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Relative hydrolysis rates of glycosidic forms of genistein and daidzein by beta‐glycosidase

Genistein and daidzein, major isoflavones found in soy foods, are present primarily as glycosidic forms such as beta‐glycoside, acetyl glycoside, or malonyl glycoside. Glycosides cannot be absorbed prior to enzymatic cleavage of the sugar moiety by intestinal beta‐glycosidase in the gut. But it is not known if acetyl or malonyl glycosides of isoflavones are substrates of beta‐glycosidase in the intestine. Isoflavones are more bioavailable from fermented soy products as compared to unfermented products and it is suggested that this is due to the higher level of aglycones in fermented soy products. In the present study we characterize the predominant forms of daidzein and genistein in unfermented soy flour and fermented soy bean paste. Daidzein 6‐malyonyl glucoside (707.1 ug g‐1 dry wt) and genistein 6‐malyonyl glucoside (572.2 ug g‐1 dry wt) are the predominant forms in soy flour. More importantly, we found when these two products are consumed, based upon total isoflavones content, the levels of beta‐glycosides consumed in soy paste are twice the levels consumed in soy flour if the 6‐OMalGlc forms are not considered. To further investigate the influence of the glycosidic moiety on bioavailablility, the relative hydrolysis rates of isoflavone glycosides by beta‐glycosidase were determined.