Abstract: Objective: Nocardiopsis sp. HY-G, a strain isolated from marine environment by our laboratory, has been found to produce large amounts of hydrolases that can transform soybean isoflavones (SI) into genistein and daidzein during fermentation. In this study, medium composition and fermentation conditions for decomposing SI by the strain were optimized. Methods: Single factor and orthogonal array design methods were combinedly used for the optimization of fermentation conditions and medium composition. Results: The optimal fermentation medium was Gausse No.1 medium fortified with 1 g/100 mL sucrose, 0.03 g/100 mL (NH4)2SO4, 200 mg/L SI and initial pH 8.0. Fermentation temperature of 40 ℃, rotation speed of 120 r/min and fermentation time of 72 h in a 250 mL shake flask with 150 mL of medium were found optimum for culturing the strain. The activity towards the substrate genistein of the resulting product was 3621 U/mL and the activity towards daidzin was 4862 U/mL. Conclusion: The optimum fermentation conditions obtained after optimization are true and reliable, which will be helpful for large scale biotransformation of SI.

Key words: Nocardiopsis sp. HY-G, soybean isoflavone, hydrolysase, fermentation condition