Optimization of ultrasound-assisted natural deep eutectic solvent extraction for the recovery of bioactive polysaccharides and phlorotannins from the brown alga Ecklonia radiata

Phlorotannins and polysaccharides are widespread in brown seaweeds. This study shows that ultrasound-assisted extraction combined with natural deep eutectic solvents (NaDES-UAE) enhances the yields of phlorotannins and polysaccharides from the brown seaweed Ecklonia radiata. Among the NaDES solvents tested, choline chloride:lactic acid (molar ratio of 1:3) supplemented with 20 % water was the most effective. The extraction parameters were further optimized using a Box-Behnken design and response surface methodology. The phlorotannin and polysaccharide yields obtained after optimized NaDES-UAE extraction were 12.8 ± 0.3 % and 26.5 ± 0.9 %, respectively-both significantly higher than yields obtained using conventional organic solvent extraction. Twenty-one phlorotannins and isomers were identified using ultra-high performance liquid chromatography coupled to high-resolution mass spectrometry, while ten different monosaccharides were detected in the polysaccharide fractions. The relative abundance of specific phlorotannins and monosaccharides varied depending on the extraction method. Compared to conventional aqueous and organic solvent extractions, NaDES-UAE-extracted phlorotannins exhibited higher antioxidant activity, while the polysaccharide fraction, mainly consisting of low-molecular-weight polysaccharides and oligosaccharides, showed enhanced antibacterial activity against both Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. In conclusion, our study reveals that the NaDES-UAE method is an efficient, eco-friendly alternative for extracting bioactive compounds from brown seaweeds, with strong potential for applications in the food, nutraceutical, biopharmaceutical, and other industrial sectors.