Use of Natural Deep Eutectic Based Green Solvents to Replace Organic Solvents to Extract Phytochemicals from Spearmint Leaves

Environmentally friendly green extraction methods, such as natural deep eutectic solvents (NADES) are being evaluated to replace the traditionally used organic solvents. In the present study, solvents prepared with choline chloride (ChCl) (propylene glycol, PG; malic acid, MA; citric acid, CA) and ethanol (60%) were used for extraction of spearmint leaves with an ultrasonic bath at 50 °C. NADES solvents had higher total phenolic yields (91.09-94.09 mg GAE g-1 DW) than ethanol (74.27 mg GAE g-1 DW). However, only ChCl:PG had higher flavonoid yield than the ethanol extraction (70.56 and 63.28 mg CAE g-1 DW, respectively). CUPRAC and DPPH assays were used to evaluate the antioxidant capacity of NADES and ethanol solvents. All NADES solvents had higher antioxidant capacity than ethanol, but antioxidant capacity varied within NADES solvents. Individual carotenoids, flavonoids and phenolic acids were quantified by UHPLC-MS/MS analyses. Only ethanol and ChCl:PG extracted β-carotene and lutein from leaves. The type of phenolic and flavonoid compounds did not differ between the solvents, but the extracted amounts differed. ChCl:PG had higher extraction yield for all compounds, followed by ethanol. Rosmarinic, chlorogenic and caffeic acids and hesperidin and rutin were the prevalent phenolic acids and flavonoids, respectively. Hierarchical cluster and greenness analysis were performed, and the solvents were differentiated based on their extraction yields and types, and NADES had higher greenness score (0.71) than ethanol (0.64). The results indicated that NADES solvents differ in their efficiency and could be alternatives to traditional solvents for extraction of plant-based phytochemicals.