Bioactivity of the crude polysaccharides from fermented soybean curd residue by Flammulina velutipes

► Soybean curd residue was reused by Flammulina velutipes to produce polysaccharides. ► Optimum fermentation conditions were obtained by response surface methodology. ► The yield of polysaccharides was improved by ultrasonic assisted extraction. ► Polysaccharides showed DPPH radical scavenging activity and SOD-like activity. ► F. velutipes polysaccharides showed a strong immunomodulatory activities. The solid-state fermentation, reusing soybean curd residue (SCR) as a solid substrate, was conducted for producing polysaccharides by Flammulina velutipes (F. velutipes). The optimal fermentation conditions were 74.5% of moisture content, 9.69 of inoculum size and 30.27 of C/N ratio by response surface methodology. 59.15mg/g of polysaccharides were obtained. F. velutipes polysaccharides were subsequently extracted from fermented SCR by ultrasonic assisted extraction. The optimal extract conditions were 30min, 80°C, 150W and 20 of water to solid ratio and 106.74mg/g of polysaccharides were obtained. Furthermore, the antioxidant and the immunomodulatory activities of polysaccharides were assessed. The results showed that polysaccharides exhibited a strong DPPH radical scavenging activity, SOD-like activity, stimulatory the proliferation of the macrophage, the production of nitric oxide, phagocytosis and the protection on Doxorubicin damage. These could lay the foundation for changing SCR into a nutritious functional food or a food additive.