saponin
Meaning of saponin: Plant compounds named for the lather they raise in water, and the reason quinoa is rinsed before it is cooked.
Definition of saponin
A saponin is a plant compound made of a sugar chain attached to a fat-soluble core, a structure that lets it behave like soap. Shaken with water it foams, and it can break open cell membranes by wedging itself into them. Saponins occur in beans, oats, quinoa, ginseng and many other plants, usually as part of the plant's chemical defense.
Why does the soap comparison work?
A soap molecule has a water-loving end and a grease-loving end, which is what lets it lift oil into water. A saponin is built the same way, with the sugars providing the water-loving end and the ring structure the greasy one. That shape gathers the molecules at the surface of water, producing foam, and drives them into the fatty membranes of cells. A membrane that holds together only by the same fatty attraction is pulled apart, which is why saponins burst red blood cells in a test tube and why many taste bitter and irritate the throat.
Where are they encountered?
| Source | Note |
|---|---|
| Quinoa | The bitter surface coating that is rinsed off before cooking |
| Soybeans and other legumes | Present in ordinary amounts in the diet |
| Ginseng | The ginsenosides are saponins |
| Soapbark tree | A purified fraction is used to strengthen vaccines |
| Yucca and soapwort | Used historically as washing agents |
The vaccine use is the best established medical application. A purified saponin fraction added to a vaccine sharpens the immune response to it, so a smaller amount of the target material does the same work.
What about the health claims?
Two claims recur. The first, that saponins lower cholesterol, has a plausible mechanism behind it: they bind bile acids and cholesterol in the gut so that less is absorbed and more is lost in the stool. The effect measured in people who eat saponin-rich foods is modest and hard to separate from the fiber those foods also carry. The second claim, of anticancer activity, rests mostly on cells grown in dishes, where a compound that damages membranes kills tumor cells and healthy cells alike. No saponin forms part of established anticancer therapy, and the membrane-breaking property that looks promising in a dish is the same one that makes these compounds toxic when injected.
Also known as
Used in a sentence
The extract was standardized to its saponin content.
Good to know
Saponins are poorly absorbed from the gut, so effects seen when they are applied directly to cells or injected in the laboratory do not carry across to the amounts present in food.
Word origin
Where “saponin” comes from
Saponin comes from the Latin sapo, meaning soap, by way of the soapwort plant whose sap lathers in water.