Quillaia is the milled inner bark or small stems and branches of the soapbark (Quillaja saponaria, Molina). Other names include Murillo bark extract, Panama bark extract, Quillaia extract, Quillay bark extract, and Soapbark extract. Quillaia contains high concentrations of saponins that can be increased further by processing. Highly purified saponins from quillaia are used as adjuvants to enhance the effectiveness of vaccines. Other compounds in the crude extract include tannins and other polyphenols, and calcium oxalate.
Quillaia is used in the manufacture of food additives, and it is listed as an ingredient in root beer and cream soda. The extract also is used as a humectant in baked goods, frozen dairy products, and puddings and as a foaming agent in soft drinks. It is used in agriculture for some "natural" spray adjuvant formulations.
The saponins from Quillaja saponaria are used in several approved veterinary vaccines (e.g., foot-and-mouth disease vaccines). Initially a crude preparation was used, but more recently purified products have been developed. Two of these (Quil A and Matrix-M) have been shown to be more effective and cause less local irritation. 
- EFSA Panel on Food Additives and Flavourings (FAF) et al. Re-evaluation of Quillaia extract (E 999) as a food additive and safety of the proposed extension of use. EFSA Journal. 06 March 2019.
- Biopesticides Registration Action Document. Saponins of Quillaja Saponaria . U.S. Environmental Protection Agency. 11 September 2009.
- Sun, Hong-Xiang; Xie, Yong; Ye, Yi-Ping (2009). "Advances in saponin-based adjuvants". Vaccine. 27 (12): 1787–1796. doi:10.1016/j.vaccine.2009.01.091. PMID 19208455. Archived from the original on 2017-10-08.
- Sarah Jane Tribble and Rachana Pradhan. Novavax’s Effort to Vaccinate the World, From Zero to Not Quite Warp Speed Kaiser Health News, Kaiser Family Foundation, July 19, 2021.