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Particle size distributions of skim milk, diluted with its own diffusate have been determined using the technique of clipped digital autocorrelation of scattered laser light. Particle size distributions found by an analysis of the observed autocorrelation functions have peaks at sizes similar to those found in recent electron microscopic and quasi-elastic light scattering investigations but the distributions are broader.

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.