Major Role of Voluminosity in the Compressibility and Sol–Gel Transition of Casein Micelle Dispersions Concentrated at 7 °C and 20 °C

The objective of this work is to bring new information about the influence of temperatures (7 °C and 20 °C) on the equation of state and sol–gel transition behavior of casein micelle dispersions. Casein micelle dispersions have been concentrated and equilibrated at different osmotic pressures using equilibrium dialysis at 7 °C and 20 °C. The osmotic stress technique measured the osmotic pressures of the dispersions over a wide range of concentrations. Rheological properties of concentrated dispersions were then characterized, respectively at 7 °C and at 20 °C. The essential result is that casein micelle dispersions are less compressible at 7 °C than at 20 °C and that concentration of sol–gel transition is lower at 7 °C than at 20 °C, with compressibility defined as the inverse to the resistance to the compression, and that is proportional to the cost to remove water from structure. From our interpretations, these two features were fully consistent with a release of soluble β-casein and nanoclusters CaP and an increased casein micelle hydration and apparent voluminosity at 7 °C as compared with 20 °C.

Hydration of Alkylammonium Salt Micelles - Influence of Bromide and Chloride Counterions

The micellization process of dodecyltrimethylammonium chloride (DTAC) and bromide (DTAB) was studied. Nuclear magnetic resonance method was used. The 1H NM R and 13C NM R spectra were taken at higher and lower concentrations than the critical micelle concentrations (CMC) of the compounds studied. Chemical shifts were analysed. The studies performed were prompted by earlier calorimetric measurements which showed that there were significant qualitative and quantitative differences in the micellization process of the compounds studied. Namely, DTAB micelle dissociation was found to be an endothermic process while that of DTAC was exothermic. The differences found must be the result of differentiated influence of bromide and chloride counterions on the micellization process, including the phenomenon of micelle hydration. The objective of the work was to check whether cationic surfactant counterions can influence the micelle hydration process. Indeed, DTAB and DTAC, as monomers, exhibit similar hydrophobic hydration, but DTAB micelles are more hydrated than DTAC ones. It seems that the differences found in micellization of both salts studied may be attributed to different physicochemical properties of bromide and chloride ions, such as their mobilities and radii of their hydrated forms. Moreover, the effect of anions on the water structure must be taken into account. It is important whether the anions can be classified as water ordering kosmotropes, that hold the first hydration shell tightly, or water disordering chaotropes, that hold water molecules in that shell loosely.

Heat stability of milk: role of β-lactoglobulin in the pH-dependent dissociation of micellar κ-casein

SummaryOn heating casein micelle systems containing β-lactoglobulin (β-lg) at 90°C for 10 min, β-lg complexed with casein micelles at pH < 6·9, probably as a result of interaction with κ-casein via sulphydryl-disulphide interchange, and co-sedimented with the micelles on ultracentrifugation. Complex formation with β-lg appeared to prevent the dissociation of micellar κ-casein on heating. However, at pH ≥ 6·9, κ-casein/β-lg complexes dissociated from the micelles on heating, thus enhancing the release of micellar κ-casein. High concentrations of β-lg (≥0·8%) induced coagulation at pH 7·3, essentially by promoting the dissociation of micellar κ-casein. It appeared that αs1-, αs2-, β- and κ-caseins dissociated from serum protein-free casein micelles to equal extents, but the presence of β-lg specifically enhanced the dissociation of κ-casein at pH values ≥ 6·9. Micelle hydration increased slightly when casein micelles were heated in the presence of β-lg at pH 6·7, while at pH 7·3 β-lg decreased the degree of hydration of casein micelles. Formation of a complex between β-lg and κ-casein appeared to stabilize the micelles in the pH range 6·5–6·7, possibly via increased micellar charge or degree of hydration or by preventing the dissociation of κ-casein.

Effect on the composition and properties of casein micelles of interaction with ionic materials

SummaryThe effect on the composition and properties of casein micelles of the binding of ionic materials which accelerate the coagulation of milk on rennet treatment, was investigated. When considered in terms of their relative charge concentration, all the materials tested caused similar effects. The casein, inorganic phosphate and Ca contents of the micelles increased slightly. Micelle hydration decreased as additive binding increased. Casein and Ca dissociation on cooling increased at low concentrations of bound material, then progressively decreased at higher concentrations. The mean size of micelles and their electrophoretic mobility was little affected by bound ionic materials. The aggregation of the casein complexes in colloidal calcium phosphate-free milk was markedly increased by adding ionic materials, the efficiencies of these additives paralleling their efficiencies in accelerating the coagulation of milk by rennet. The results suggested that the ionic materials were bound in the interior of the casein micelles and promoted aggregation after rennet treatment by shielding charged groups, thus increasing the micellar hydrophobicity.