Saponins are glycoside compounds that are often referred to as natural detergents because of their ability to form foaming solutions in water. The majority of naturally occurring saponins are of the triterpenoidal type, with the steroidal based saponins forming a much smaller class. The steroidal saponins are based on a backbone of a (C30) triterpenoid saponin nucleus attached via C3 and an ether bond to a sugar side chain, whereas the steroidal are based on a choline (C27) steroid backbone. The aglycone of the triterpenoidal derivative is known as a sapogenin, whereas the steroidal aglycone derivatives are known as saraponins.

The non-saccharide portion (aglycone) of the saponin molecule is called the “genin” or “sapogenin”. Saponins are divided into three main classes depending on the type of sapogenin present:

• Triterpene glycosides - there are over more than 350 sapogenins and more than 750 triterpene glycosides in the triterpene glycoside class.
• Steroid glycosides  
• Steroid alkaloid glycosides  

The ability of a saponin to foam is caused by the combination of the non-polar sapogenin and the water-soluble side chain present on the molecule. The foams tend to be stable and have been used in fire extinguishers as the foaming agent. They are also used to produce foam in beer and are responsible for the natural foam in root beer. They have been used as the foaming agent in toothpaste and are employed by local people where the plants occur as a shampoo and laundry detergent.

Typical soap plants include Yucca (Yucca schidigera), Soapwort (Saponaria officinalis), Soapbark (Quillaia saponaria), Soaproot (Chlorogalum pomeridianum) and Soapnut (Sapindus spp).

Source: Dweck, Anthony. Handbook of Formulating Natural Cosmetics (Dweck Books 1)