Thermal conversion kinetics and solubility of soy isoflavones in subcritical water extraction

The conversion of isoflavones during subcritical water extraction were studied using first-order reaction kinetics modeling. Isoflavones were extracted from soybean using temperatures from 100 degrees C to 180 degrees C for 3 to 30 min. It was found that malonylgenistin was the most thermally unstable, with little being detected above 100 degrees C. The optimal extraction temperatures for acetylgenistin (AG), genistin (G), and genistein (GE) were 120 degrees C, 150 degrees C, and 180 degrees C, respectively. A larger sum of the numbers of both hydroxyl groups and oxygen molecules was associated with a lower melting point and optimal extraction temperature. Kinetics modeling of reaction rate constant k and activation energy Ea showed that all of the reaction rates tended to increase with temperature, with the rela-tionship fitted well by a first-order model in nonlinear regression. For temperatures between 100 degrees C and 150 degrees C, AG -> G and AG -> GE conversions showed the highest rate constants, but G -> GE and G -> D3 (degraded G) conversions became dominant at 180 degrees C.Chemical compounds studied in this article: genistein (PubChem CID: 5280961), genistin (PubChem CID: 5281377), 6 ''-O-malonylgenistin (PubChem CID: 15934091), 6 ''-O-acetylgenistin (PubChem CID: 5315831)