Objective  The polysaccharides extracted from Sargassum fusiforme with different ethanol volume were studied.

  Method  Polysaccharides were extracted from S.fusiforme using an acid solution, then precipitated with different volume fractions of ethanol. The resulting extract was analyzed for its physicochemical properties, monosaccharide composition, molecular weight, structural characteristics, functional group identification, spatial conformation, apparent morphology, and rheological properties using ultraviolet spectrophotometer, precolumn derivatization-reverse-phase high performance liquid chromatography, GPC-MALLs, FT-IR, SEM, and Huck MARS Ⅲ rheometer. The antioxidant activity of the extracts, including the oxygen radical absorption capacity and DPPH· and ABTS· scavenging ability, were determined.

  Result  Extracts, SFP-50 and SFP-70, were found to contain higher contents of sugar, sulfate and uronic acid than the other samples. All 5 extracts contained mannose, glucose, glucuronic acid, galactose, arabinose, and fucose but differed in ratio, with fucose being the highest at about 50% of them combined. The molecular weights of SFP-40, SFP-50, SFP-60, SFP-70, and SFP-80 were estimated to be 97.63, 90.51, 97.19, 81.52, and 90.93 kDa, respectively. The Congo red assay showed that the polysaccharides had a triple helical structure. The solution of the extracts was non-Newtonian fluid with elasticity. The scanning electron microscopy images showed an uneven, loose and porous surface. All samples had varying degrees of oxygen radical absorption capacity and DPPH· and ABTS· scavenging ability.

  Conclusion  The volume fraction employed for the ethanol precipitation affected the physicochemical properties, monosaccharide composition, and molecular weight as well as the antioxidant activity, but not the conformation, microstructure or rheological properties, of the polysaccharides obtained.