Green extraction of phenolic compounds from foxtail millet bran by ultrasonic-assisted deep eutectic solvent extraction: Optimization, comparison and bioactivities

An ultrasonic-assisted extraction (UAE) protocol using deep eutectic solvent (DES) was employed to extract phenolic compounds from foxtail millet bran (FMB). DES composed with betaine and glycerol in a 1:2 M ratio was selected basing on the total phenolic content (TPC) extraction yield, with the optimal extraction technology investigated using response surface methodology (RSM) with Box-Behnken design (BBD). The optimized process obtained was as follows: DESs with water content of 29 mL/100 mL, ultrasonic power at 247 W, extraction temperature of 61 °C, and extraction time of 31 min. The TPC of the extract was 7.80 ± 0.09 mg ferulic acid equivalent (FAE)/g under the optimum extraction conditions, with the result corresponding well with the model prediction. DES-based UAE produced higher total phenolics, total flavonoids, in vitro antioxidant activity and acetylcholinesterase inhibitory activity than the conventional solvent extraction. The phenolic extract from FMB with DES-based UAE was mainly composed of fifteen phenolic compounds, with p-coumaric acid, apigenin-C-dihexoside, and N′, N″-di-p-coumaroylspermidine being the predominant phenolic compounds. Additionally, 1-O-p-coumaroylglycerol was detected for the first time in FMB. The microstructure differences of the FMB samples following extraction were confirmed using scanning electron microscopy (SEM).