Soap Bark Tree, Quillaja Saponaria

The inner bark of this tree has been used traditionally as medicine, soap, and a food additive. It contains saponins that form a soap-like foam when shaken with water.

In cocktails, soap bark has been used as a foamer for many years. It is cited in "The Standard Manual of Soda and Other Beverages" from 1906 [link]  Some modern cocktail foaming agents (as alternatives to egg whites) most likely contain a solution of quillaja saponaria. 

Note that aquafaba also contains saponins. 

Legality

According the United States FDA: 

Quillaia (soapbark); Quillaja saponaria Mol is allowable "Sec. 172.510 Natural flavoring substances and natural substances used in conjunction with flavors." [link]

Safety

According to the United States Environmental Protection Agency [link]

There is reasonable certainty that no harm will result from aggregate exposure to residues of Quillaja saponins to the U.S. population, infants, and children. This includes all anticipated dietary exposures and all other exposures for which there is reliable information. The Agency arrived at this conclusion based on the low level of toxicity of Quillaja extract and the already widespread exposure to Quillaja saponins without any reported adverse effects on human health. The risks from aggregate exposure via oral, dermal and inhalation exposure are a compilation of three low-risk exposure scenarios and are negligible. Since there are no threshold effects of concern, the provision requiring an additional margin of safety does not apply. Moreover, Quillaja extracts are classified by the Food and Drug Administration (FDA) as ‘Generally Recognized as Safe” (GRAS), and are also a part of the human diet when used as emulsifiers in baked goods, candies, frozen dairy products, gelatin, and puddings (WHO, 2002). Humans have had frequent physical contact with Quillaja saponaria with no negative health effects. Therefore, the Agency has not used a margin of exposure (safety) approach to assess the safety of saponins of Quillaja saponaria.

 

Further Reading:

"Quillaja Saponaria Extract; Exemption from the Requirement of a Tolerance" Environmental Protection Agency [link]

"Quillaja" Drugs.com [link]

Aquafaba (CocktailSafe)