Osmometric and viscometric study of levan, β-lactoglobulin and their mixtures

Abstract

The current study investigated the molecular interactions of the exopolysaccharide levan (low and high molecular weight), β-lactoglobulin and their mixtures at different NaCl concentrations (2.5 mM and 100 mM, pH 7) using viscometry and membrane osmometry. Both methods were used to predict the phase behavior of the mixed polymer systems. A positive cross-virial coefficient indicates repulsive pair interactions between levan and β-lactoglobulin due to the excluded volume effect at both salt concentrations. A higher molecular weight of levan seemed to enhance the repulsive forces, while the NaCl concentration had only a minor influence. For the phase behavior, the charge of the β-lactoglobulin and therefore the NaCl concentration plays an important role. At low salt concentrations the repulsive, electrostatic forces between the individual β-lactoglobulin molecules counteract the concentration of the protein in one phase. Therefore, both polymers seem to be co-soluble regardless of the molecular weight of levan. At 100 mM NaCl, the electrostatic repulsion between the β-lactoglobulin molecules is screened and attractive protein - protein interactions predominated. This facilitates the formation of two phases and segregative phase separation due to the excluded volume effect becomes more likely.