Isolation of soybean isoflavones and hydrolysis of their conjugates

Abstract

Contents of isoflavone in whole soybeans of 6 traditional Korean strains and 1 imported soybean were in the range 193∼386 mg/100g cotyledon. In all the beans tested, malonylglycosides accounted for more than 70% of the total isoflavone, followed by glycosides. Hypocotyls contained 6∼17 times more isoflavone than corresponding cotyledons of the soybean, and thus hypocotyls was one of the most excellent source for the production of isoflavones. Isoflavones of soybean were extracted from soybean hypocotyls with aqueous ethanol. 60% of aqueous ethanol was the most suitable solvent for extracting isoflavone from the hypocotyls. Optimum temperature and time for the extraction was 60℃ and 1hr. At alkaline pH range, extraction yield was drastically reduced. Volume of extraction solvent/hypocotyl ratio (v/w) should be over 6. Isoflavones extracted from hypocotyls were purified by several methods. Adsorption chromatography could recover isoflavone and saponin. The pH was critical in the chromatographic process and the adsorption of isoflavones was maximum at pH 4.0. Adsorption of genistin on the resin was stronger than that of daidzin. Elution rate and height/diameter ratio also affected the recovery yield. Under the optimal conditions, about 85% of genistin and 70% of daidzin could be recovered. The draw back of chromatographic method was that saponins were also recovered as well as isoflavones. Ultrafiltration concentrated isoflavone when the crude extract was solubilized in water. When pH of concentrated solution of crude isoflavones was adjusted below 4, isoflavones were precipitated. At pH 1.5, more than 80% of isoflavones was precipitated and the malonylglycosides were found to be most sensitive to pH. When the precipitates were suspended in 40% aqueous ethanol, only isoflavone and saponin were solubilized while contaminants including proteins, lipids and other substances were precipitated. For the removal of saponins, divalent cations such as $Ca^{2+}$, $Mg^{2+}$ ...