Antioxidant activity and isolation from soybeans fermented with Aspergillus spp.

Steamed soybeans (SSB) were fermented with 30 kinds of Aspergillus strains which have been used in manufacturing Japanese fermented foods and beverages. Methanol extracts from the fermented soybeans were tested for antioxidative activity. Extract from soybeans fermented with Aspergillus saitoi, which had the strongest activity, was subjected to a silica gel column followed by preparative HPLC using ODS columns. Antioxidative principle (AS-4) was isolated and identified as 2,3-dihydroxybenzoic acid (2,3-DHBA) by UV, EI-MS, and H-1- and C-13-NMR techniques. Among the six DHBA isomers tested in both oil and lipid/aqueous systems, 2,3-, 2,5-, and 3,4-dihydroxybenzoic acids exhibited the greatest antioxidative activities. Since 2,3-DHBA was not found in unfermented soybeans (SSB), it was produced during the incubation with A. saitoi.