in A1A1 samples. The positive effect of A1 variant on milk technological traits might be related to the fact that A1A1 genotype was associated to higher proportions of β-CN. These results suggested that, beside κ-CN, also β-CN plays a role on the cheese-making process and in particular the A1 allele seems to have no detrimental effects on the quality of milk destined for cheese making. Acknowledgements. The research was part of the FARM-INN AGER project funded by the Fondazione Cariplo.