From Foxtail Millet Husk (Waste) to Bioactive Phenolic Extracts Using Deep Eutectic Solvent Extraction and Evaluation of Antioxidant, Acetylcholinesterase, and α-Glucosidase Inhibitory Activities

Abstract

Foxtail millet husk (FMH) is generally removed and discarded during the first step of millet processing. This study aimed to optimize a method using deep eutectic solvents (DESs) combined with ultrasonic-assisted extraction (UAE) to extract phenols from FMH and to identify the phenolic compositions and evaluate the biological activities. The optimized DES comprised L-lactic acid and glycol with a 1:2 molar ratio by taking the total flavonoid content (TFC) and total phenolic content (TPC) as targets. The extraction parameters were optimized to maximize TFC and TPC, using the following settings: liquid-to-solid ratio of 25 mL/g, DES with water content of 15%, extraction time of 41 min and temperature of 51 °C, and ultrasonic power at 304 W. The optimized UAE-DES, which produced significantly higher TPC, TFC, antioxidant activity, α-glucosidase, and acetylcholinesterase inhibitory activities compared to conventional solvent extraction. Through UPLC–MS, 12 phenolic compounds were identified, with 1-O-p-coumaroylglycerol, apigenin-C-pentosyl-C-hexoside, and 1-O-feruloyl-3-O-p-coumaroylglycerol being the main phenolic components. 1-O-feruloyl-3-O-p-coumaroylglycerol and 3,7-dimethylquercetin were identified first in foxtail millet. Our results indicated that FMH could be exploited by UAE-DES extraction as a useful source of naturally derived antioxidants, along with acetylcholinesterase and α-glucosidase inhibitory activities.

Advances in the novel and green-assisted techniques for extraction of bioactive compounds from millets: A comprehensive review

“…The DES is then prepared by thoroughly mixing the selected components in the desired ratio, typically through heating and stirring to ensure proper dissolution [ 149 ]. After which, the raw material containing the bioactive compounds undergoes preparation, which may include grinding [ 150 ], drying, or other pre-treatment methods [ 22 , 151 ]. The prepared sample is mixed with the DES, and selected extraction techniques are applied.…”

“…(63 % higher than UAE). [ 22 ] UAE + DES Foxtail millet bran DES: Choline chloride/ l -proline/Glycine/Betaine/Glycerol + Oxalic acid/Lactic acid/Glycerol/ l -lactic acid/Citric acid, Sodium acetate molar ratio: 1: 1, 1: 2, 1: 3, 2: 5, 1:1, 1:5, 3: 1 Water in DES(v/v): 20–40 % SSR: 1:10 ratio Temperature: 50–70 °C Time: 20–40 min US intensity: 200–300 W -The highest yield was observed at betaine/glycerol in a 1:2 M DESs containing 29 %(v/v) water, 247 W, 61 °C for 31 min. -Corresponding yields of TPC was 7.8 mg FAE/g d.m.…”

“…The foxtail millet husk (FMH) had a notably higher level (5–17 times) of phenolics compared to the dehulled millet used for cooking [ 140 ]. Chunqing Wang et al [ 22 ] employed DES with the UAE to extract phenols from foxtail millet husk, along with flavonoids, α-glucosidase, and acetylcholinesterase inhibitors. To extract a significant quantity of bioactive compounds, the UAE and DES were combined [ 22 , 157 ].…”

“…Chunqing Wang et al [ 22 ] employed DES with the UAE to extract phenols from foxtail millet husk, along with flavonoids, α-glucosidase, and acetylcholinesterase inhibitors. To extract a significant quantity of bioactive compounds, the UAE and DES were combined [ 22 , 157 ]. A significantly higher TPC (67.7 %), TFC (63.49 %), antioxidant activity, α-glucosidase, and acetylcholinesterase inhibitory activities (IC 50 ) was obtained for combined UAE and DES extracted mass compared to UAE alone.…”

“…Currently, there exists a substantial literature addressing innovative extraction methods for extracting bioactive compounds from millets and their by-products. For example, recently the novel and green-assisted extraction techniques including DES assisted technique for phenolic [ 20 ] and flavonoid compounds in sorghum [ 21 ], foxtail millet [ 20 , 22 ], Supercritical fluid extraction of phenolic compounds in Foxtail millet [ 23 , 24 ] and sorghum [ [25] , [26] , [27] , [28] ], US assisted technique for phenolic and flavonoid compounds in sorghum [ [29] , [30] , [31] , [32] ] and proso millet [ 33 ], Microwave-assisted technique for 3-DXA (3-deoxyanthocyanidins) flavonoid [ 34 ]and phenolic compounds in sorghum [ [34] , [35] , [36] , [37] ] and kodo millet [ 38 ], enzyme assisted technique for polyphenol and flavonoid compounds in pearl millet [ 39 , 40 ], finger [ 39 , 41 , 42 ], sorghum [ 39 , 43 ], foxtail [ 44 ]and proso millet [ 45 ] and accelerated assisted technique for phenols compounds in sorghum millet [ 46 ], have been employed for the extraction of millet bioactive components. However, after an extensive review of the literature, and to the author's best knowledge, no comprehensive review articles have been identified that offer an in-depth analysis of novel extraction techniques and their potential applications in the context of millet grains.…”