Optimizing the ultrasound-assisted deep eutectic solvent extraction of flavonoids in common buckwheat sprouts

Highlights

• Choline chloride (CC)-based deep eutectic solvent (DES) extraction was evaluated.

• 80% CCTG (CC:triethylene glycol + 20% water), MeOH and EtOH had comparable efficacies.

• The ultrasound-assisted DES (80% CCTG) extraction of flavonoids was optimized.

• Flavonoids in buckwheat sprouts were efficiently extracted by the optimized method.

• Flavonoids in the DES extract were easily recovered with C18 solid-phase extraction.

Abstract

In this study, deep eutectic solvents (DESs) coupled with ultrasound-assisted extraction (UAE) were evaluated and optimized for the extraction of major flavonoids from common buckwheat sprouts. Initially, 18 different choline chloride (CC)-based DESs were evaluated as potential extraction solvents, and 80% CCTG (CC-based DES composed of triethylene glycol and 20 vol% water) extracted significantly higher (p < 0.05) amounts of flavonoids than other DESs studied. The extraction efficiency of 80% CCTG was even higher (p < 0.05) than that of methanol for the extraction of vitexin and quercetin-3-O-robinobioside. Procedure using 80% CCTG and UAE were optimized; an extraction temperature of 56 °C and extraction time of 40 min were considered optimal. The optimized extraction procedure was reliable and efficient for the extraction of major flavonoids from common buckwheat sprouts. In addition, flavonoids could be efficiently recovered from DES extracts with high recovery yields (>97%) by using a C18 solid-phase extraction.

Introduction

Common buckwheat (Fagopyrum esculentum Möench) is one of the most widely grown and consumed buckwheat species worldwide (Suvorova & Zhou, 2018). As a new vegetable, buckwheat sprouts have received much attention from the sprout market because of their abundant protein, vitamin, mineral, and health-associated phenolic compound contents, as well as their attractive fragrance and slightly crispy texture (Kreft, 2016, Zhou et al., 2016). Buckwheat sprouts contain a significantly greater amount of major flavonoids, including orientin, isoorientin, vitexin, isovitexin, quercetin-3-O-robinobioside (Q3R), and rutin, compared to buckwheat seeds (Lim et al., 2012, Nam et al., 2015, Zhang et al., 2012). Moreover, orientin, isoorientin, vitexin, and isovitexin were found abundant only in common buckwheat sprouts but not in tartary buckwheat sprouts (Kim et al., 2008); these compounds showed good antioxidant activity (Terpinc, Cigić, Polak, Hribar, & Požrl, 2016). In addition, Q3R was only detected in common buckwheat sprouts (Nam, Lee et al., 2015), and it showed immunoenhancing activities when combined with its isomer (rutin) (Ajaghaku et al., 2018).

Almost all recent studies dealing with the extraction of the above-mentioned flavonoids in buckwheat and its sprouts have been performed using organic solvents or water-mixed organic solvents due to the low solubilities of some flavonoids (e.g. vitexin and rutin) in water (Jiang et al., 2015, Nam et al., 2015, Terpinc et al., 2016, Zhu, 2016). Despite organic solvents have excellent properties for the dissolution and extraction of phenolic compounds, they possess many intrinsic drawbacks, such as low boiling points that lead to their accumulation in the atmosphere, flammability, high toxicity, non-biodegradability, and cost (Ruesgas-Ramón, Figueroa-Espinoza, & Durand, 2017). In addition, based on an environmental perspective, these solvents pose significant challenges in green chemistry. Therefore, the development of a new alternative green extraction technique and the search for alternative solvents are imperative.

Deep eutectic solvents (DESs) are emerging as alternatives to conventional organic solvents and have attracted attention in many fields due to their unique advantages. DES is a mixture of a halide salt or another hydrogen bond acceptor (HBA) and one or two hydrogen bond donors (HBDs). The most common DESs are formed by using choline chloride (CC), which is a non-toxic quaternary ammonium salt, as the HBA, along with natural uncharged compounds, such as alcohols, amines, carboxylic acids, sugars, and vitamins, as the HBDs (Ruesgas-Ramón et al., 2017). These compounds have good prospects for wider use in green extraction technologies (Belwal et al., 2018). The high kinematic viscosities (typically 200–500 mm²/s at 40 °C) of DESs, however, lead to some practical problems including time-consuming solvent-transfer operations and slow mass transfer during dissolutions or extractions (Dai, van Spronsen, Witkamp, Verpoorte, & Choi, 2013). Diluting DESs with water has proved to be an efficient way of decreasing their viscosities and adjusting the solubilities of compounds (Dai, Witkamp, Verpoorte, & Choi, 2015). To enhance the efficiencies of DESs in the extraction of phenolic compounds, DESs have been combined with other extraction methods, such as ultrasound-assisted extraction (UAE), heating extraction (HE), microwave-assisted extraction (MAE), and stirring (Cunha and Fernandes, 2018, Paradiso et al., 2016). The HE and MAE methods were found to be less effective than UAE, and both may cause losses or changes in the properties of bioactive molecules due to thermal degradation, ionization, hydrolysis, or oxidation during extraction (Bubalo et al., 2016, Nam et al., 2015, Wang et al., 2017). Despite these recent advancements, the application of DESs to the extraction of phenolic compounds, especially flavonoids, in buckwheat remains limited to a single analyte (rutin) (Huang et al., 2017).

Considering the biological importance of flavonoids like orientin, isoorientin, vitexin, isovitexin, Q3R, and rutin in common buckwheat sprouts, it would be very interesting to develop a DES-based extraction method for the extraction of multiple flavonoid analytes and their recovery intended for biochemical and pharmaceutical applications. We propose that multiple flavonoid analytes can be extracted from common buckwheat sprouts using an appropriately optimized DES. Accordingly, our study aimed to evaluate and optimize the efficiency of ultrasound-assisted CC-based DESs on the extraction of orientin, isoorientin, vitexin, isovitexin, Q3R, and rutin in common buckwheat sprouts. After initially screening different DESs, the extraction efficiency of the selected DES was optimized at different extraction temperatures and times using response surface methodology (RSM). Finally, the reliability of the optimized extraction method and the recovery of flavonoids from the DES extract were evaluated.