Extracts+Vanillin (8%, w/v) + Sulphuric acid (72%, w/v) + incubated at 60C° for 10 min + cooled in ice water bath for 15 min + read at 538 nm. the total saponin content is expressed in Quillaja saponin equivalents (QS µg/mg extract).
Reference:
Hiai, S.; Oura, H.; Nakajima, T. Color reaction of some sapogenins and saponins with vanillin and sulfuric acid. Planta Med. 1976, 29, 116–122.
Shiau, I.L.; Shih, T.L.; Wang, Y.N.; Chen, H.T.; Lan, H.F.; Lin, H.C.; Yang, B.Y.; Ko, C.H.; Murase, Y. Quantification for saponin from a soapberry in cleaning products by a chromatographic and two colorimetric assays. J. Fac. Agr. Kyushu. Univ. 2009, 54, 215–221.