Saponins — Self-assembly and behavior at aqueous interfaces

Saponins are interfacially active ingredients in plants consisting of a hydrophobic aglycone structure with hydrophilic sugar residues. Variations in aglycone structure as well as type and amount of sugar residues occur depending on the botanical origin. Saponins are a heterogeneous and broad class of natural substances and therefore the relationship between molecular structure and interfacial properties is complex and, yet, not completely understood. A wide range of research focused either on structural elucidation of saponins or interfacial properties. This review combines recent knowledge on structural features with interfacial properties and draws conclusions on how saponin structure affects interfacial properties. Fundamental understanding on interfacial configuration of individual saponin molecules at the interface distinctly increased. It was shown that interfacial configuration may differ depending on botanical origin and thus structure of the saponins. The formation of strong viscoelastic interfacial films by some saponins was attributed to hydrogen bonds between neighboring sugar residues. Few studies analyzed the relationship between botanical origin and interfacial rheology and derived main conclusions on important structural features. Saponins with a triterpenoid structure are most likely to form viscoelastic films, which result in stable foams and emulsions. The aglycone subtype may also affect interfacial properties as triterpenoid saponins of oleanane type formed most stable interfacial networks. But for more reliable conclusions more saponins from other aglycone subtypes (dammarane, ursolic) have to be analyzed. To-date only extracts from Quillaja saponaria Molina are approved for food products and many studies focused on these extracts. From experiments on interfacial rheology a reasonable model for supramolecular structure of Quillaja saponins was developed. It was further shown that Quillaja saponins may form micelles loaded with hydrophobic substances, nano-emulsions and stable foams. In combination proteins an increase in interfacial film stability may be observed but also negative phenomena like aggregation of oil droplets in emulsions may occur.