Mixtures of Quillaja saponin and beta-lactoglobulin at the oil/water-interface: Adsorption, interfacial rheology and emulsion properties

Aim of the present study was to investigate the interfacial properties of mixed films of Quillaja saponins (QS) and beta-lactoglobulin (β-LG) at the oil/water-interface. It was hypothesized that due to the differences in the physical characteristics of the dispersed phase molecular interactions and film characteristics at the oil/water interface substantially differ from the air/water-interface. Furthermore QS/β-LG-interactions will affect stability of emulsions in a concentration-dependent manner. Oscillating drop experiments were performed with subsequent analysis of raw data (Lissajous-plots) to discover non-linear behavior upon compression and expansion. Interfacial shear rheology as well as dynamic interfacial tension and two-fluid needle experiments were performed to comprehensively characterize interfacial properties of mixtures of QS/β-LG. Finally, emulsions were prepared and their stability, oil droplet size and ζ-potential were determined. It became obvious that QS dominates the interfacial film in a binary mixture as indicated by dynamic interfacial tension and dilational rheology. Strain stiffening was observed for mixed QS/β-LG interfacial layers upon dilational expansion at an amplitude above 2.8%. Intermolecular interactions increased in mixtures of QS/β-LG as indicated by shear rheology. Emulsion experiments showed extensive aggregation of oil droplets in QS/β-LG-emulsions with high content of β-LG. Aggregation of oil droplets increased velocity of creaming and after 7 days a distinct creaming layer was visible. Changes in dispersity were influenced by concentration and ratio of QS and β-LG.