Structure formation and phase-separation behaviour of aqueous casein–alginate emulsions in the presence of strong polyelectrolyte

Rheo-small angle light scattering (SALS), optical microscopy (OM), rheology, phase composition analysis, and environment scanning electron microscopy (ESEM) were applied to characterize the effect of strong polyelectrolyte on the structure formation and phase-separation behaviour in aqueous sodium caseinate–sodium alginate (W–SC–SA) water-in-water emulsion with the droplet morphology in the concentration ranges both near and far from the critical point (CP). Addition of 0.04–1.5 wt% of dextran sulfate sodium salt (DSS) to the emulsion placed on the phase diagram close to the CP leads at pH 7.0 and ionic strength (I) = 0.002 to a considerable decrease in the compatibility of SC with SA, disappearance of the droplet morphology, change in solvent distribution between coexisting phases, and a dramatic increase in moduli (G′ and G″) and viscosity, due to formation of water soluble intermacromolecular SC–DSS associates with the pronounced network structure. The maximal relative changes in rheological properties take place at the 10/1 “molar” ratio SC/DSS, and at 6–10 wt% content of SC in the water-in-water emulsion. Beta casein interacts with DSS in less degree than other casein fractions. Near the critical point of phase separation, when the concentration of SC is not so high (6 wt%), the complexes are formed and stabilized via electrostatic interactions, but at higher SC concentrations the contribution of secondary hydrogen bonds and hydrophobic interactions becomes obvious at a higher ionic strength (I = 1.13). Addition of DSS to the water-in-water emulsion placed on the phase diagram far from the CP leads to reinforcement of the SC–DSS network, partial inclusion of SA in the SC–DSS network, and as a consequence, increases in cosolubility of SC with SA. The molecular origin of the unusually strong interaction of SC with DSS at a high concentration range on the “wrong side” of pH values, far from isoelectric point of caseins is discussed. Experimental observations suggest that use of small amount of sulfate polysaccharide in aqueous water-in-water biopolymer emulsions is a promising tool for regulation their structure, phase behaviour, and rheological properties.