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

Caprine alpha(s1)-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 alpha(s1)-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 alpha(s1)-casein gene. The aim of this study was to provide more detailed information on how alpha(s1)-casein polymorphism affects composition and rennet coagulation properties of milk from Norwegian dairy goats. Class of alpha(s1)-casein significantly affected milk composition; crude protein, casein and pH. The mean casein micelle size was larger in milks containing alpha(s1)-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. alpha(s1)-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 alpha(s1)-casein variant. These results indicate that environmental and compositional factors not considered in this study may be important in the curd formation process.