Green Strategies for Recovery of Bioactive Phenolic Compounds from Agro-Industrial Wastes (Pomegranate Peels, Almond Hulls, and Elderberry Pomace) Using Natural Deep Eutectic Solvents

Natural deep eutectic solvents (NADES) based on choline chloride and organic acids were applied for the recovery of phenolic compounds from three agro-industrial residues: pomegranate peels, almond hulls, and elderberry pomace. Box–Behnken designs were conducted to reach the best extraction conditions, which were 50 °C, 20 min, and 20 w/w NADES/solid for pomegranate peels, 30 °C, 60 min, and 20 w/w NADES/solid for almond hulls, and 40 °C, 60 min, and 30 w/w NADES/solid for elderberry pomace. Punicalagin was the major phenolic in the NADES-based pomegranate extract, with a concentration about 87% higher than that found in the ethanolic extract obtained under the same conditions. The same was valid for (+)-catechin, a major phenolic in almond extracts, and cyanidin-3-sambubioside, a major anthocyanin in elderberry extracts, with concentrations around 27 and 68% higher in the NADES extracts, respectively. These findings show that the extraction of valuable compounds from agro-industrial waste using NADES was improved with accuracy during ultrasound-assisted extractions.