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

Exopolysaccharide (EPS) fractions P-1/5, P-2/5, P-1, P-2 and P-5 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. P-1/5 and P-2/5 were mainly composed of polysaccharides with negligible protein content and a large molecular or particle size (intrinsic viscosity [eta] 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, P-5 was composed of mainly protein with an average molecular weight 16 kDa, [eta] 4.3 mL/g and R-h 23.5 nm. The antioxidant activities of EPS fractions showed a significant dependence on the protein content, being negligible of P-1/5 and P-2/5, low to moderate of P-1 and P-2, and very strong of P-5. 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. (C) 2013 Elsevier Ltd. All rights reserved.