Light scattering study of sodium caseinate + dextran sulfate in aqueous solution: Relationship to emulsion stability

Using combined static and dynamic light scattering, various structural and thermodynamic parameters have been determined for the complex particles formed from sodium caseinate (0.5 wt/v%) + dextran sulfate (0.01, 0.1 or 1.0 wt/v%) in aqueous solution at pH = 6.0. The polysaccharide contents refer, respectively, to three polysaccharide/protein molar ratios (R = 1, 10 and 100) calculated on the basis of the measured values of the weight-average molar masses of sodium caseinate particles and dextran sulfate molecules. The complexes were prepared by mixing together the two biopolymer components in bulk solution or bringing them together at the interface in a protein-stabilized foam. The results indicate dissociation of the original sodium caseinate particles in response to associative interactions with excess amounts of negatively charged polysaccharide. A significant difference was observed between properties of complexes formed in solution and those formed at the interface, especially for R = 100. We identify a possible correlation between the structures of these complexes and the recently reported stability properties of oil-in-water emulsions containing the same biopolymers (Jourdain, L., Leser, M. E., Schmitt, C., Michel, M., & Dickinson, E. (2008). Food Hydrocolloids, 22, 647–659). In particular, we infer that the greater hydrophilicity and the more open/bulky architecture of complexes formed in the bulk aqueous phase are better able to provide effective steric/electrostatic stabilization of the so-called “mixed” emulsions, as compared with the interfacial complexes formed in the so-called “bilayer” emulsions. We also present results on the effect of ionic strength on the structural parameters of the complexes, and we attempt to interpret the data in the context of previously determined stability behaviour of the corresponding emulsions.