Interfacial Properties of Saponin Extracts and Their Impact on Foam Characteristics

Saponins from various botanical origins highly differ in molecular structure. Little is known of the influence of structural differences between the different saponins on interfacial tension, short-term adsorption and foam properties at the air-water interface (a/w). In this study five triterpenoid saponins, with three of these being monodesmosidic and two bidesmosidic as well as one steroid saponin, were analyzed. Interfacial tension isotherms were measured using a tensiometer with a Wilhelmy plate and were fitted using the modified Frumkin model. For characterization of the short-term adsorption at the a/w-interface, two-fluid needle experiments were performed. Foaming, foam stability and foam structure were analyzed using a foaming device. A new method for semi-quantitative analysis of different foam structures was established. Additionally the impact of pH and ionic strength (addition of NaCl) on interfacial tension and foam properties were determined. The short-term adsorption of all saponins was limited by an additional barrier and was not diffusion-limited. Extracts from Quillaja saponaria Molina (QS), Gypsophila (GYP), Camellia oleifera Abel (TS) and Aesculus hippocastanum (ESC) lowered the interfacial tension to 37-42 mN/m and produced stable foams. The steroid saponin from Tribulus terrestris (TT) and the monodesmosidic saponin from Glycyrrhiza glabra (GA) had only poor interfacial and foam properties. Foams made from QS and GYP were only little affected by changes in pH and ionic strength. A reduction of the pH from 5 to 3 increased stability of foams made from GA significantly. Foams made from ESC and TS were negatively affected by increasing ionic strength.