Kinetic Study on the Conversion of Soy Isoflavones in Soymilk during Ohmic Heating

Ohmic heating is an emerging thermal process technology that enables rapid and uniform heating. The conversion of isoflavones in soymilk during ohmic heating at 80–95 °C for up to 240 min was investigated. Within the temperature range, only the conversion of malonyl forms to their corresponding glucoside forms was observed and no degradation of isoflavones was detected. The first-order kinetic model describing the conversion from malonyl forms to glucoside forms showed good agreement with the experimental data. The reaction rate constants for the conversion of malonyldaidzin to daidzin (kD) were calculated as 0.00358–0.01002 min–1 over 80–95 °C, respectively. The corresponding rate constants for malonylgenistin to genistin (kG) were 0.00327–0.01061 min–1. Additionally, both kD and kG followed the Arrhenius equation. The activation energies and the frequency factors were determined to be 73.2 and 80.4 kJ/mol and 2.30 × 108 and 2.52 × 109 min–1, respectively. These results provide useful kinetic insights.