Response surface methodology for the fermentation of polysaccharides from Auricularia auricula using Trichoderma viride and their antioxidant activities

Fermentation technology was used to improve the antioxidant activities of Auricularia auricula polysaccharide (AAP). Response surface methodology (RSM) was used to optimize the fermentation conditions. The effects of 4 independent factors: water content (X1: 40–80%), inoculation amount (X2: 2–20%), temperature (X3: 24–32 °C), and time (X4: 4–6 d) on the biological degradation efficiency were evaluated. The RSM results showed that the optimal fermentation conditions were: X1: 61.7%, X2: 12.4%, X3: 31.0 °C, X4: 5.5 d. Verification tests showed no significant differences between the practical and the predictive values for each response. Under the optimal conditions, the degradation rate was 26.89 ± 0.14%, without significant differences with the predicted value (27.03%). The degradation products were classified to different molecular weight (Mw) polysaccharide fragments using membrane separation technology. The FT-IR analysis and monosaccharide composition analysis of degraded AAP (D-AAP-VI) showed that D-AAP-VI was a furan type polysaccharide, which was different from the total AAP (pyran type). In addition, compared to total AAP, the antioxidant activities in vitro of D-AAP-VI were significantly improved (p < 0.05) and D-AAP-VI showed the strongest antioxidant activity. These results indicated that biological degradation may be a suitable way to improve the antioxidant activities of natural polysaccharides.