Optimization of ultrasonic-assisted extraction of polysaccharides from Ulva rigida and evaluation of their antioxidant activity

Ultrasonic-assisted extraction (UAE) of sulfated polysaccharides from the cell walls of Ulva rigida green macro-algae was performed with a central composite design using the independent variables (pH 2-10, extraction temperature 40-80 degrees C, time 50-130 min and liquid/solid ratio of raw material 15-75 mL/g) with response surface methodology. Optimum conditions were obtained as pH 4.92, extraction temperature 73 degrees C, time 126 min, and the liquid/solid ratio of the raw material as 68 mL/g. Under these optimum conditions, the experimental yield was obtained as 13.22 %, and the results were close and in harmony with the estimated value of 12.91 %. FT-IR results, especially at the peaks corresponding to wave numbers 850 cm-1 and 1256 cm -1, the tension vibrations of sulfated esters in C-O-S and S-O bonds were more clearly observed in the extracted ulvan components. As a result of the SEM images obtained by considering the surface morphology, it was observed that the sulfated polysaccharides from the cell wall were more prominently removed after the lipids were removed and as a result of the precipitation process. While the extracted sulfated polysaccharides showed higher anti-oxidant activity, although the results were close to each other. The total phenolic content was higher in the extracted sulfated polysaccharides.