In order to determine the fungal strain and fermentation period which produce the highest antioxidant activity of fermented soy product, soybean was fermented with Aspergillus oryzae NL 5 isolated from Korean traditional meju produced in Sunchang Yeastopia Land in Korea, and was subjected to analyzing isoflavone profiles and assaying antioxidant activity. Antioxidant potential of the samples was evaluated by ferric reducing antioxidant power assay, 2,2'-azinobis (2-ethylbenzthiazoline-6-sulfonic acid) radical scavenging activity and total phenolic contents. In addition, antioxidant assay using fluorescent probes, 2',7'-dichlorofluorescin diacetate and dihydroethidium, in HepG2 cells was conducted for the samples. The results showed that both high isoflavone variety Aga 3 and medium isoflavone variety Daewon of soybean fermented with Asp oryzae NL5 for 4 to 5 d showed the highest increase in free isoflavone contents, together with the highest antioxidant activity. In conclusion, the antioxidant activity of fermented soy was proportional to the concentration of free isoflavones, and was significantly influenced by total isoflavone content of soybean variety and fermentation period. Practical Application: Isoflavone profile and antioxidant capacity of soybean were widely variable during fermentation with Asp oryzae, and thereby it is recommended that meju (or koji) preparation condition, in particular, fermentation time is carefully optimized to maximize the antioxidant capacity of soy products fermented by Asp oryzae.
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