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The enthalpies of reactions between α s1 -casein and Ca 2+ in solution were measured using a gradient layer calorimeter. The reactions are exothermic between 0 and 4.3 mM CaCl 2 . In the region of 4.3 mM CaCl 2 there is a change to an endothermic reaction corresponding to micellisation. A calcium-binding curve has been obtained under the same conditions as the calorimetry experiments and this shows two sigmoidal binding phases. Turbidity measurements show that there is an association process corresponding to the second sigmoidal phase. A tentative interpretation of the heat curve in the region before micellisation is given in terms of the site binding of Ca 2+ , conformational changes in the protein and association. The main thermal processes are taken to be exothermic intramolecular hydrogen bonding induced by calcium binding and endothermic hydrophobic bonding.

The average molecular weight, particle radius and size distribution of particles in skim milk from eight cows in mid-lactation have been measured by means of elastic and quasi-elastic light scattering techniques. The properties of sub-micellar casein particles in the milk of each cow were also studied. Particular attention has been given to the effects of particle size heterogeneity in the interpretation of results. The weight average molecular weight of the particles from the different cows varied from 2.6·10 8 to 15·10 8 and the corresponding average particle radius varied between 90 and 130 nm. An unusual feature of these particles is their high water content, which was found to vary from 2.4 to 6.4 ml/g with a positive correlation between average particle density and average particle mass. Variations in particle water content can be most readily understood in terms of a gel-like casein micelle.

A method of measuring an average particle radius in a highly polydisperse dispersion using the wavelength dependence of turbidity is described. For particles which are no larger than 0.3 of the wavelength of light used, a polynomial representation of the scattering cross-section can be used. For larger particles, more extensive numerical calculations are required. The use of the method is illustrated by determining the average particle radius of casein micelles by both elastic and quasi-elastic light scattering techniques. A polydisperse homogeneous sphere model is found to be a reasonably accurate representation of casein micelles. Several modifications of the model which would improve the agreement between the two techniques are mentioned briefly.

Calcium in cow's milk is mainly in the form of calcium phosphate-phosphoprotein complexes known as casein micelles. These micelles, in contrast to other phosphoprotein complexes in bone and other tissues, can be readily isolated and studied, but conventional techniques have given ambiguous and conflicting evidence on the structure of milk calcium phosphate. Extended X-ray absorption fine structure and near-edge structure measurements at the newly commissioned Synchrotron Radiation Source at Daresbury indicate that it closely resembles brushite, CaHPO4 X 2H2O. This result, and chemical analysis, requires that phosphate groups from the matrix phosphoproteins be incorporated in the brushite lattice, probably in the surface, suggesting that these organic phosphate groups act as heterogeneous nucleation sites for phase separation of the calcium phosphate from solution.