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Comparison of microwave and ultrasound-assisted extraction techniques for leaching of phenolic compounds from nettle

In this study, extraction of phenolic compounds from nettle by microwave and ultrasound was studied. In both microwave and ultrasound-assisted extractions, effects of extraction time (5-20 min for microwave; 5-30 min for ultrasound) and solid to solvent ratio (1:10, 1:20, and 1:30 g/mL) on total phenolic content (TPC) were investigated. Effects of different powers (50 % and 80 %) were also studied for ultrasound-assisted extraction. In microwave-assisted extraction, the optimum TPC of the extracts (24.64 +/- 2.36 mg GAE/g dry material) was obtained in 10 min and at 1:30 solid to solvent ratio. For ultrasound-assisted extraction, the condition that gave the highest TPC (23.86 +/- 1.92 mg GAE/g dry material) was 30 min, 1:30 solid to solvent ratio, and 80 % power. Extracts obtained at the optimum conditions of microwave and ultrasound were compared in terms of TPC, antioxidant activity (AA) and concentration of phenolic acids with conventional extraction and maceration, respectively. Microwave reduced extraction time by 67 %. AA of extracts varied between 2.95 +/- 0.01 and 4.48 +/- 0.03 mg DPPH/g dry material among four methods. Major phenolic compounds were determined as naringenin and chlorogenic acid in nettle.