Effects of long-term soaking on nutraceutical and taste characteristic components in Thai soybeans

Changes in nutraceutical and palatable components of Thai soybean (Chiang-Mai 60, CM60) and black soybean (Sukhothai 3, SK3) were investigated during long-term soaking at 35 degrees C for 48 h. Isoflavone, saponin, free sugar, free amino acid including gamma-aminobutyric acid (GABA) and folate, and also antioxidant capacity (H-ORAC) were evaluated. Isoflavone composition in CM60, malonyiglycoside and glucoside were degraded to their aglycone forms while components in SK3 did not change greatly during long-term soaking. Total amount of soybean saponins including DDMP-saponins and group B saponins (group B) did not change significantly but the ratio of DDMP-saponins and group B changed; DDMP-saponins decreased while group B increased with soaking longer than 24 h. The soaking process decreased the contents of free sugars; sucrose, stachyose, and raffinose. Total amount of free amino acids in CM60 showed a decreasing tendency especially for amino acid groups showing undesirable bitter taste characteristics after 48 h soaking. GABA increased after 12 h soaking in both samples. Folate content increased up to 12 h soaking but then reduced to the original amount or lower with long-term soaking. Antioxidative activities of SK3 decreased after 24 h soaking. Results showed that long-term soaking affected the amounts of isoflavones, group B saponins, GABA, stachyose, raffinose and folate.