Heating affects the content and distribution profile of isoflavones in steamed black soybeans and black soybean koji

Steamed black soybeans and black soybean koji, a potentially functional food additive, were subjected to heating at 40-100 degrees C for 30 min. It was found that steamed black soybeans and black soybean koji after heating at 80 degrees C or higher generally showed reduced contents of malonylglucoside, acetylglucoside, and aglycone isoflavone and an increased content of beta-glucoside. A lower reduction in malonylglucoside and acetylglucoside isoflavone but greater reduction in aglycone content was noted in steamed black soybeans compared to black soybean koji after a similar heat treatment. After 30 min of heating at 100 degrees C, steamed black soybean retained ca. 90.3 and 83.8%, respectively, of its original malonylglucoside and acetylglucoside isoflavone, compared to lower residuals of 80.9 and 78.8%, respectively, for black soybean koji. In contrast, the heated black soybeans showed an aglycone residual of 68.0%, which is less than the 80.0% noted with the heated black soybean koji.