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 (beta-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/beta-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/beta-LG. Finally, emulsions were prepared and their stability, oil droplet size and zeta-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/beta-LG interfacial layers upon dilational expansion at an amplitude above 2.8%. Intermolecular interactions increased in mixtures of QS/beta-LG as indicated by shear rheology. Emulsion experiments showed extensive aggregation of oil droplets in QS/beta-LG-emulsions with high content of beta-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 beta-LG. (C) 2017 Elsevier B.V. All rights reserved.