Isolation, structure, and surfactant properties of polysaccharides from Ulva lactuca L. from South China Sea

•Two sulfated polysaccharides were isolated from Ulva lactuca L. from the South China Sea.•The extraction conditions were optimized, getting the highest yield of 17.57%.•The structure of the polysaccharides was characterized.•The polysaccharides expressed good surfactant property.•The polysaccharides are potential as natural additives in food. Two polysaccharides (ULP1 and ULP2) were isolated through ultrasonic-assisted extraction from green seaweed Ulva lactuca L. which was collected from the South China Sea. The highest yield of 17.57% was obtained under the conditions of 2% NaOH, 90°C, material/water mass ratio 1:80, liquid extraction 5h and subsequent ultrasound-assisted extraction 1h. The structure of ULPs were characterized with periodate oxidation followed by Smith degradation, 1H NMR, 13C NMR spectroscopy, FTIR, and GPC. The molecular weights of ULP1 and ULP2 were 189kDa and 230kDa, respectively. The structural characteristics of ULP1 and ULP2 were quite similar. They were composed of rhamnose, xylose, glucose, and glucuronic acid. The content of rhamnose, xylose, glucose, glucuronic acid, sulfate was 51.2%, 12.3%, 20.1%, 16.4%, 12.0% for ULP1, respectively, and 60.8%, 14.2%, 8.2%, 16.8%, 26.8%, respectively, for ULP2. Both ULP1 and ULP2 showed good surface activity. 5mg/mL ULP1 (2.62×10−2mmol/L) decreased the water surface tension to 51.63mN/m. The critical micellar concentration of ULP1 and ULP2 was 1.01mg/mL (5.3×10−3mmol/L) and 1.14mg/mL (5.0×10−3mmol/L), respectively.