Conversion/degradation of isoflavones and color alterations during the drying of okara

- We evaluated isoflavones and color of okara during oven-drying at 50, 60 and 70 °C.
- Okara browning occurred during drying for all temperatures.
- At 50 °C, conversion of glycosides to aglycones was observed up to 6 h of drying.
- Drying at 70 °C led to higher isoflavones loss.

The aim of this work was to investigate the convective drying of okara, a by-product from soymilk processing. The specific objectives were as follows: to model the drying kinetics using the Fick and Page models; and to evaluate the color and isoflavone content during drying. An oven-dryer with hot air circulation at 50, 60 and 70 °C was used. Samples were taken at each predetermined time to analyze the isoflavone profile by UHPLC and the color. The Page model fitted the drying kinetics data well. Throughout drying, there was a decrease in the color parameters (lightness and hue angle), showing sample browning. A significant reduction in the isoflavone content was observed at 70 °C, possibly due to thermal degradation. During the initial stage of drying at 50 °C, daidzin and genistin were converted to the aglycones daidzein and genistein, respectively, probably due to residual β-glucosidase activity. However, isoflavone degradation was observed during the final stages of drying at 50 °C.