Abstract:The extraction process of polyphenols from apple peels by ultrasound-assisted natural deep eutectic solvent (NADES) was established, and the mechanism of NADES was analyzed. After evaluating the polyphenol content of apple peel and antioxidant activity extracted with six NADES and 80% methanol, choline chloride-ethylene glycol (ChCl-EG, 1:4) was identified as the optimal extraction solvent. The process was optimized using single factor and orthogonal experiments, resulting in a solid-liquid ratio of 1:50, a water content of 30%, an extraction duration of 50 min, and an extraction temperature of 60 °C. The extract exhibited significantly higher levels of total phenolic content (TPC), DPPH, ABTS+ free radical scavenging activity, and the ferric ion reducing antioxidant power at 33.16 ± 0.43 mg GAE/g DW and 52.60, 38.30, and 30.76 mg TE/g, respectively, compared to the 80% methanol extracts. These values experienced significant increases of 160.69%, 122.41%, 138.04%, and 102.37%, respectively, following the optimization process. These results indicate that ChCl-EG can extract polyphenols efficiently from apple peels. A total of 17 phenolic compounds were identified from ChCl-EG and methanol extracts using UPLC-MS/MS, with the primary components being consistent. NMR hydrogen spectrum revealed the formation of hydrogen bonds between ChCl and EG molecules. FT-IR analysis indicated that the hydrogen bonding interactions between ChCl-EG and extract molecules was stronger compared to that of the methanol extracts. The analysis of the conductor-like screening model for real solvents (COSMO-RS) revealed that ChCl-EG exhibited superior binding properties with apple polyphenols, with the solubility and interaction energy being 1.39 and 1.12 times those of methanol, respectively. This elucidates the efficient mechanism of ChCl-EG at the molecular level. The results of this study provide a scientific basis for the green extraction technology of polyphenols from apple peel and the efficient mechanism of NADES.
