The properties of adsorbed films of αs1-, β- and κ-casein and sodium caseinate are compared at the planar oil–water interface, at the surface of emulsion droplets, and at the surface of polystyrene latex particles. Attention is directed towards understanding (i) the relationship between the molecular characteristics of the caseins and their surface activities and surface viscosities at the oil–water interface, (ii) the nature of competitive adsorption between the caseins (particularly αs1- and β-), and (iii) the effect of Na and Ca ion concentrations on electrophoretic mobilities and flocculation behaviour.

Glycosylation has been recognized as enhancing some functional properties of caseinates. The present work was aimed at elucidating the structure–function relationship of the functionality of caseinates and, more particularly, the influence of hydration on these properties. Measurements of non-frozen water content derived from sorption isotherms are presented. Using spin-probing Electron Spin Resonance we studied the mobilization pattern of solutes as hydration changed.The protein interactions with bound water were not significantly affected by sugar binding. Changes in the functional properties as, for example, in the water-holding capacity, reflected alterations in the structure, which was looser and more flexible after glycosylation, and in the associative properties (increased solubility) of caseinates. Disaccharides seemed more effective than monosaccharides in inducing these changes.

β-Casein, was enzymically modified by incubation with plasmin to yield γ-caseins and proteose peptones. Whole γ-, γ1-, γ2/γ3-caseins and whole proteose peptone (pp) were isolated from the hydrolysate mixture. The time dependence of surface tension at the air-water interface of solutions of β-casein and its plasmin derived fragments, at concentrations of 10−1 to 10−4% (w/v) protein, pH 7.0, was determined, at 25 °C, using a drop volume apparatus. The ranking of the proteins with respect to rate of reduction of surface tension, during the first rate determining step, at 10-2% (w/v) protein, was γ2/γ3 ≫ pp > whole γ- > γ1- > β-casein. The ranking of the proteins with respect to surface pressures attained after 40 min (π40) was concentration dependent. γ2/γ3-Caseins were found to be very surface active, decreasing surface tension rapidly and giving a high π40. γ1 Casein decreased surface activity somewhat faster than β-casein, but generally reached a lower π40. Whole γ-casein reflected the properties of both γ1 and γ2/γ3-caseins. Proteose peptone was found to decrease surface tension rapidly during the initial rate determining step; it gave a relatively high π40 at a bulk phase concentration of 10−3% (w/v) protein, but, it was the least surface active protein at 10−1 and 10−2% (w/v) protein.

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.