Bioconversion of γ-aminobutyric acid and isoflavone contents during the fermentation of high-protein soy powder yogurt with Lactobacillus brevis

This study evaluated the changes in γ-aminobutyric acid (GABA) and isoflavone aglycone contents from soy powder yogurt (SPY) due to sprouting of soybean (1 cm) and fermentation with Lactobacillus brevis. The levels of GABA and the aglycone form increased, and the glutamate decarboxylase and β-glucosidase activities increased; however, the isoflavone glycoside and malonylglycoside contents decreased after fermentation for 72 h. In particular, after 60 h, the SPY presented the highest GABA content (120.38 mg/100 mL). The highest daidzein (179.93 µg/g), glycitein (44.10 µg/g), and genistein (126.24 µg/g) contents were present after 72 h of fermentation. In addition, the 2,2-diphenyl-1-picrylhydrazyl, 2,2-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt, and hydroxyl radical scavenging activities increased from 69.65, 97.94, and 70.90% during this fermentation, respectively. This result suggests that SPY may be used for the preparation of high-protein soybean with high GABA and isoflavone aglycone contents, which can then be used as a natural ingredient of functional foods.