The formation of electrostatic complexes of whey protein (WP) and a nongelling carrageenan (CG) was investigated as a function of pH, ionic strength, temperature, and protein-to-polysaccharide (Pr:Ps) ratio. On lowering the pH, the formation of soluble WP/CG complexes was initiated at pH(c) and insoluble complexes at pH(phi), below which precipitation occurred. The values of the transition pH varied as a function of the ionic strength. It was shown that at [NaCl] = 45 mM, the value of pH(phi) was the highest, showing that the presence of monovalent ions was favorable to the formation of complexes by screening the residual negative charges of the CG. When CaCl(2) was added to the mixtures, complexes of WP/CG were formed up to pH 8 via calcium bridging. The electrostatic nature of the primary interaction was confirmed from the slight effect of temperature on the pH(phi). Increasing the Pr:Ps ratio led to an increase of the pH(phi) until a ratio of 30:1 (w/w), at which saturation of the CG chain seemed to be reached. The behavior of WP/CG complexes was investigated at a low Pr:Ps ratio, when the biopolymers were mixed directly at low pH. It resulted in an increase of the pH of the mixture, as compared to the initial pH of the separate WP and CG solutions. The pH increase was accompanied by a decrease in conductivity. The trapping of protons inside the complex probably resulted from a residual negative charge on the CG. If NaCl was present in the mixture, the complex took up the Na(+) ions instead of the H(+) ions.

Whey proteins (WPs) and the exopolysaccharide B40 (EPS B40) form electrostatic complexes under specific conditions. EPS B40 is a natural thickener in yogurt-like products. It is a phosphated polysaccharide and thus has a strong polyelectrolyte character. When the WP and the EPS B40 were mixed at pH values near or below the isoelectric point (pI) of the protein, soluble complexes were formed at pHc and phase separation took place below pHφ. The formation and the structure of those complexes were studied by various methods, including turbidity, dynamic and static light scattering (DLS and SLS), and viscosity measurements. The results showed that the strength of the interaction was strongly pH- and salt-dependent. The ζ-potential of the protein at pHc and pHφ was linearly dependent on the square root of the ionic strength (√I), showing the electrostatic nature of the interaction. Light scattering and viscosity measurements provided new results on the behavior of the complexes at the molecular level. In the region where the complexes were still soluble and at low ionic strength, the DLS radius measured in the WP/EPS B40 mixture was smaller than the coil size in the EPS B40 solution but the apparent molar mass was increased. The increase of the molecular mass was attributed to the complexation of WP on the EPS B40 chain, which, at low salt, induced a reduction of the intramolecular repulsion and led to the compaction of the polysaccharide. Also, the ratio of protein to polysaccharide was varied in order to get more insight into the dynamics, the structure, and the apparent stoichiometry of the EPS B40/WP complexes. The results illustrated that phase separation was a consequence of charge neutralization of the complexes and that the apparent stoichiometry of the complexes depends on the order of mixing of the compounds. In time, the complexes rearranged to form neutralized complexes and free EPS B40. The concept of cooperative binding was highlighted in the case studied.

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This study investigated the competitive adsorption between milk proteins and model milk membrane lipids at the oil-water interface and its dependence on the state of the lipid dispersion and the formation of emulsions. Both protein and membrane lipid surface load were determined using a serum depletion technique. The membrane lipid mixture used was a model milk membrane lipid system, containing dioleoylphosphatidylcholine, dioleoylphosphatidylethanolamine, milk sphingomyelin, dioleoylphosphatidylserine, and soybean phosphatidylinositol. The model composition mimics the lipid composition of natural milk fat globule membranes. The interactions were studied for two proteins, beta-lactoglobulin and beta-casein. The mixing order was varied to allow for differentiation between equilibrium structures and nonequilibrium structures. The results showed more than monolayer adsorption for most combinations. Proteins dominated at the oil-water interface in the protein-emulsified emulsion even after 48 h of exposure to a vesicular dispersion of membrane lipids. The membrane lipids dominated the oil-water interface in the case of the membrane lipid emulsified emulsion even after equilibration with a protein solution. Protein displacement with time was observed only for emulsions in which both membrane lipids and beta-casein were included during the emulsification. This study shows that kinetics controls the structures rather than the thermodynamic equilibrium, possibly resulting in structures more complex than an adsorbed monolayer. Thus, it can be expected that procedures such as the mixing order during emulsion preparation are of crucial importance to the emulsification performance.