Biotransformation of saponins to astragaloside IV from Radix Astragali by immobilized Aspergillus niger

•Immobilized A. niger M85 exhibited 10-fold increase in yield than traditional methods.•Immobilized A. niger M85 had good stability and biotransformation ability.•It is an economical and environment-friendly technology for AG IV production.

Eight different strains of yeasts, Aspergillus niger, Aspergillus oryzae and White-rot fungus were tested for biotransformation of saponins to astragaloside IV in potato medium with Radix Astragali. Among these strains, immobilized cells of A. niger M85 exhibited the best performance on the biotransformation of saponins to astragaloside IV from Radix Astragali. After the biotransformation with immobilized A. niger M85, the contents of astragaloside IV and total astragalosides in Radix Astragali increased 10.7-fold (2.326 mg/g) and 8.6-fold (2.56 mg/g) to the solvent extraction sample, respectively. Optimization was done at incubation period 5 days, the amount of Radix Astragali powder 2 g, medium volume 50 mL/g, incubation temperature 30 °C and initial pH 6.0. The immobilized A. niger M85 can be reused seven times and retained 71% of its residual activity. The A. niger M85 was found to be a highly efficient producer of astragaloside IV.