The casein micelles of bovine milk are large aggregates of roughly spherical shape consisting of the four phosphoproteins αs1-, αs2-, β- and κ-casein (CN). They are polydisperse colloidal particles with weight-average diameters between about 50 and 500 nm (Horne, 1984). Since αs1-, αs2- and β-CN are sensitive to calcium it is assumed that they are located predominantly within the micelle, whereas κ-CN is mainly on the surface of the micelles and serves as a stabilizer for the aggregates owing to its hydrophilic C-terminus. Inorganic calcium phosphate interacts with the phosphate groups of the caseins. This conformation of caseins, calcium and phosphate also contains small amounts of magnesium and citrate. Of the dry matter of the micelle, ∼93% is casein and ∼7% inorganic components, mainly calcium and phosphate (McMahon & Brown, 1984; Holt & Horne, 1996).

In several studies it has been demonstrated that milk with the κ-CN B variant has on average smaller micelles than κ-CN A milk (Morini et al. 1975; Nuyts Petit et al. 1997). Compared with κ-CN variants, there are only a few studies on the association of αs1-, αs2- and β-CN variants with casein content and micelle size. In the milk of β-casein-deficient mice, casein micelles were smaller than in normal mouse milk but still present. The lack of β-CN was partly compensated by an increased whey protein content (Kumar et al. 1994). A comparison of the cheesemaking properties of Tarentaise milks containing the A (A1 or A2, without distinguishing between these variants) and C variants of β-CN revealed that the most significant characteristic for discriminating the two types of milk is the average diameter of the casein micelles: 293 nm in β-CN C type milk, 187 nm in the A type (Delacroix-Buchet & Marie, 1994).

The objective of the present study was to investigate the micelle size and the casein composition of milk from half sibs within Simmentals cross bred with Red Holsteins (Simmentaler Fleckvieh, SI×RH) sires heterozygous for A1A2 of β-CN.