Ultrasound-Assisted Deep Eutectic Solvent Three-Phase Partitioning System for Extraction of Polysaccharides from Longan Shell: Process Optimization, Physicochemical Properties, Structural Characterization, and Antioxidant Activities

In this study, a methodology that combines ultrasound-enhanced extraction with the use of hydrophobic deep eutectic solvents (DESs) and three-phase partitioning (TPP) was presented for the green isolation of polysaccharides from longan shells (LSP). The extraction system was a DES composed of an equal molar ratio of dodecanoic acid and octanoic acid, which was used as the separation medium. Generally, the main phase separation mechanisms involved in the purification of the polysaccharide were investigated. The ideal operational parameters were found through systematic optimization by the single-variable experiment with the response surface methodology, i.e., the extraction temperature of 63.8 °C, the phase volume ratio of 1:1.04 (v/v), and the ammonium sulfate concentration of 26.3%. The extraction efficiency is 2.42 ± 0.03% for LSP when the above operational parameters are used. The structural characterization showed that the isolated LSP is an acidic heteropolysaccharide rich in galacturonic acid and arabinose. It was also shown that the molecular architecture of LSP includes both types of glycosidic bonds, which are also of importance for its physicochemical properties. The polysaccharide exhibits an open fibrous network structure. Notably, the DES maintained stable performance over five successive reuses without significant degradation. Concerning the antioxidant capacity, LSP at 0.4 mg/mL showed 96.6 ± 2.0% inhibition of ABTS radical, and showed an iron-reducing capacity of 68.67 ± 2.02 micromol Trolox per gram (concentration-dependent effect). These results are present a new method for the sustainable extraction of bioactive macromolecules.