Extracting isoflavones from okara using deep eutectic solvents: An optimization study of operating conditions

Okara is a solid residue generated after obtaining soy extract and is rich in phytochemicals such as phenolic acids and, mainly, isoflavones. These compounds are phenolic compounds that are associated with several health effects, mainly in the relief of menopausal symptoms and prevention of breast and prostate cancer. These observations make okara an interesting residue to extract these bioactive compounds and apply them in products that promote human well-being. Taking this into account, six soy isoflavones daidzein, genistein, glycitein, daidzin, genistin, and glycitin were extracted under different conditions. The deep eutectic solvent (DES) composed with choline chloride (ChCl) and acetic acid (AA) (ChCl:AA, 1:2) was used as solvent. The isoflavones were quantified by UHPLC-MS. Initially, a screening of temperature (°C), % water in DES, solid-liquid ratio and stirring speed was performed applying a fractional design (24-1) followed by optimization applying a central composite rotational design (CCRD) (2² + 2 axial points). Total isoflavone content was used as the response variable. From the screening results, temperature and % water in DES were statistically significant effects (p<0.05) on isoflavone extraction. In the DCCR, temperature was evaluated from 40-70 °C and % water in DES from 40-100%. From the CCRD analysis, only % water in DES showed significant effect on the levels studied, being the maximum extraction at 40 °C, 500 rpm, solid-liquid ratio of 10 mg/mL and 61.5% water in DES. In this condition, the composition of isoflavones in the extract, expressed in ug/g of dried okara, was116.61 for daidzein, 151.31 for genistein, 27.98 for glycitein, 52.0 for daidzin, 74.74 for genistin and 28.26 ug of glycitin. In conclusion, it was possible to optimize the extraction of isoflavones from a low-value waste with the use of an environmentally friendly alternative solvent in line with current concerns for reducing environmental impacts.