COSMO-RS-assisted extraction of pectin from fingered citron (Citrus medica L. var. sarcodactylis) using natural deep eutectic solvents: Structural characteristics, antioxidant activities and pectin–κ-carrageenan gel properties

Fingered citron is a pectin-rich citrus resource, but extraction of fingered citron pectin (FCP) using natural deep eutectic solvents (NADES) remains limited. This study combined conductor-like screening model for real solvents (COSMO-RS) prediction with experimental validation to screen NADES for FCP extraction. Thirty-five NADES systems were evaluated using monosaccharides and pectin fragments as molecular models, and five candidates were selected for ultrasound-assisted extraction. Betaine-maltose (Bet-Mal) achieved the highest yield (11.86%), followed by choline chloride-maltose (ChCl-Mal) (9.57%), both higher than water extraction (4.88%). Structural analyses showed that NADES changed the domain composition, molecular size, methyl esterification, surface charge, conformation, and aggregation behavior of FCPs. L-proline-maltose (LP-Mal), Bet-Mal, and choline chloride-fructose (ChCl-Fru) extracted galacturonic acid-rich pectins with high homogalacturonan proportions, whereas glucose-maltose (Glc-Mal) yielded a highly branched rhamnogalacturonan-I-rich fraction with the highest molecular weight (945.2 kDa), largest particle size, and strongest nanoscale aggregation. ChCl-Mal-extracted pectin showed the highest DPPH scavenging activity, while the water-soluble pectin (WSP) had the strongest superoxide anion scavenging activity. The κ-carrageenan composite gels prepared from pectins extracted with different solvents exhibited distinct textural, rheological, thermal, and microstructural characteristics. The LP-Mal-extracted pectin formed the strongest water-retentive gel, with hardness of 1.54 N and water holding capacity of 92.34%, whereas ChCl-Mal and ChCl-Fru showed higher small-deformation viscoelastic moduli. These results provide insight into the solvent-dependent structure–function relationships of FCP and offer a COSMO-RS-guided NADES extraction strategy for obtaining fingered citron pectins with distinct food-related functionalities.