Saponins

Saponins are a diverse class of naturally occurring plant glycosides that produce a characteristic foaming in aqueous solutions. They consist of a lipophilic aglycone, called a sapogenin, linked to one or more hydrophilic sugar chains. The combination gives amphipathic properties that disrupt lipid membranes and allow saponins to act as detergents and emulsifiers in biological systems and in industry.

Two major structural types are triterpenoid saponins and steroidal saponins, depending on the core sapogenin. The sugar moieties are usually glucose, galactose, rhamnose, or arabinose, and their number and linkage determine solubility and bioactivity. Saponins occur widely in plants, notably in legumes such as soybeans and chickpeas, in quinoa seeds, as well as in many herbs and shrubs.

Biologically, saponins serve as defense compounds against herbivores, fungi, and bacteria, and can affect plant germination and soil microbiota. They can bind cholesterol and interact with membranes, which contributes to their characteristic foaming and potential toxicity. In humans, intake can irritate the gastrointestinal tract in sensitive individuals; some saponins have been studied for anti-inflammatory or anticancer properties, though effects are highly compound-specific. Certain saponins are used commercially as emulsifiers, foaming agents, or vaccine adjuvants (for example, QS-21 derived from Quillaja saponaria). Some also have pharmaceutical applications or serve as precursors for steroid synthesis.

Examples include glycyrrhizin from licorice (a triterpenoid saponin), soyasaponins from soy, and diosgenin-derived sapogenins used in steroid chemistry. Processing steps such as washing, soaking, or decortication often reduce saponin content in foods, mitigating bitterness and antinutritional effects.