Natural variation in casein composition of milk

Bovine milk contains 3-4 % protein and almost 80% of the milk protein fraction consist of four caseins; α_s1-casein, β-casein, α_s2-casein and κ-casein. Most of the caseins in milk are assembled in casein micelles, which consist of several thousands of individual casein molecules and salts. The unique structure of casein micelles allows the delivery of large amounts of calcium and phosphate to the neonate. Considerable natural variation in casein content and composition exists between milk samples from individual cows, however the impact of these variations on casein micelle structure and technological properties of dairy products are largely unknown. This study showed that the expression of caseins and their post-translational modification as well as inclusion of calcium in casein micelles are well-balanced processes. Analysis of the genetic background of phosphorylation of α_s1-casein with eight or nine phosphate groupsshowed that α_s1-casein phosphorylation was regulated by a different set of genes. Also, variation in α_s1-casein phosphorylation resulted in changes in the core of casein micelles and was associated with a difference in degradation efficiency by chymosin in milk gels. Another post-translational modification, glycosylation of κ-casein, resulted in changes in the surface of casein micelles. Natural variation in α_s1-CN phosphorylationand glycosylation of κ-casein are both relevant factors to consider for optimization of dairy products and the design of future breeding strategies.