Results
One metabolite, 3,4-dihydroxyphenylacetic acid, was produced by both C. perfringens and B. fragilis transforming quercetin. No other metabolites were detected in the other fermented broths. The antioxidant activities of all strains fermenting quercetin reached the highest values at the concentration of 1 mg/mL quercetin in broth; the fermented products of C. perfringens and B. fragilis presented stronger activities than those of other strains at most concentrations of quercetin in broth. Additionally, all of the fermented broths presented a decline of the antioxidant activities compared to quercetin. Therefore, the antioxidant activity of quercetin will be lost when it reaches the human colon because of the gut bacterial fermentation.