Extraction of pigments from chestnut (Castanea mollissima) shells using green deep eutectic solvents: Optimization, HPLC-MS identification, stability, and antioxidant activities.

The chestnut processing industry produces a large amount of chestnut shells, which are usually discarded, resulting in a significant waste of biomass resources. In this study, chestnut shell pigments (CSP) were extracted using ultrasound-assisted deep eutectic solvent (DES) extraction. The choline chloride-lactic acid was selected as the optimal solvent. The total polyphenols content (TPC), DPPH, and FRAP were 43.83 mg GAE/g DW, 91.31 %, and 0.86 μmol/mL under the optimum conditions (61 °C, 21 %, 61 min). A total of 36 compounds were qualitatively determined in CSP using HPLC-MS. Calomelanol F, dihydrokaempferol, catechin, procyanidin B2, procyanidin A2, ellagic acid, isorhamnetin-3-O-glucoside, and coumarin derivatives were the dominant compounds. Compared with DES-3 (choline chloride -urea) and ethanol, DES-1 extract exhibited significantly higher color stability and antioxidant activity. FTIR and SEM analysis revealed that DES-1 formed more hydrogen bonds and showed stronger erosion effect, thereby enhancing the stability and extraction rate. This study provides a green and efficient alternative method for extracting natural pigments from chestnut shells.