Fractionation, characterization and antioxidant activity of exopolysaccharides from fermentation broth of a Cordyceps sinensis fungus

Exopolysaccharide (EPS) fractions P1/5, P2/5, P 1, P2 and P5 were isolated from the fermentation medium of a medicinal fungus Cordyceps sinensis by gradient precipitation with ethanol at 1/5, 2/5, 1, 2, and 5 volume ratios to the liquid medium. P1/5 and P2/5 were mainly composed of polysaccharides with negligible protein content and a large molecular or particle size (intrinsic viscosity [η] 2025 mL/g; hydrodynamic radius R h 905 nm). The fraction attained at a higher ethanol volume ratio had a much higher protein and lower carbohydrate content, and a smaller molecular size. In particular, P5 was composed of mainly protein with an average molecular weight 16 kDa, [η] 4.3 mL/g and Rh 23.5 nm. The antioxidant activities of EPS fractions showed a significant dependence on the protein content, being negligible of P1/5 and P2/5, low to moderate of P1 and P2, and very strong of P5. Gradient ethanol precipitation was proven a simple and workable method for initial fractionation of polysaccharides, proteins and their complexes with different molecular sizes and for further identification of the bioactive components.Department of Applied Biology and Chemical Technolog