In order to optimize the use of caseins as surfactants, the surface tension, foaming capacity and stability were measured as a function of pH, ionic strength, protein concentration and polarity (modified by covalent binding of carbohydrates). We found that the caseins differ in their behaviour at the air/water interface with β-casein showing the greatest ability to decrease surface tension and to produce foams, due probably to its amphipathic structure. In experiments carried out at pH values close to pI, with low ionic strength and constant solubility (optimal conditions for foam formation), we observed a high surface hydrophobicity, a good accessibility and flexibility of peptidic side chains (evaluated by proteolysis), and a high foaming capacity parallelled by increased surface pressure. Foam stability of caseins was low compared to those of globular proteins such as β lactoglobulin.

Chemical and physical changes that occur in Na caseinate (at 1 or 2% in water, pH 7·0) on heating in the range 120–150 °C were investigated by polyacrylamide gel electrophoresis, ion-exchange and gel-permeation chromatography, light scattering, u.v. spectroscopy, amino acid analysis, and the formation of pH 4·6 and 12% TCA-soluble N and 12% TCA-soluble P. The electropherograms of heated samples were smeared and indistinct suggesting intermolecular aggregation which was not reversed by 6 M-urea or SDS and indicating covalent bond formation; αs2-casein was especially sensitive. Aggregation was confirmed by ion-exchange chromatography and light scattering. Fragmentation of the caseins also occurred on heating, as indicated by the formation of pH 4·6 and 12% TCA-soluble N and by gel filtration. Formation of soluble N and dephosphorylation followed first-order kinetics. Concentrations of available lysine and available methionine were reduced by 10–15% on heating at 140 °C for 30min; chemical assays for arginine and tryptophan indicated increases, suggesting interference. Ultraviolet spectroscopy indicated a slight apparent increase in aromatic residues after heating at 140 °C for up to 60 min.

The heat-induced changes in sait balance between the colloidal phase of milk and its serum were studied using an ultrafiltration technique. Milk permeate was isolated at the heating temperature by means of a hollow fibre ultrafiltration cartridge coupled with a stainless steel heat exchanger unit. The milk samples initially at 4 °C were heated to 20, 40, 60, 80 or 90 °C. Ca, P, Mg and citrate contents of the permeates were determined. The decreases in Ca and P were proportional to the increase in temperature. Smaller losses in Mg and citrate were observed. An initial sharp decrease in concentration occurred within the first seconds of holding time and was followed by a slower and smaller decrease. The possible occurrence of a two-stage mechanism for the heat-induced salt precipitation is discussed. The precipitation of dicalcium phosphate is believed to occur together with some tricalcium citrate precipitation.

We have investigated the role of pH in the destabilization of milk and sodium caseinate solutions by ethanol. Ethanol shifted the pH of minimum stability of both caseinate and milk to higher values. A linear relation between this pH and the reciprocal dielectric constant of the mixture was observed. The addition of calcium to these alcoholic mixtures also increased the pH of minimum stability. The results are discussed in terms of the alcohol-induced shifts in the pK values of important constituent amino acids and of the effect of alcohol on soluble Ca levels.

The rheological properties of rennet-induced skim milk gels at 30 °C are compared with casein gels formed by acidification to pH 4·6 at 2 °C and subsequent heating to 30 °C. Both types of gels are viscoelastic. However, the gels formed by rennet action are relatively more viscous over time scales longer than 1 s. This implies that on average the protein-protein bonds in these gels are relatively more mobile than in the acid casein gels. It is thought that this is primarily due to differences in the structure of the casein particles and in the type of the main interaction forces in and between the casein molecules and particles.

The concepts of fractals and fractal dimension are introduced and applied to the description of the disordered structures of colloidal aggregates. It is demonstrated that the structure of the aggregates produced by ethanol de-stabilization of casein micelles can be quantitatively characterized by a fractal dimension. The values measured are compared to literature predictions from various computer studies simulating different models of the aggregation process.