High-efficiency Extraction of Phenolics from Wheat Waste Biomass (Bran) by Combining Deep Eutectic Solvent, Ultrasound-assisted Pretreatment and Thermal Treatment

The aim of this study was the examination of the effect of ultrasonication pretreatment and thermal treatment of soft wheat bran (SWB) with a novel acidic, tertiary deep eutectic solvent, composed of glycerol/citric acid/glycine, on the polyphenol extraction yield and antioxidant activity. This was achieved by sequentially deploying ultrasonication pretreatment and prolonged thermal treatment (extraction). Ultrasonication prior to stirred-tank extraction was shown to afford significantly higher total polyphenol yield compared to aqueous control solvents. Furthermore, thermal treatment had a striking effect on SWB polyphenol extraction, which increased from 6.31 ± 0.06 mg ferulic acid equivalents (FAE) g− 1 dry mass (dm) (50 °C, 2.5 h) to 57.12 ± 1.14 mg FAE g− 1 dm (90 °C, 24 h). Combination of ultrasonication pretreatment and thermal treatment boosted even more extraction yield at 94.62 mg FAE g− 1 dm. Comparison of glycerol/citric acid/glycine with other common deep eutectic solvents demonstrated that this particular solvent was an exceptionally high-performing system. Liquid chromatography-tandem mass spectrometry analyses showed that the SWB extract produced with glycerol/citric acid/glycine deep eutectic solvent was dominated by polar compound(s), as opposed to acid and alkaline hydrolyzates, which contained virtually only ferulic acid. It was concluded that this investigation could be expanded by designing and screening task-specific deep eutectic solvents (DES) for improved extraction efficiency under less severe thermal treatment conditions. This would contribute towards the establishment of less energy-demanding processes for wheat bran polyphenol recovery.