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

Exopolysaccharide (EPS) fractions P1/5, P2/5, P1, P2and P5were 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. P1/5and P2/5were mainly composed of polysaccharides with negligible protein content and a large molecular or particle size (intrinsic viscosity [η] 2025 mL/g; hydrodynamic radius Rh905 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, P5was composed of mainly protein with an average molecular weight 16 kDa, [η] 4.3 mL/g and Rh23.5 nm. The antioxidant activities of EPS fractions showed a significant dependence on the protein content, being negligible of P1/5and P2/5, low to moderate of P1and 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.