In this study, sufu, a Chinese traditional fermented product of soybean, was prepared by ripening salted tofu cubes in the mash of Aspergillus oryzae-fermented rice-soybean koji possessing various hydrolytic enzymes at 25 degrees C, 37 degrees C and 45 degrees C. Antioxidant activity including 2,2-diphenyl-2-picylhydoxyl (DPPH) radical-scavenging activity, Fe(2+)-chelating ability and reducing power exerted by the methanol extract of sufu was determined and compared with that of the non-fermented tofu extract. It was found that antioxidant activity of the sufu extracts was, generally, higher than the non-fermented tofu extract. Ripening temperature and the duration of ripening period affected the antioxidant activity of the sufu extracts. Taking into account of extraction yields, the sufu product ripened at 45 C for 16 days showed the most profound enhancement in the DPPH radical-scavenging effect and Fe(2+)-iron-chelating ability, which is 3.4 and 11.5 folds, respectively, that noted with the non-fermented tofu. (C) 2011, The Society for Biotechnology, Japan. All rights reserved.