, Volume 91, Issue 1, pp 39-51
Date: 02 Aug 2010

Extreme frequencies of the αs1-casein “null” variant in milk from Norwegian dairy goats— implications for milk composition, micellar size and renneting properties

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Abstract

Caprine αs1-casein polymorphism is one of the key factors which determines important technological properties of milk, such as rennetability and cheese yield. The indigenous Norwegian goat breed is characterized by a remarkably high frequency (>70%) of a “null” variant of αs1-casein unique to this population. This mutation, referred to as the exon 12 D allele, involves a single base deletion in exon 12 of the αs1-casein gene. The aim of this study was to provide more detailed information on how αs1-casein polymorphism affects composition and rennet coagulation properties of milk from Norwegian dairy goats. Class of αs1-casein significantly affected milk composition; crude protein, casein and pH. The mean casein micelle size was larger in milks containing αs1-casein “null” variant when compared to “strong” milks; in contrast, the heat-induced changes in micelle size appeared to be least pronounced in this group. Compositional differences between “strong” and “null” milks may explain these differences. αs1-Casein class had a significant effect on gel strength recorded 30 min after the addition of rennet, with the “strong” milks giving the greatest firmness. However, a considerable fraction of the milk samples in this study were unable to form a strong clot, regardless of the αs1-casein variant. These results indicate that environmental and compositional factors not considered in this study may be important in the curd formation process.

摘要

山羊αs1-酪蛋白遗传多态性是决定乳重要技术特性(如凝乳特性和干酪产量) 的重要因素之一。 来自本土挪威山羊品种的乳非常独特、 这个群体中αs1-酪蛋白“零” 型变异体出现的频率高达70% 以上。 参照第12 外显子D 等位基因、 这个突变体涉及到第12 外显子中单个碱基的缺失。 本研究目的在于提供、 关于αs1-酪蛋白遗传多态性对挪威乳山羊乳组成和凝乳特性影响更为详尽的信息。 αs1-酪蛋白的类型显著影响着乳组成、 粗蛋白、 酪蛋白和 pH。 与含有αs1-酪蛋白“强”型变异体的乳相比、 含“零”型变异体的乳、 形成的酪蛋白胶束的平均直径更大、 且由热诱导对胶束尺寸的影响更小。 包含“强” 型和“零” 型变异体的乳组成的差异可以解释上述区别。 αs1-酪蛋白类型对在加入凝乳酶30 分钟后的胶强度具有显著的影响。 含有“强” 型变异体的乳具有最大的硬度。 然而、 本研究中、 无论含有何种αs1-酪蛋白变异体类型、 乳样中相当大比例不能形成强的凝块。 这些结果表明本研究中没有考虑的环境和组成因素、 可能对凝块的形成过程起到重要作用。