Green approach for obtaining bioactive compounds from Pterocaulon polystachyum through ultrasound-assisted deep eutectic solvent extraction

A green approach for the extraction of bioactive compounds from Pterocaulon polystachyum was studied. The extraction process was optimized regarding time, temperature, solid/liquid ratio and technique, either ultrasound-assisted extraction (UAE) or heating and stirring. Eleven deep eutectic solvents (DES) based on choline chloride, menthol and betaine were evaluated to identify the most effective solvent to extract bioactive compounds from P. polystachyum. The solvent was selected based on the total phenolic content (TPC) extracted. For all DES systems, their relative polarity, the water content, viscosity and density were determined. The results showed that the best extraction condition regarding extraction yield was 30 min, 40 °C, S/L ratio of 1:20 (m/m) and using UAE technique. The best DES for TPC extraction was Lac:Gly:Wat (3:1:3), with a TPC of 50.68 ± 2.87 (mg GAE/g P. polystachyum), and the Bet:Et (1:3) DES was the best extraction system without water and lactic acid in its composition, which are two components that can directly affect cell viability in cytotoxicity assays. Additionally, using the same extraction conditions, valorization of biomass wastes after steam distillation and supercritical fluid extraction was successfully achieved. In the cytotoxicity assays towards spontaneously immortalized human keratinocyte (HaCaT) cells, certain inconclusive results were obtained for the Lac:Gly:Wat (3:1:3) extracts, mainly because the color of the extract interfered with the absorbance measurement. For Bet:Et (1:3) extracts this interference was not observed and the half-maximal effective concentration value (EC₅₀) was determined as 227 ± 64.29 mg/mL for Bet:Et (1:3) pure solvent, and between 114 ± 9.49 and 206.9 ± 15.61 mg/mL for Bet:Et (1:3) extracts. These results suggest that the P. polystachyum extracts obtained from the different plant materials are non-toxic to cells at concentrations typically used in cosmetic applications, highlighting the potential for creating new products using an environmental-friendly process.