Sustainable extraction of phenolics from beetroot leaves via hydrophobic DES

Abstract This study evaluates the efficiency of ultrasound-assisted extraction (UAE) for recovering total phenolic compounds (TPC) from dried beetroot leaves (Beta vulgaris subsp. cicla var. flavescens), an agro-industrial by-product with high bioactive potential. A hydrophobic deep eutectic solvent (DES) composed of menthol and lactic acid (1:1 molar ratio) was assessed as a sustainable alternative to absolute ethanol (99.8%). The DES was synthesized via sonication at 40 ± 5°C for 1 h and characterized using FTIR, TGA, and UV–Vis spectroscopy to confirm eutectic formation and thermal stability. A full 2³ factorial design evaluated the effects of solvent type, liquid-to-solid ratio (12 and 16 mL/g), and extraction time (5 and 10 min) at a constant temperature of 50°C. TPC was determined using the Folin–Ciocalteu method at 765 nm. Factorial ANOVA revealed significant main effects and interactions (p < 0.05), with the DES significantly outperforming absolute ethanol across all conditions. The highest TPC yield achieved was 5912 ± 234 mg GAE/100 g dry leaves using DES at a 1:16 ratio for 5 min. These findings demonstrate that menthol–lactic acid DES combined with UAE provides an efficient extraction approach with potential for the sustainable valorization of beetroot leaf residues.