Extraction of Polyphenols and Anthocyanins From Apple Pomace With Natural Deep Eutectic Solvents: Evaluation of Their Antioxidant and Antibacterial Activities

The aim of this study was to evaluate the antioxidant and antibacterial properties of apple pomace (AP) extracts using natural deep eutectic solvents (NADES). Six NADES, based on choline chloride (ChCl), were used as environmentally friendly solvents. Four of these were combined with organic acids, while the other two were combined with urea; they were then used to extract bioactive compounds from AP. ChCl:urea mixture proved to be the solvent with the highest total polyphenol content (TPC), with 13.15 ± 4.70 mg gallic acid equivalent/mL. Antioxidant activity and total anthocyanidin content (TAC) were also assessed. ChCl:oxalic acid recorded the highest values with 35.59 ± 9.53 mg extract/mL and 64.81 ± 4.65 malvidin-3-glucose equivalent μg/mL, respectively. Solvent pH plays a crucial role in selective extraction; an acidic pH facilitates selective anthocyanidin extraction, while a basic pH does not. Anthocyanidin extraction correlated with extract antioxidant activity and solvent viscosity. In addition, the antibacterial activity of the extracts against Bacillus cereus, Listeria innocua, and Escherichia coli strains was studied. All extracts showed antibacterial properties against the strains tested. The ChCl:oxalic acid extracts showed particularly low minimum inhibitory concentrations (e.g., 25 mg_extract/mL for B. cereus) and EC₅₀ values (e.g., 6.0 ± 0.3 mg_extract/mL for B. cereus).