Effects of deep eutectic solvent systems on the extraction of antioxidant components from peanut (Arachis hypogaea L.) hulls

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Peanut hulls are rich agricultural and agro-industrial wastes with various biological activities. This research explored the use of deep eutectic solvents (DESs) combined with ultrasound in the extraction of active substances from peanut hulls. 5,7-dihydroxychromone, eriodictyol, and luteolin were isolated by high-speed countercurrent chromatography. Twenty-one DESs and three conventional solvents were compared, and L-propine-acetic acid (Pro-Aa) was the best. The various factors at a 1:4 molar ratio (Pro-Aa), which included molar ratios of DES (1:2–1:6), water contents in the DES/water mixture (0–70%), solid/liquid ratios (1:12–1:50), extraction temperatures (30–70 °C) and extraction times (20–60 min), were systematically investigated. The optimal conditions for two flavonoids through response surface methodology (RSM) were established with a 56% water content in DES/H2O for 41 min and a 16:1 mL/g liquid/solid ratio. The dry weight of the peanut hull flavonoids obtained from Pro-Aa was 2.51 mg/g, which was 1.7 and 3.8 times greater than that of methanol and ethanol, respectively. SEM, 1H NMR, and DFT calculations were performed to gain insight into the molecular mechanisms and characteristics of DESs and flavonoids in peanut hulls. Peanut hull extracts have the potential to be used in the pharmaceutical or health care industry or as functional foods.