Influence of pH and iota-carrageenan concentration on physicochemical properties and stability of beta-lactoglobulin-stabilized oil-in-water emulsions

The influence of pH and iota-carrageenan concentration on the properties of beta-lactoglobulin (beta-Lg)-stabilized oil-in-water emulsions was investigated by measuring the particle charge, particle size distribution, and creaming stability. Emulsions containing droplets stabilized by beta-Lg were produced by homogenization, and then, iota-carrageenan was added. At pH 3, the droplet charge did not change for iota-carrageenan concentrations <or=0.1 wt % but decreased rapidly at high concentrations, while the mean particle diameter increased slightly as the iota-carrageenan concentration was increased. These results suggest that the interaction between iota-carrageenan and beta-Lg was weak at pH 3 probably because some sulfate groups were protonated (pK(a) = 2). At pH 4 and pH 5, the droplet charge decreased dramatically as the iota-carrageenan concentration was increased from 0 to 0.15 wt %, but droplet aggregation and creaming occurred in the emulsions, indicating that interfacial complexes between iota-carrageenan and beta-Lg could not stabilize the emulsions, probably due to bridging flocculation. At pH 6, the droplet charge in the primary emulsions was negative and became more negative as the iota-carrageenan concentration was increased. The mean particle diameter was relatively small at all iota-carrageenan concentrations, and emulsions were stable to creaming after 1 week of storage. We propose that carrageenan adsorbed to the droplet surfaces and increased the electrostatic repulsion between droplets. At pH 7 and pH 8, the droplet charge did not change as the iota-carrageenan concentration was increased, but these emulsions became unstable to creaming above a critical carrageenan concentration, which was attributed to depletion flocculation.