Optimization of ultrasonic-assisted deep eutectic solvent for the extraction of polysaccharides from Indocalamus tessellatus leaves and their biological studies

To the valorization of Indocalamus tessellatus（I.tessellatus） leaves for the functional food industry, the process of ultrasonic-assisted deep eutectic solvent (DES) extraction of I.tessellatus leaves polysaccharides (U-DES-ICP) was optimized by response surface methodology. In comparison to ultrasonic extracted polysaccharides (UICP) and water extracted polysaccharides (WICP), their physicochemical properties were analyzed by HPLC, HPGPC-MALLS, and FTIR, and the structure morphology was observed by SEM. Their antioxidant activities were evaluated based on DPPH and ABTS + assay as well. The results showed that the best DES system for U-DES-ICP was a mixture of choline chloride and malonic acid in the molar ratio of 1:4 (10% water content). The highest yield of U-DES-ICP (0.6812%) was obtained by following optimized conditions: 60 min of sonication time, 17 mL/g of liquid-solid ratio, 500W of sonication power, and 63.5 °C of sonication temperature, which was significantly higher than that of the conventional methods. The structural analysis indicated that U-DES-ICP had more uronic acid with higher molecular weight compared with UICP and WICP, the relative content of uronic acid of U-DES-ICP reached 8.364%, which was twice as much as the other two polysaccharides; its molecular weight reached up to 6.201 × 103 KD, and the highest values of molecular weight of UICP and WICP were 1.684 × 103 KD and 995.4 KD, U-DES-ICP which were confirmed by SEM. The antioxidant assays reflected that U-DES-ICP possessed stronger scavenging capacities both on DPPH and ABTS + radicals with IC50 (0.48 mg/mL and 0.61 mg/mL) than the other two biomacromolecules. IC50s for DPPH and ABTS radical scavenging by UICP were 0.51 and 0.73 mg/mL, while 0.82 and 0.94 mg/mL were IC50s for both radical scavenging by WICP, respectively. Therefore, this study evidenced that a novel DES system could boost the extraction of I.tessellatus leaves polysaccharides, which produced a higher yield of product with stronger biological activities compared with traditional techniques.