Stability of emulsions containing sodium caseinate and dextran sulfate: Relationship to complexation in solution

Abstract

The effect of addition of the anionic polysaccharide dextran sulfate (DS) on the properties of n-tetradecane-in-water emulsions containing sodium caseinate (CN) has been studied under different pH and ionic strength conditions. Two methods were used for preparation of the emulsions (20 vol% oil, 0.5 wt% CN, 0.1–1 wt% DS). In ‘bilayer emulsions’, the DS was added to a CN-stabilized emulsion after emulsification; in ‘mixed emulsions’, the oil was directly emulsified into the mixed biopolymer solution. At pH=6, DS was found to adsorb onto CN-coated droplets, and extensive aggregation due to bridging flocculation occurred at 0.1 wt% DS. Polymeric stabilization was observed at higher polysaccharide concentration (1 wt% DS). In the mixed emulsions, no bridging flocculation was observed at any DS concentrations. These contrasting results for the two preparation methods indicate that strong interactions are present already at a pH value above the isoelectric point of the protein, and that the structure of the adsorbed layer at the oil–water interface is different for the two sets of systems. When the pH of emulsions containing 0.5 wt% DS or less was lowered towards acidic values, precipitation was observed. The mixed and bilayer emulsions containing 1 wt% DS were found to be more stable at pH=2 than the emulsion without DS. Mixed emulsions containing 1 wt% DS exhibited good shelf-life stability on quiescent storage for 3 weeks, especially when the emulsion was prepared at low pH. Complexes were found to dissociate when 0.1 or 0.5 M NaCl was present, suggesting that associative interactions between CN and DS are mainly electrostatic.