Ulvan from green algae Ulva intestinalis: optimization of ultrasound-assisted extraction and antioxidant activity

Response surface methodology was employed to optimize the ultrasound-assisted extraction of crude polysaccharides from the cell wall of Ulva intestinalis. The impacts of four independent variables including extraction temperature (X1 50–90 °C), extraction time (X2 20–40 min), the ratio of water to raw material (X3 50–70), and pH (X4 7–9) were investigated through Box–Behnken experimental design. The experimental data were adequately fitted into a second-order polynomial model, and optimized conditions were as follows: extraction temperature 66 °C, extraction time 40 min, water to raw material ratio 50, and pH 7.0. Fourier transform infrared (FT-IR) results revealed the presence of sulfate esters indicated by the peaks at 850 and 1256 cm−1 derived from the bending vibration of C–O–S and stretching vibration of S–O of sulfate. The experimental yield achieved under these conditions was 8.30 %, which was well in close agreement with the value (8.36 %) predicted by the response surface methodology (RSM) model. The extracted polysaccharides exhibited appreciable 2,2-diphenylpicrylhydrazyl (DPPH) radical scavenging and reducing power as well as macrophage-stimulating capacity indicating their potential value for health and food industry.