Abstract:
Wild thyme (Thymus serpyllum L.) herbal dust has been recognized as a potential underutilized
resource for the recovery of antioxidants. The aim of this paper was to optimize natural
deep eutectic solvent (NADES) extraction of polyphenols to obtain improved antioxidant activity
of extracts determined by selected in vitro assays (DPPH, FRAP, and ABTS). Twenty different
NADES systems were investigated in the first step of the screening of the extraction solvent and
L-proline (Pro)–glycerine (Gly) based solvents provided the best results. Preliminary experiments
organized by 25−1 fractional factorial design narrowed down the number of extraction factors from
five (temperature, extraction time, NADES type, water content and L/S ratio) to three and determined
their experimental domain for the final step. A face-centered central composite design with
temperature (40–55–70 °C), extraction time (60–120–180 min) and L/S ratio (10–20–30 g NADES/g
sample) was applied for influence analysis and process optimization. Multi-response optimization
suggested a temperature of 65 °C, time of extraction of 180 min and L/S ratio of 28 g NADES/g DW
as optimal extraction parameters. Experimental validation confirmed good agreement between
experimental and predicted results in the extract obtained at optimal conditions and the interactions
in the most suitable NADES (N16; Pro–Gly–H2O; 1:2:1) were confirmed by the 1H-NMR.
