Mixtures of saponins and beta-lactoglobulin differ from classical protein/surfactant-systems at the air-water interface

Interactions between surfactants and proteins have been intensively studied in the past because their interactions in food and cosmetic products can tremendously alter the product properties. Recent studies have shown that Quillaja saponins (QS) have very different interfacial properties in comparison to common low-molecular weight surfactants. It was reported that QS forms highly elastic interfacial films, adsorbs more slowly to the interface and cannot as easily be classified as an ionic or non-ionic surfactant. However, the mechanism of interaction between QS and proteins like beta-lactoglobulin (β-LG) is still to be understood. For this purpose the present study aimed to explore the interactions between Quillaja saponin and beta-lactoglobulin in the bulk and at the air/water-interface. At this purpose, interfacial properties were characterized with dynamic interfacial tension, short-term adsorption, shear and dilational oscillation experiments and the results were compared to foam properties. To study molecular interactions fluorescence quenching was analyzed and sequential two-fluid needle experiments were performed to get more insights on depletion of β-LG by QS. The presence of β-LG lowered the dilational viscoelasticity of the mixed films. Although the interfacial film was weakened by β-LG the dilational elasticity was still very high and did not result in reduced foam stability. Interfacial shear results suggest that QS and β-LG can interact at the interface probably through hydrogen bonds and/or hydrophobic interactions. We determined 1.7 binding sites on β-LG in the bulk which are accessible for complex formation for QS by fluorescence quenching experiments. The findings of this work can contribute to a better understanding of the properties of complex systems with mixtures of natural surfactants and proteins.