Selective Enrichment of Chlorogenic Acid and Related Phenolic Acids from Spent Coffee Grounds by Ultrasound-Assisted Extraction with Deep Eutectic Solvents

Spent coffee grounds (SCGs), a major by-product of coffee consumption, remain an underused source of chlorogenic acid (CGA) and other phenolic constituents. This study investigated an ultrasound-assisted extraction strategy using deep eutectic solvents (DESs) to improve the recovery and phenolic-acid enrichment of SCGs. Among the tested DES formulations, the betaine-acetic acid system gave the best CGA extraction performance and was therefore used for further optimization by response surface methodology. The optimized process, conducted at a liquid-to-solid ratio of 28 mL/g, 75 °C, and 50 min, produced a CGA yield of 15.18 mg CGA/g dried SCG powder, markedly exceeding that achieved with 70% ethanol under comparable conditions. Structural and chemical characterizations helped explain this improvement. Scanning electron microscopy revealed that the DES-based process caused more evident disruption of the SCG matrix, which favored solvent penetration and mass transfer. Fourier-transform infrared spectroscopy confirmed the formation of a hydrogen-bonding network between betaine and acetic acid. Ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry further revealed that the betaine-acetic acid extract was mainly composed of CGA and hydroxycinnamic acid derivatives. The purified extract also displayed strong in vitro antioxidant capacity. Overall, the betaine-acetic acid DES combined with ultrasound provides an effective green approach for recovering CGA-rich phenolic extracts from SCGs.