Surface activity of the oat, horse chestnut, cowherb, soybean, quinoa and soapwort extracts - Is it only due to saponins?

Saponins - amphiphilic molecules produced by plants, can be relatively easily extracted and applied as replacements of synthetic surfactants, e.g. in cosmetic, pharmaceutical or food products. In this contribution, surface activity of aqueous extracts of oat (Avena sativa), horse chestnut (Aesculus hippocastanum), quinoa (Chenopodium quinoa)), soapwort (Saponaria officinalis), cowherb (Vaccaria hispanica) and soybean (Glycine max) plants has been assessed by analysis of surface tension, surface compression (dilational) rheology, foamability and foam stability. We observed that the extracts significantly differ in their surface activity, depending on the plant, its organ, and extraction method. The UPLC-MS analysis did not show strong correlation between the total saponin content and the observed surface activity. For some allegedly saponin-rich plants (especially oat, soybean and cowherb), the saponin concentration in aqueous extracts was found too low and variable to explain their surface activity, pointing to the crucial role of non-saponin extract components.