Novel lactic acid-based natural deep eutectic solvents: Efficiency in the ultrasound-assisted extraction of antioxidant polyphenols from common native Greek medicinal plants

• Synthesis of novel bio-based eutectic mixtures.
• Polyphenol extraction from selected common Greek medicinal plants.
• Process assessment using polyphenol recovery and antioxidant tests.

Selected native Greek medicinal plants, including dittany, fennel, marjoram, mint and sage, were used to test the efficiency of some novel lactic acid-based natural deep eutectic solvents (NADES) to extract polyphenolic compounds. Extractions were performed under ultrasonication and the eutectic mixtures, tested as 80% (v/v) aqueous solutions, were lactic acid:choline chloride, lactic acid:sodium acetate, lactic acid:ammonium acetate and lactic acid:glycine:water, with corresponding molar ratios of 3:1, 3:1, 3:1 and 3:1:3. The three latter NADES are reported for the first time. Water and 60% (v/v) aqueous ethanol were also used as control solvents. The results obtained evidenced that lactic acid:glycine:water exhibited high efficiency, but in some instances lactic acid:sodium acetate and lactic acid:ammonium acetate were equally efficient. The data also suggested that extracts with high polyphenol concentration may also possess higher antiradical activity and reducing power. The NADES tested are non-toxic, renewable and exceptionally efficient solvents for polyphenol recovery from medicinal plants. The findings of this study were interpreted on the ground of assumptions regarding the polarity of the NADES tested.