Role of Solvent and Citric Acid-Mediated Solvent Acidification in Enhancing the Recovery of Phenolics, Flavonoids, and Anthocyanins from Apple Peels

Apple peels are rich in phenolic acids, flavonoids, and anthocyanins, particularly in red-skinned varieties. However, they are commonly underutilized. Their re-evaluation as a source of natural antioxidants and functional ingredients represents an important opportunity to reduce waste and improve sustainability across the apple processing chain. The aim of this study is to evaluate the effect of solvent type (aqueous methanol vs. ethanol) and citric acid-mediated acidification (0.1 to 1.0 g/mL) on the extraction of total phenolics, flavonoids, and anthocyanins from dried Red Delicious apple peels using ultrasound-assisted extraction (50 °C, 10 min). Results indicate that moderate solvent acidification (0.5 g/mL) optimized the recovery of total phenolics and anthocyanins, whereas high acidity negatively affects flavonoid recovery, indicating compound-specific sensitivity to low pH. Ethanol proved superior for flavonoid and anthocyanin extraction, achieving comparable or higher yields than methanol under acidified conditions. At the individual compound level, HPLC analysis showed that only a subset of the phenolic compounds—especially cyanidin-3-O-glucoside—responded strongly to changes in sample conditions, while others, such as catechin and rutin, remained largely unaffected. These findings provide a practical and sustainable strategy for valorizing apple peel residues, highlighting citric acid as a safe, inexpensive acidifying agent and ethanol as a GRAS solvent suitable for scalable recovery of high-value antioxidants.