Thermal dynamic properties of isoflavones during dry heating

P>Thermal stabilities of major soya isoflavones at different dry-heating temperatures were determined in this study. The conversion of glucoside isoflavones to aglycone isoflavones was monitored as well. The thermal degradation of the isoflavones: glucoside form daidzin, glycitin, and genistin and aglycone form daidzein, glycitein, and genistein followed first-order reaction kinetics at heating temperatures 100, 150, and 200 degrees C. The degradation rate constants of the six isoflavones were not significantly different at 100 degrees C. However, the constants increased with increasing heating temperature. The half-life for the glucoside and aglycone isoflavones was from 144 to 169 min and 139 to 176 min at 100 degrees C, respectively. They decreased rapidly to 15.7-54.7 and 36.0-90.7 min when temperature increased to 150 degrees C. When heated at 200 degrees C, they further decreased to 5.8-6.0 and 15.7-21.2 min, respectively. The order of thermal stability from lowest to highest was glycitin < genistin < daidzin < glycitein < genistein < daidzein at temperature below 150 degrees C. However, their thermal stabilities were not different at 200 degrees C. The conjugated glucosides were cleaved from the isoflavones to produce their corresponding aglycones when heated at 150 degrees C or higher. The production of glycitein increased constantly and was the highest among the three aglycone isoflavones.