Cascade approach to valorize soybean by-product by using subcritical water. First step: bioactive compounds

Abstract

Okara is the soybean by-product generated in the production of soymilk, tofu, and soy protein isolate, at the filtration stage. It’s quite expensive to dry it (necessary for its conservation), so it is generally discarded; however, its nutritional value makes it very interesting. It presents fiber, protein, good amino acid profile, healthy lipids and great isoflavone content; being these last compounds particularly noteworthy.[1, 2] Subcritical water (subW) was the green technology selected as the main hydrolytic agent in this study, focusing on the isoflavone fraction. OKW extracts were subjected to independent subW extractions in six assays from 110 °C to 180 °C, using N2 to pressurize up to 5 MPa, for 5 h,in a batch reactor of 0.5 L capacity. It was analyzed total isoflavone content at different treatment times for each selected temperature. 8 isoflavones (daidzein, genistein, glycitein and its derivates) and the difference in extraction of its malonyl forms, β-glucosides and aglycones, were analyzed. Different assays were carried out on OKW (oven-dried washed okara) and best conditions were applied posteriorly to OKA (okara as received without pretreatment). Total isoflavone extraction (Fig. 1) and antioxidant capacity were analyzed in all of these kinetics. At low temperatures (lower than 140 °C), there was observed a maximum in the extraction of isoflavones, being the highest at 120 ° C, after 30 min of treatment (27.8 μg/mL; 585.6 μg/g OKW). On the contrary, at the highest temperatures essayed, 160 and 180 °C, there was observed a continuous decrease due toits degradation at these temperatures. Opposite kinetics were found for antioxidant capacity, where the highest temperatures (mainly 160 and 180 °C) report highest results, although lower isoflavone content was determined. This could be attributed to Maillard reaction between other compounds release at those temperatures (such as reducing sugars and amino groups). Focusing on isoflavone extraction, 120 °C was selected to treat OKA (as received, with a humidity content of 82.5 %). At 35 min, it was reported 1229.2 μg isoflavones /g OKA, being 210 % higher than using OKW. The study of the composition of the isoflavones chemical forms was also carried out (Fig. 2), showing how malonylglucosides decrease losing their malonyl group and forming βglucosides, that increase even faster, due to the high hydrolizing power of subW; at the same time, aglycones increase slowly without showing high content in any moment. Other novel extraction technologies were applied to OKA, Microwave (895.2 μg /g OKA) and ultrasound extraction (927.5 μg /g OKA), reporting high values of total isoflavone extraction, but lower results (72.8 % and 75.4 %, respectively) than subW (Fig. 3). Besides, subW results present higher ratio βglucoside/malonylglucoside, being the first more noteworthy. As a way to reach an integral valorisation of this by-product, additional subW hydrolysis can be carried out on the solid residue after subW at 120 °C, with the isoflavones from OKA already extracted. Higher temperatures on subW will lead to valorize structural components of the okara such as carbohydrates and valuable proteins that cannot be hydrolyzed at the isoflavone extraction optimum conditions.