Department of Animal Science, University of Padova, viale dell'Università 16, 35020, Legnaro, Padova, Italy.
J Dairy Sci. 2010 Aug;93(8):3809-17. doi: 10.3168/jds.2009-2779.
The aim of this study was to investigate the effects of CSN2-CSN3 (beta-kappa-casein) haplotypes, BLG (beta-lactoglobulin) genotypes, content of milk protein fractions, and protein composition on coagulation properties of milk (MCP). Rennet coagulation time (RCT) and curd firmness (a(30)) were measured using a computerized renneting meter, and the contents of major milk protein fractions were quantified by reversed-phase HPLC in individual milk samples of 2,167 Simmental cows. Cow genotypes at CSN2, CSN3, and BLG were ascertained by reversed-phase HPLC, and CSN2-CSN3 haplotype probabilities were estimated for each cow. Phenotypes for MCP were regressed on CSN2-CSN3 haplotype probabilities using linear models that also included the effects of herd-test-day, parity, days in milk, pH, somatic cell score, renneting meter sensor, sire of the cow, BLG genotype, and content of major protein fractions or, alternatively, protein composition. When the statistical model did not account for protein fraction contents or protein composition, haplotypes carrying CSN3 B were associated with shorter RCT and greater a(30) compared with those carrying CSN3 A. Haplotypes carrying CSN2 B had the effect of decreasing RCT and increasing a(30) relative to haplotype A(2)A. When effects of protein fractions content or protein composition were added to the model, no difference across haplotypes due to CSN3 and CSN2 alleles was observed for MCP, with the exception of the effect of CSN2 B on RCT, which remained markedly favorable. Hence, the effect of CSN3 B on MCP is related to a variation in protein composition caused by the allele-specific expression of kappa-casein, rather than to a direct role of the protein variant on the coagulation process. In addition, the favorable effect exerted by CSN2 B on a(30) was caused by the increased beta-casein content in milk. Conversely, CSN2 B is likely to exert a direct genetic effect on RCT, which does not depend upon variation of beta-casein content associated with CSN2 B. Increased RCT was observed for milk yielded by BLG BB cows, even when models accounted for protein composition. Rennet clotting time was favorably affected by kappa-casein content and percentage of kappa-casein to total casein, whereas a(30) increased when contents and percentages of beta-CN and kappa-CN increased. Changes of milk protein composition and allele frequency at casein and whey protein genes affect variation of MCP.
本研究旨在探讨 CSN2-CSN3（β-κ-酪蛋白）单倍型、BLG（β-乳球蛋白）基因型、乳蛋白组分含量和蛋白组成对乳凝固特性（MCP）的影响。使用计算机凝乳仪测定凝乳时间（RCT）和凝乳强度（a（30）），并在 2167 头西门塔尔牛的个体乳样中通过反相高效液相色谱法定量测定主要乳蛋白组分的含量。通过反相高效液相色谱法确定 CSN2、CSN3 和 BLG 奶牛的基因型，并估计每个奶牛 CSN2-CSN3 单倍型的概率。使用线性模型将 MCP 表型回归到 CSN2-CSN3 单倍型概率上，该模型还包括群体-测试日、胎次、泌乳天数、pH 值、体细胞评分、凝乳仪传感器、奶牛的 sire、BLG 基因型以及主要蛋白组分的含量或蛋白组成的影响。当统计模型不考虑蛋白组分含量或蛋白组成时，与携带 CSN3A 的单倍型相比，携带 CSN3B 的单倍型具有较短的 RCT 和较大的 a（30）。与 A（2）A 单倍型相比，携带 CSN2B 的单倍型具有降低 RCT 和增加 a（30）的作用。当向模型中添加蛋白组分含量或蛋白组成的效应时，由于 CSN3 和 CSN2 等位基因，除了 CSN2B 对 RCT 的有利影响仍然明显外，在 MCP 方面没有观察到单倍型之间的差异。因此，CSN3B 对 MCP 的影响与κ-酪蛋白的等位基因特异性表达引起的蛋白组成的变化有关，而不是与蛋白变体对凝固过程的直接作用有关。此外，CSN2B 对 a（30）的有利影响是由于乳中β-酪蛋白含量增加所致。相反，CSN2B 可能对 RCT 产生直接的遗传影响，而这种影响不依赖于与 CSN2B 相关的β-酪蛋白含量的变化。即使在模型考虑蛋白组成的情况下，BLG BB 奶牛所产牛奶的 RCT 也会增加。凝乳时间受κ-酪蛋白含量和κ-酪蛋白与总酪蛋白的百分比的有利影响，而当β-CN 和κ-CN 的含量和百分比增加时，a（30）增加。乳蛋白组成和干酪蛋白和乳清蛋白基因等位基因频率的变化会影响 MCP 的变化。
