Higher antioxidant properties of Chungkookjang, a fermented soybean paste, may be due to increased aglycone and malonylglycoside isoflavone during fermentation

Chungkookjang (CKJ), a popular fermented soybean paste in Korea, has been considered to be more healthful than soybeans. To verify this, we compared in vitro and in vivo antioxidant effects of CKJ and unfermented steamed soybeans (SOY) and investigated the possible effective substance. Chungkookjang and SOY were extracted with ethanol and fractioned sequentially with hexane, dichloromethane, ethylacetate, butanol, and water. In vitro antioxidant activity was examined by lipid peroxidation inhibition and 1,1-diphenyl-2-picrylhydrazyl radical scavenging abilities. Total polyphenol content of CKJ (22.19 mg/g dry weight [wt]) was much higher than that of SOY (8.81 mg/g dry wt). All the fractions from CKJ showed significantly higher antioxidant activities than those from SOY. The ethylacetate fraction showed the strongest antioxidant effect and has the most isoflavone content of 5 fractions. The proportion of aglycone and malonylglycoside in total isoflavone content were 77.5% and 12.9%, respectively, in ethylacetate fraction from CKJ, whereas it was 19.3% and 0.0%, respectively in that from SOY. For in vivo study, about 10% of dried CKJ or SOY was fed to Sprague-Dawley rats with a high fat diet for 6 weeks. Chungkookjang diet elevated hepatic superoxide dismutase activity compared to SOY diet and high-fat control diet. Chungkookjang or SOY diet significantly reduced hepatic thiobarbituric acid-reactive substance and increased catalase activity slightly, compared to control diet. From these results, we conclude that CKJ has a much stronger antioxidant activity than unfermented steamed soybeans, probably due to its increased aglycone isoflavone and malonylglycoside isoflavone which resulted from fermentation as well as total polyphenol contents.