Microstructure and rheology of β-lactoglobulin–galactomannan aqueous mixtures

The effect of the addition of a galactomannan (locust bean gum, LBG, or tara gum, TG) on the microstructure and rheological properties of a globular protein (β-lactoglobulin, β-Lg) solution was studied at pH 7.0, when the protein bears a net negative charge. Confocal laser scanning microscopy was used to explore the microstructure. Steady shear and dynamic oscillatory measurements were performed with a controlled stress rheometer AR2000 (TA Instruments) fitted with a cone-and-plate geometry. Mixtures were prepared with 6.5 wt% β-lactoglobulin concentration and 0.31–0.82 wt% LBG or 0.23–0.71 wt% TG concentration. All mixed systems were two-phase. The microstructure was clearly dependent on the concentration of the galactomannan in the mixture: the systems evolved from a continuous matrix of β-lactoglobulin enriched phase containing some small inclusions of the galactomannan, to a matrix of galactomannan-enriched continuous phase containing aggregates of β-lactoglobulin. Modifications of the flow and viscoelastic properties with respect to the individual components were clearly evidenced for the mixed systems. Phase inversion detected by microscopy could also be detected by rheology as a modification in the flow/viscoelastic behaviour.