title: Glycosylation of κ-casein: genetic and nongenetic variation and effects on rennet coagulation properties of milk.
abstract: The aims of this study were to investigate genetic and nongenetic variation in the degree of glycosylation of κ-casein (κ-CN) and to estimate the effects of glycosylated (G-κCN) and unglycosylated (U-κCN) κ-CN contents on milk coagulation properties of Simmental cows. Measures of contents of the main casein fractions, G-κCN, and U-κCN, and assessment of genotypes at CSN2, CSN3, and BLG were obtained by reversed-phase HPLC analysis of 2,015 individual milk samples. Content of total κ-CN (κ-CNtot, g/L) was the sum of G-κCN and U-κCN, and the glycosylation degree of κ-CN (GD) was measured as the ratio of G-κCN to κ-CNtot. Rennet coagulation time (RCT) and curd firmness were measured by using a computerized renneting meter. Measures of curd firmness were adjusted for RCT before statistical analysis. Variance components of κ-CNtot, G-κCN, U-κCN, and GD were estimated by Bayesian procedures and univariate linear models that included the class effects of the herd-test-day, parity, days in milk, genotypes at milk protein genes, and animal. These class effects, those of G-κCN, U-κCN, and content of other caseins, and the linear effect of milk pH were accounted for by models investigating the influence of κ-CN glycosylation on coagulation properties. The GD ranged from 22 to 76%, indicating that variation in G-κCN depends on the variation both in κ-CNtot and in the efficiency of κ-CN glycosylation. Genotype CSN3 BB exhibited high G-κCN and U-κCN relative to that of CSN3 AA. Heritability of G-κCN, U-κCN, and GD was high and ranged from 0.46 to 0.56. A large proportion of the additive genetic variation in G-κCN and U-κCN was attributable to influence of CSN and BLG, but these genes did not affect variation in GD, and across-genotypes differences in the trait were small or trivial. Average RCT of the milk class having the highest G-κCN was, on average, 2min (standard deviation 0.5) shorter than that of the lowest class. Conversely, U-κCN and content of other caseins were not associated with any effect on RCT, except for a slight delay in coagulation when U-κCN was very high. Curd firmness increased when the contents of both κ-CN fractions and other caseins increased. This study provides evidence that the positive association between RCT and κ-CN content is exclusively attributable to the glycosylated fraction of the protein. Because exploitable additive genetic variation in G-κCN exists, improvement of κ-CN composition through selective breeding might be an effective way to enhance milk coagulation properties.
authors: Department of Comparative Biomedicine and Food Science (BCA), University of Padova, viale dell'Università 16, 35020, Legnaro, Padova, Italy. Electronic address: valentina.bonfatti@unipd.it.
authors: V
authors: V
authors: Bonfatti
authors: Department of Comparative Biomedicine and Food Science (BCA), University of Padova, viale dell'Università 16, 35020, Legnaro, Padova, Italy.
authors: G
authors: G
authors: Chiarot
authors: Department of Comparative Biomedicine and Food Science (BCA), University of Padova, viale dell'Università 16, 35020, Legnaro, Padova, Italy.
authors: P
authors: P
authors: Carnier
mesh_terms: D000818:Animals; D002364:Caseins / Q000378:metabolism*; D002417:Cattle / Q000235:genetics*; D012085:Chymosin / Q000378:metabolism*; D005260:Female; D014644:Genetic Variation; D006031:Glycosylation; D008892:Milk / Q000737:chemistry*
chemical_list: D002364:Caseins; C025350:rennet; D012085:Chymosin
keywords: casein; coagulation; glycosylation; heritability