Antioxidant activities of the water-soluble fraction in tempeh-like fermented soybean (GABA-tempeh)

Tempeh is a traditional fermented soyfood native to Central Java, Indonesia; tempeh is prepared by salt-free aerobic fermentation using Rhizopus. Similar to miso, tempeh is also reported to be antioxygenic. In this study, we used a tempeh-like fermented soybean. First, soybean was incubated aerobically and then successively incubated anaerobically. Because the gamma-amino butyric acid (GABA) content was increased remarkably, we named this tempeh GABA-tempeh. The contents of other free amino acids and peptides were significantly higher in GABA-tempeh than in conventional tempeh. In this study, we compared the antioxidant activity of the water-soluble fraction in GABA-tempeh with that of soybean and conventional tempeh. The order of the antioxidant activity was shown as follows: GABA-tempeh > conventional tempeh > soybean. The components responsible for the antioxidant activity of GABA-tempeh were isoflavone aglycones, free amino acids, and peptides, which increased during aerobic and anaerobic fermentations.