Effect of ultrasound on the extraction of bioactive aglycone isoflavones for the green valorization of black soybean residue (okara)

The effect of ultrasound on the extraction of bioactive aglycone isoflavones for the valorization of black soybean okara was investigated. A high extraction efficiency for total isoflavones was achieved with the total isoflavones extracted using water being 77.8% of that using 80% methanol as the solvent. The fraction of bioactive aglycones (daidzein and genistein) in total isoflavones using ultrasound (40 kHz/300 W) for 10 min was 81.7%, and further enhanced to 86.7% for 30 min. The profile of isoflavones was significantly influenced by temperature with the amounts of 351.00 μg/g-okara of aglycones and 91.43 μg/g-okara of glycosides using ultrasound at 60°C for 3 hr, and the fractions of aglycones in isoflavones were 96.4, 89.6, and 79.3% for 35, 50, and 60°C, respectively. An extraction model was proposed to successfully correlate the extraction rate coefficients and the limit concentrations of the four isoflavones in water. Practical applications The bioactive aglycone isoflavones are beneficial for human health. The okara produced from food industries contains high-quality isoflavones, which have to be recovered for the potential use in functional foods. The process developed in this study allows the recovery of high-content aglycone isoflavones from black soybean okara without further biotransformation of glycoside isoflavones, and provides the effective valorization of okara and process sustainability in food applications.