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.
Shi et al. (Thu,) studied this question.