Ultrasound-Assisted Extraction of Methylxanthines from Cocoa Bean Shells Using Glycerol–Urea Natural Deep Eutectic Solvent (NADES): Optimization and Greenness Assessment

This study reports the optimization of ultrasound-assisted extraction of theobromine and caffeine from cocoa bean shells using glycerol-urea natural deep eutectic solvents. The solvents were characterized by rheological analysis and infrared spectroscopy. A Doehlert design was applied to optimize sonication time (60-300 s) and water content (30-70% v/v) in the glycerol-urea natural deep eutectic solvents formulations. Theobromine and caffeine concentrations, quantified by high-performance liquid chromatography, were used as response variables, and simultaneous optimization was performed using the desirability function. Under optimized conditions (45.5% water and 4 min sonication), the method yielded 29.18 ± 0.07 mg g^-1 DM of theobromine, 2.46 ± 0.04 mg g^-1 DM of caffeine, and 91.79 ± 3.77 mg GAE g^-1 DM of total phenolics. Antioxidant capacity was also high, with values of 230.65 ± 5.41 mg TE g^-1 DM (ABTS) and 74.34 ± 3.84 mg TE g^-1 DM (DPPH). The high-performance liquid chromatography method demonstrated good linearity, precision, accuracy, and detection limits. Greenness assessment indicated low environmental impact (AGREE = 0.55/1.00; AGREEprep = 0.67/1.00), confirming the method's alignment with green chemistry principles. Overall, the proposed approach represents a sustainable and efficient alternative for the recovery of bioactive compounds from cocoa processing residues. The workflow uses inexpensive, food-relevant components, short sonication times, and moderate temperatures, supporting transferability to continuous ultrasound reactors. The resulting methylxanthine-rich extract is suitable for downstream formulation of food, beverage, or nutraceutical ingredients after routine polishing/dilution steps. The solvent can be prepared from bulk commodities and reused, aligning with circular economy strategies for cocoa processing.