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Quillaia is the milled inner bark or small stems and branches of the soapbark (Quillaja saponaria, Molina). Other names include China bark extract, Murillo bark extract, Panama bark extract, Quillai extract, Quillaia extract, Quillay bark extract, and Soapbark extract. It contains a high concentration of saponins which is increased by processing; highly purifed quillaia is used to enhance vaccines. Other compounds in the extract include tannins and other polyphenols, and calcium oxalate. It is listed as an ingredient in root beer and cream soda.
The extract of quillaia is used in the manufacture of food additives (E number 999). It is used as a humectant in baked goods, frozen dairy products, and puddings and as a foaming agent in soft drinks. It is also applied in some "natural" spray adjuvant formulations for agriculture.
In particular, the saponins from Quillaja saponaria are used in veterinary vaccines as adjuvant (e.g., foot-and-mouth disease vaccines, helping to enhance the immune response). Initially the crude fraction was used. Later on, a purified mixture called Quil A was developed by Dalsgaard, which was more effective and caused less local side reactions. Still, Quil A is a mixture of more than 25 different saponin molecules. One of them, the saponin QS21, is being investigated for possible beneficial adjuvant effects on the human immune system.