Development and optimization of green extraction process of spent coffee grounds using natural deep eutectic solvents

The aim of this study was the development and optimization of a green extraction process of spent coffee grounds (SCG) using high energy techniques (such as microwaves and ultrasonic power)and various Natural Deep Eutectic Solvents (NADES) as green extraction solvents. Six NADES were investigated and among them the one derived by betaine and glycerol was selected as the optimum for further investigation. For comparison reasons the SCG extraction using a hydroethanolic solution as extraction solvent was also performed and optimized. A 3 level-4 factor Taguchi design of experiments was implemented in order to optimize the parameters of the ultrasound-assisted (UAE) and the microwave-assisted extraction (MAE). The effects of NADES-to-water ratio, solid-to-liquid ratio, extraction time as well as temperature in the case of MAE and ultrasonic power in the case of UAE were tested. A L9 orthogonal experiment plan was applied with combinations among factors and their levels in order to study their effect on phenolic compounds extraction. The metric of signal-to-noise (S/N) ratio was employed to find the optimum conditions. The extracts' total phenolic (TPC) and total flavonoid content (TFC) were used as the response factors. Regression analysis was used for results prediction in conditions within and beyond the design levels. Confirmation experiments were performed at optimum conditions and were in accordance to the predicted values. Finally, the optimum extraction conditions were implemented in various varieties of coffee grounds and TPC and TFC were determined before and after brewing. Finally, the extract's caffeine content was measured by GC-MS analysis.