Ultrasound and deep eutectic solvent as green extraction technology for recovery of phenolic compounds from Dendrobium officinale leaves

Ultrasound and deep eutectic solvent (DES) are two green and efficient techniques in extraction. In this study, an eco-friendly extraction of phenolic compounds from Dendrobium officinale leaves (DOLs) was investigated. Ten DESs containing choline chloride, betaine, organic acids, polyols and amide, were tailored for extraction. Choline chloride-oxalic acid showed the most promising efficiencies for phenolics as 9.84 ± 0.19 mg GAE/g DW. Ultrasound-assisted DES (UADES) were optimized by a Box-Behnken design of response surface methodology. The optimum conditions were ultrasonic power 330 W, extraction temperature 60 ℃, and liquid-solid ratio 53:1 mL/g with a total phenolic content (TPC) of 17.50 ± 0.35 mg GAE/g DW. Compared with a single or triple frequency ultrasound, a dual-frequency of 40/60 kHz treatment for 15 min made TPC up to be 25.15 ± 1.46 mg GAE/g DW. About 13 phenolics were identified by UPLC-Triple-TOF/MS as the crucial phenolics in extracts, such as rutin and apigenin flavonoids. In addition, leave surface changed dramatically by UADES. These phenolic extracts had eminent antioxidant capacities, such as DPPH• scavenging rate of 65.4% and FRAP value of 0.234 mmol/L FeSO4 at TPC of 4 mg/mL. Accordingly, dual-frequency UADES is a meaningful and green technology for extraction of phenolic compounds.