Rheology as a probe of protein-polysaccharide interactions in oil-in-water emulsions

The influence of the anionic polysaccharide dextran sulphate on the dynamic oscillatory rheology of concentrated oil-in-water emulsions (40 vol % n-tetradecane, 2.6 wt % protein, droplet diameter similar to 0.6 mu m) prepared with bovine serum albumin (BSA), beta-lactoglobulin or sodium caseinate has been investigated at 30 degrees C. At neutral pH and low ionic strength, bridging flocculation of the BSA emulsion at low polymer additions leads to the formation of a characteristic strong gel structure, but no discernible bridging flocculation of beta-lactoglobulin or caseinate emulsions. Under the same solution conditions but higher added polysaccharide contents, while the globular protein emulsions form weak gels of low complex modulus, there is substantial thickening of the caseinate emulsions to form viscoelastic solutions of high complex modulus. For the mixed systems containing BSA. or beta-lactoglobulin + dextran sulphate, the frequency-dependent storage and loss moduli of the emulsions are broadly consistent with mechanisms inferred from previously reported measurements of emulsion droplet-size distributions, surface rheology of adsorbed layers, droplet electrophoretic mobilities, and foam stability. Changes in emulsion rheological properties with increase in ionic strength or reduction of pH, together with new electrophoretic mobility data at low pH values, can be interpreted in terms of the changing nature and strength of electrostatic protein-polysaccharide interactions at the emulsion droplet surface. This research demonstrates the considerable potential of small-deformation rheology for sensitively monitoring the flocculation behaviour of complex food emulsions containing mixtures of proteins and polysaccharides.