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.