Changes in the profile of genistein, daidzein, and their conjugates duringthermal processing of tofu

Profiles of genistein, daidzein, genistin, daidzin, and their acetyl- and m alonyl-beta -glycosides were determined in tofu as affected by temperature and time. Tofu was heated in water at 80, 90, and 100 degreesC for 0 (contr ol), 10, 20, 30, and 40 min, and the contents of the isoflavones of interes t were quantified using reversed-phase HPLC. Total isoflavone content decre ased most likely due to leaching of isoflavones into the water. Because the content of the isoflavones of the genistein series was little affected by the treatments, the decrease in the total isoflavone content was almost exc lusively due to a decrease of the daidzein series. Changes in the profile o f the daidzein series suggest little decarboxylation of the malonylglycosid e to the acrtylglycoside, but considerable de-esterification of the malonyl - and acetylglycoside to the beta -glucoside. Strongly temperature dependen t decreases of the aglycon suggest possible thermal degradation of daidzein in addition to losses due to leaching.