Variations in Isoflavone During Soybean Maturation and Their Thermal Process-Dependent Conversion

Immature soybean seeds, after steaming or boiling, are widely consumed in Northeast Asia. However, changes in isoflavones and antioxidant activities during processing at different seed stages remain underexplored. In this study, soybean seeds at four maturity stages (R5–R8) were analyzed for 12 isoflavones and evaluated after steaming. Total isoflavone content increased from R6 to R7 and remained stable to R8, presenting a 10-fold increase in R7 than in R6. Levels of malonyl derivatives, such as malonylgenistin, malonyldaidzin, and malonylglycitin, consistently decreased with longer steam treatment at all seed stages. In contrast, β-glycoside forms, such as genistin and daidzin, increased after steaming, with notably high content at R7. Additionally, ABTS radical scavenging activity and total phenolic content showed strong positive correlations with total and major isoflavones, whereas DPPH radical scavenging activity showed no correlation with maturity stage or steam treatment. These findings indicate that isoflavone stability and conversion are strongly affected by seed maturation and that the R7 stage offers a favorable balance for high isoflavone and antioxidant intake in soybean seeds.