Effect of the A and B variants of both alpha s1- and kappa-casein on bovine casein micelle solvation and kappa-casein content.

Casein micelle solvation, a micelle characteristic that is sensitive to many factors, has been measured by a centrifugation technique at 30 degrees C for a series of uncooled fresh skim milks at pH 6.3, 6.6, 6.9 and 7.1. The relative alpha s-(alpha s1-plus alpha s2-), beta- and kappa-casein contents of all centrifuge pellets and supernatants were determined by a standardized electrophoretic method. The calcium and phosphate contents of a number of the pellets and milk samples were also determined. Solvation of micelles from milks with various genetic variants of beta-lactoglobulin (A and B), alpha s1-casein (A and B) and kappa-casein (A and B) was often found to be lower for milks containing either the B variant of alpha s1-casein or the A variant of kappa-casein. It was also found that these two variant caseins were associated with a lower kappa-casein. It was also found that these two variant caseins were associated with a lower kappa-casein content of the milks and the micelles, which is consistent with the lower solvation as kappa-casein is associated with smaller micelle size and greater solvation. The solvations also seemed to increase during the lactation period. It is possible that some of the other features of milk and its products that have been ascribed to the differences in functional character between the A and B variants of alpha s1-casein may be partly caused by the increased level of kappa-casein. The reason for the association of the A variant of alpha s1-casein with higher concentrations of kappa-casein (and micelle solvation) is not obvious but possibly the haplotype alpha s1-casein A, beta-casein A1, kappa-casein A contains a controlling sequence in the chromosomal DNA that enhances expression of the kappa-casein gene.