Enhanced extraction of phenolic compounds using choline chloride based deep eutectic solvents from Juglans regia L.
Introduction
Deep eutectic solvents (DES) were first introduced in 2003 by Abbott and co-workers (Abbott et al., 2003) reporting mixtures of urea and quaternary ammonium salts, in particular choline chloride (CC). Since then many other DES have been reported and their use proposed for a wide range of applications (Francisco et al., 2012; Smith et al., 2014). A DES is a mixture between two (or more) starting materials (hydrogen bond acceptor – HBA and hydrogen bond donor – HBD) where the eutectic temperature of the mixture is considerably lower than that of either of the constituents (Abbott et al., 2004). Some authors prefer to call them low transition temperature mixtures (Francisco et al., 2013; Jancheva et al., 2017).
The replacement of volatile and toxic organic solvents by greener and more performant solvents is one the most important challenges of our days for the chemical industry in general, and biorefineries in particular (Pena-Pereira and Namieśnik, 2014). Choi et al. (2011) introduced the DES as an alternative media to extract secondary metabolites, such as phenolic compounds, from natural matrices instead of conventional organic solvents. Nowadays, this idea is being expanded to create designer solvents, using several combinations of HBA and HBD, with tunable properties to selectively dissolve and extract natural and bioactive compounds from plants, oils or biomass, valorizing natural products or wastes as a source of valuable compounds (Dai et al., 2016; Nam et al., 2015; Paradiso et al., 2016). A very recent review on the application of deep eutectic solvents for the extraction of phenolic compounds can be found in literature, describing the possibility of using DES both as solvent and formulation media with potential cosmetics, pharmaceutical, or food applications (Ruesgas-Ramón et al., 2017).
This new generation of solvents entails several characteristics, which support their insertion in different industries. First, they are simple to prepare from cheap starting materials (Dai et al., 2016). Then, the HBA and HBD can be selected to be less toxic than organic volatile solvents and also biodegradable (Mbous et al., 2017). Moreover, in general, these solvents have low volatility and flammability (Dai et al., 2016). The addition of water to DES, for extraction purposes, is a well-established procedure to reduce the viscosity of the solvents and improve the mass transfer of bioactive molecules from the natural matrices (García et al., 2016; Gu et al., 2014). Regarding temperature, its increase leads to a lower viscosity and, consequently, to higher extraction yields (Dai et al., 2016; Jancheva et al., 2017).
In the present study, the performance of choline chloride (CC) based DES combined with carboxylic acids (CA) is investigated for the extraction of phytochemical compounds from the leaves of walnut trees. Walnut leaves were chosen as a plant model as they stand out as a significant source of bioactive compounds such as phenolic acids and flavonols (Santos et al., 2013; Vieira et al., 2017). These leaves are well known in traditional medicine due to their health benefits arising from their antioxidant, antitumor (Santos et al., 2013), antiproliferative (Carvalho et al., 2010), anti-inflammatory and antinociceptive (Erdemoglu et al., 2003) activities. A DES screening of mixtures of CC and CA is carried by heat-assisted extraction (HAE). Fifteen acids were selected to assess the effect of the alkyl chain length, the number of carboxylic acid groups, and the additional presence of hydroxyl and/or phenyl groups over the extraction performance. The best two DES selected will be used to optimize the extraction conditions of time (t), temperature (T) and water content (S) by response surface methodology (RSM). The work here presented can be better comprehended as a continuation of a previous work from other authors (Vieira et al., 2017), in which they optimized the extraction of bioactive compounds from walnut leaves using hydro-alcoholic mixtures.
Section snippets
Standards and reagents
HPLC-grade acetonitrile and anhydrous citric acid were from Fisher Scientific (Lisbon, Portugal). The phenolic compounds standards 5-O-caffeoylquinc acid (Extrasynthèse, Genay, France) and quercetin 3-O-glucoside, as well as glutaric acid (99 wt%) and glycolic acid (99 wt%) were purchased from Sigma-Aldrich. Butyric acid (99 wt%), lactic acid (88–92 wt%) and valeric acid (99 wt%) were purchased from Riedel-de Haen. Acetic acid (99.5 wt%) was obtained from Labsolve JMGS, propionic acid (99 wt%)
DES screening to investigate the effect of CA molecular structure on the extraction of phytochemical compounds
Phenolic compounds are complex molecules, and their extraction from a solid matrix requires compatible solvents. The responses used to evaluate the efficiency of the extraction were the specific quantification of the phenolic compounds performed by HPLC-DAD (P1, P2 and P3) as described before (Vieira et al., 2017). The phenolic profile of J. regia leaves extracts is in good agreement with the previously reported by Santos et al. (2013) (supplementary material). These compounds were identified
Conclusions
The combined effects of three independent variables were studied to maximize the intrinsic responses on the J. regia chemical composition. A CCCD of three factors with five-levels of 16 combinations and 6 replicates at the center of the experimental domain was successfully applied for the solvent optimization by RSM. Polynomial responses were designed and experimentally verified on 3-O-caffeoylquinic acid (P1), quercetin 3-O-glucoside (P2), quercetin O-pentoside (P3) and total HPLC (sum of the
Acknowledgements
The authors thank the Foundation for Science and Technology (FCT, Portugal) and FEDER under Programme PT2020 for financial support to CIMO (UID/AGR/00690/2013), V. Vieira (SFRH/BD/108487/2015) grant and L. Barros contract. To project POCI-01-0145-FEDER-006984 – Associate Laboratory LSRE-LCM funded by FEDER through COMPETE2020 – Programa Operacional Competitividade e Internacionalização (POCI) – and by national funds through FCT – Fundação para a Ciência e a Tecnologia. To project CICECO‐Aveiro
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