Ionic Liquids as Additives for Extraction of Saponins and Polyphenols

Article pubs.acs.org/IECR

Ionic Liquids as Additives for Extraction of Saponins and Polyphenols from Mate (Ilex paraguariensis) and Tea (Camellia sinensis) Bernardo D. Ribeiro,†,* Maria Alice Z. Coelho,† Luis Paulo N. Rebelo,‡ and Isabel M. Marrucho‡,§,* †

Escola de Química Universidade, Federal do Rio de Janeiro, 21941-598 Rio de Janeiro, RJ, Brazil Instituto de Tecnologia Química e Biologica, Universidade Nova de Lisboa, Av. Republica, 2780-157 Oeiras, Portugal § Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal ‡

S Supporting Information *

ABSTRACT: Extracts from plant tissue are a rich source of lead compounds for nutraceutical or pharmaceutical applications. Nevertheless, the concentration of added value compounds in the plants extracts is usually lower than 1−2%. As consequence, a search for new improved, more efficient technologies combined with solvent engineering has emerged in the last years. In this work, we evaluate the performance of ionic liquids (ILs) as additives in the extraction of saponins and polyphenols from tea and mate. Two families of ILs, imidazolium- and cholinium-based, combined with a wide variety of hydrophilic anions, have been tested. The influence of several parameters such as ionic liquid chemical structure, water content, solvent/raw material ratio, temperature, and contact time period was evaluated. The best performing IL in the extraction of saponins and polyphenols was chosen to pursue to the concentration step using ILs-based aqueous biphasic systems (ABS) were tested. Finally, the saponins and polyphenols existent in the ABS IL-rich phase were recovered through the addition of a second more hydrophobic IL.

1. INTRODUCTION Tea (Camellia sinensis, Theaceae)1−3 and mate (Ilex paraguariensis, Aquifoliaceae)4−7 leaves are used to prepare fermented and nonfermented beverages, which are among the most consumed worldwide. Both these plants present in their composition numerous active compounds, which are responsible for their health benefits. Among them, two major classes of compounds are polyphenols and methylxanthines, followed by alkaloids, volatile oils, polysaccharides, amino acids, vitamins, inorganic elements, etc. Saponins are a structurally diverse class of compounds that are largely distributed in plant kingdom and associated to defense functions in plants. Chemically, they are referred to as triterpenic or steroidal glycosides and they consist on nonpolar aglycones coupled to one or more polar monosaccharide moieties. It is this combination of polar and nonpolar structural elements that explains their surface active properties. Many different saponins have been isolated from a variety of plant sources and display interesting physicochemical (foam production, emulsification, solubilization, sweetness, and bitterness) and biological (hemolytic, antimicrobial, insecticide, and moluscacide) properties that are commercially explored in many applications such as food, cosmetics, pharmaceutics, and bioremediation.8−12 Saponins and polyphenols are considered key ingredients in traditional Chinese medicines and responsible for most of the observed biological effects.13 Several authors1,3−5,7 reported the existence of triterpenic saponins in tea and mate (0.68 and 0.65% in tea and mate, respectively) with chemical structures displayed in Figure 1. Sagesaka et al.1 and Kobayashi et al.3 found the existence of aromatic groups, as cinnamoyl and angeloyl, in tea saponins. These groups are responsible for the more hydrophobic character of these saponins. On the other hand, Gosmann et al.,4 Gnoatto et al.,5 and Sugimoto et al.7 characterized mate saponins as bidesmosides oleananes and ursane-types, with a © 2013 American Chemical Society

Figure 1. Chemical structures of saponins existent in (1) tea (teasaponin B1) and (2) mate (matesaponin).

glucose or dissacaride esterifying C-28, indicating a more hydrophilic character of these saponins. Heck and De Mejia6 reported that the polyphenols found in mate are caffeic acidderivatives, while Sharangi2 described tea flavanols as mainly epicatechin-derivatives. The method commonly used to extract saponins and phenolic compounds from vegetable matrices makes use of solvents as water, methanol, ethanol, or their mixture.14 Tian et al.15 used a 30% aqueous solution of ethanol, at 50 °C, during 1 h in simple orbital extraction of glycyrrhizic acid and glabridin (saponin and polyphenol) from licorice, recovering 89.7% of Received: Revised: Accepted: Published: 12146

February 18, 2013 July 8, 2013 July 20, 2013 July 20, 2013 dx.doi.org/10.1021/ie400529h | Ind. Eng. Chem. Res. 2013, 52, 12146−12153

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inorganic salts, K3PO4 and Na2CO3, were tested. These inorganic salts were chosen due to their high salting out capacity.36 Finally, taking advantage of the complete miscibility of most of the ILs mixtures, the saponins and phenolic compounds existent in the ABS IL rich phase were recovered through the addition of a second hydrophobic IL promoting the formation of two immiscible liquid phases, where the saponins and the phenolic compounds are recovered in the aqueous phase.

saponins. Regarding saponins and phenolic compounds concentration, the most used method is the partition of those compounds from aqueous solutions into butanol.11 Another option is the use of aqueous biphasic systems (ABS). These systems are formed when two mutually incompatible, though both miscible with water, polymer/polymer, polymer/salt, or salt/salt systems are employed. Some authors reported the use of ABS as efficient method for saponin concentration: Han et al.16 used polyethyleneglycol 4000 and K2HPO4 in aqueous extract of Momordica charantia, to recover concentrated saponins in the saline phase. Tan et al.17 used an ethanol aqueous solution and KH2PO4 to recover glycyrrhizin, obtaining partition coefficients of 26. Since the last two decades, a new class of compounds has been emerging. Ionic liquids (ILs) are composed of ions, organic cations combined with organic or inorganic anions, which present melting point under the conventional temperature of 100 °C. These compounds are known to have an unusual combination of properties such as negligible vapor pressure, high thermal stability, and dissolution of chemically diverse compounds, ranging from simple model molecules to complex metabolites present in microorganisms and plants.18−20 In fact, the use of ionic liquids as solvents is by far the most important application in terms of the number of studies carried out. Ionic liquids have been shown not only to increase the solubility of poorly soluble compounds but also to selectively discriminate one compound over others, by finetuning their chemical structure and thus their properties. In 2003, Gutowski and co-workers21 published the first article on the use of ionic liquids (ILs) to promote ABS. Since then, the use of IL-based ABS to recover biomolecules,22−26 enzymes,27 and other solutes28−31 from aqueous solutions has been successfully explored. However, most of the published works where ionic liquids are used as extractants end at the point where the extraction is accomplished, leaving the solute inside the ionic liquid. Few papers report the recovery of the solute from the ionic liquid.32 Due to the high specific intermolecular forces, and despite the low volatility, it is very difficult to recover the solutes through distillation. Other more efficient techniques have been proposed, such as the change of pH for acids.31 The present work aims at evaluating the extraction efficiency of saponins and phenolic compounds from dried leaves and aerial parts of mate and tea, using ionic liquids aqueous solutions extraction schemes. Two major families of ionic liquids were tested as extractants: (i) imidazolium- and (ii) cholinium-based ionic liquid. Imidazolium-based ILs are probably the most used family of IL and the cholinium-based ILs represent an emerging class of cheap, benign, nontoxic, and, in the case of the anions used in this work, biodegradable ILs.33,34 Choline chloride, a water-soluble essential nutrient which supports several biological functions.35 The influence of several parameters such as ionic liquid chemical structure, water content, solvent/raw material ratio, temperature, and contact time period were evaluated. The results here obtained were compared to those when the conventional water + ethanol mixture was used. The best performing IL in the extraction of saponins and phenols from the plant matrices was chosen to pursue to the concentration step. A central composite statistic method was used to calculate the system composition which simultaneously maximizes the extraction of both saponins and phenolic compounds. The next step is the concentration of the saponins and phenol using ILs-based ABS systems, where two

2. MATERIALS AND METHODS 2.1. Materials. Green tea and mate were kindly provided by the company Leão Jr (Rio de Janeiro, Brazil). The ionic liquids 1-butyl-3-methylimidazolium chloride ([C4mim]Cl), 1-hexyl-3methylimidazolium chloride ([C6mim]Cl), 1-octyl-3-methylimidazolium chloride ([C8mim]Cl), 1-(2-hydroxyethyl)-3methylimidazolium chloride ([C2OHmim]Cl), 1-allyl-3-methylimidazolium chloride ([Amim]Cl), 1-benzyl-3-methylimidazolium chloride ([Bzmim]Cl), 1-ethyl-3-methylimidazolium dicyanamide ([C2mim][dca]), 1-ethyl-3-methylimidazolium ethylsulfate ([C2min][EtSO4]), 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([C2min][OTf]), and choline bis(trifluoromethylsulfonyl)imide ([Ch][NTf2]) were supplied by Iolitec (Heilbronn, Germany). Diosgenin, tannic acid, potassium phosphate tribasic, sodium carbonate anhydrous, 1ethyl-3-methylimidazolium chloride ([C2mim]Cl), and choline chloride ([Ch]Cl) were acquired from Sigma-Aldrich. The ionic liquids 1-ethyl-3-methylimidazolium acetate ([C2mim][Ac]) and 1-ethyl-3-methylimidazolium lactate ([C2mim][Lac]) were synthesized by anionic exchange of [C2mim]Cl solution in Amberlite IRN78 resin and then reacting slowly with the corresponding acid, while choline acetate ([Ch][Ac]) and choline hexanoate ([Ch][Hex]) were made by metathesis reaction between choline bicarbonate (Sigma-Aldrich) and the respective acid. To reduce the water and volatile compound contents to negligible values, each of the IL individual samples was dried under constant agitation at vacuum and moderate temperature (80 °C) for a minimum of 48 h. After this procedure, the purity of each IL was further checked by 1H and 13 C NMR spectra and found to be >99% (w/w) for all samples. The water used was ultrapure, double distilled, passed through a reverse osmosis system, and further treated with a Milli-Q plus 185 water purification apparatus. 2.2. Methods. The saponins and polyphenols content were determined by the well established vanillin-sulfuric acid method37 and Folin Denis method,38 respectively, using spectrophotometry as analytical technique. 2.2.1. Extraction of Saponins. Both dried leaves and aerial parts from mate and tea were grinded and sieved to 32 Mesh Tyler, and 50 wt % aqueous solutions of ILs were used. The choice of concentration of the ILs in the aqueous solution, 50 wt %, was done, as it is an intermediate value neither favoring water nor IL. Since we used a central composite experimental design to ascertain the system composition that leads to the best results, we thought that having data for 50 wt % composition would be most adequate. These reasons were at the basis for our choice to work with Ch Cl aqueous solutions. The experimental extraction parameters were as follows: temperature, 40 °C; agitation speed, 1400 rpm; extraction time, 2 h; and solvent (IL/water (1/1) wt) to raw material (tea and mate grinded leaves) ratio, 10. The response variable was the extraction efficiency (%) of saponins and polyphenols, defined as the ratio between the quantity of metabolite extracted using 12147

dx.doi.org/10.1021/ie400529h | Ind. Eng. Chem. Res. 2013, 52, 12146−12153


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evaluated and selected for the maximization of the global desirability function, D, which is defined as the geometric mean of all individual functions di.39 2.2.2. Concentration and Recovery of Saponins. Using the optimized extraction conditions, K3PO4 and Na2CO3 were added as salting out salts to the extracts with aqueous solutions of [Ch]Cl and of ethanol, in order to obtain 20% (w/v) of salt, promoting the formation of an aqueous biphasic system. The ABS efficiency was evaluated through the calculation of the partition coefficients and the concentration factor. The partition coefficient (Ki), defined as the ratio of concentration of the i solute (saponin or phenolic compounds) between top (IL-rich) and bottom (inorganic salt-rich) phases, and concentration factor (α), which is ratio of saponin concentration in phase rich in saponins (IL rich phase, top) and in the initial [Ch]Cl aqueous solution, were calculated according to the following equations:

the IL solution and its maximum quantity extracted using the Soxhlet method and water. Since the quantity of saponins and polyphenols in both tea and mate is unknown, optimal conditions for the Soxhlet method using water was used for 24 h. Although this last method allows the maximum extraction of metabolites, high extraction time and low industrial feasibility prevents its common use. The most efficient IL was selected to set up a central composite experimental design (Table 1), in Table 1. Optimization of Saponin Extraction for [Ch]Cl Using a Central Composite Experimental Designa parameters

−1.41

−1

0

+1

+1.41

IL/S wt % H2O

3.78 14.64

10.00 25.00

25.00 50.00

40.00 75.00

46.21 85.35

a

The mass ratio of IL and substrate (IL/S) and the mass fraction of water in the ionic liquid (wt % H2O) were used as model parameters.

order to determine the IL/raw material ratio (IL/S) and water content in IL (wt % H2O), which leads to the maximum extraction efficiency. The results of experimental design were analyzed employing the software STATISTICA 6.0 (Statsoft Inc., Tulsa, U.S.A.) by the desirability approach, which converts each response variable in an individual desirability function di, ranging from 0 to 1. Then, these function di values are

Ki =

Ci ,top(g/L) Ci ,bottom(g/L)

αSAP =

(1)

C top(g/L) C initial(g/L)

(2)

Figure 2. Influence of the IL cations in the extraction of mate (A) and tea (B) saponins and polyphenols. 12148



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Figure 3. Influence of the IL anions in the extraction of mate (A) and tea (B) saponins and polyphenols.

the imidazolium cation seems to be more important in the case of mate than tea, since a decrease in the efficiency was obtained when the alkyl side chain increases from [C2mim]Cl to [C8mim]Cl. The introduction of the OH group in the alkylside chain of the imidazolium yielded higher tea saponins extraction efficiency, probably indicating an interaction between hydroxyl group of IL and acetyl groups of saponin. Comparing the introduction of functionalities in the imidazolium alkyl side chain using the ILs [C2mim]Cl, [amim]Cl, and [C2OHmim]Cl, it can be concluded that the introduction of polarity helped in the extraction of tea saponins and had the opposite behavior for mate saponins. In Figure 3, the effect of changing the anion for the two studied families of ionic liquids is presented. In what concerns cholinium-based ionic liquids, the use of the chloride anion yields the best results for the two matrixes. For the imidazolium-base ILs, most ILs perform equally well for the extraction of both mate (with the exception of [C2mim][Lac]) and tea saponins (with the exception of [C2mim][Ac]). An interesting result is obtained when the lactate anion is used, since the highest and lowest extractions of saponins were obtained from tea and mate, respectively. The use of acetate anion, combined with both the cholinium and the imidazolium cations, enabled high extraction efficiency of saponins from mate, clearly indicating that the this anion is responsible for this result. Regarding the extraction of polyphenols, the use of the Folin Denis method is not adequate in the presence of [C4mim]Cl, [C6mim]Cl, [C8mim]Cl, and [Bzmim]Cl, since a blue precipitate occurs. We have repeated this experiment several

After the separation of the aqueous phases, [Ch][NTf2] was added to the IL rich phase to achieve the formation of an IL phase, composed mainly by [Ch][NTf2] and [Ch]Cl, and an aqueous phase where saponins and phenols remain. Equations 1 and 2 were also used to evaluate the efficiency of this procedure, where the top phase is the water-rich phase, the bottom phase is the mixture of ILs-rich phase, and the initial solution is the IL-rich phase from the ABS.

3. RESULTS AND DISCUSSION 3.1. Extraction of Saponins from Tea and Mate. The results of the extraction of saponins and phenolic compounds from two natural sources, tea and mate, using 50 wt % aqueous solutions of ILs can be seen in Figure 2 and 3 and are listed in Table S1 (Supporting Information). The ionic liquids were chosen to provide a comprehensive study of the effect of the ionic liquid cation and anion chemical structure on the extraction efficiency. In Figure 2, the influence of the IL cation in the extraction of saponins from tea and mate can be observed. The chloride anion was kept constant for all the ionic liquids used. Note that the use of aqueous solutions of ionic liquids generally provides greater extraction than the method traditionally used based on 30 wt % aqueous solution of ethanol. Generally, higher saponin extraction was obtained for tea than mate. [Bzmim]Cl presented the highest saponin extraction efficiencies from both the raw materials, probably due to aromatic interactions between the aromatic group and the aglycone. However, also note that high extraction efficiencies were obtained when [Ch]Cl is used to extract both mate and tea saponins. The effect of the alkyl side chain of 12149



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Figure 4. Optimization diagrams of saponin extraction from mate (1) and tea (2) using [Ch]Cl, using a central composite experimental design.

times in order to confirm this result. NMR experiments of the pure ILs were also carried out in order to ensure that these ILs are pure. We think that a complex between tungstatemolybdenium and the IL was formed and precipitated, making this method not suitable for these ILs. In this way, these results were eliminated from Table S1 (Supporting Information) and not represented in Figures 2 and 3. The first observation that can be made is that the extraction efficiency of phenolic compounds from both mate and tea is smaller than that of saponins. Comparing the effect of the ionic liquid cation, it can be observed that [Ch]Cl and [C2mim]Cl yielded the highest extraction efficiency for both mate and tea, while [Bzmim]Cl yielded the lowest. Again the influence of the anion does not seem to be very relevant (exception for [C2mim][dca] for mate) with the chloride anion performing equally well as the other anions. The evaluation of the extraction results lead to the conclusion that [Ch]Cl is a good choice of a solvent for the extraction of both saponins and polyphenolic compounds, since besides its good extraction efficiency for both metabolites, it is nontoxic and environmentally acceptable. A central composite experimental design was used to optimize the extraction of saponin and phenolic compounds from tea and mate. In the central composite design, the mass ratio IL/raw material (IL/S) and water content in IL (wt % H2O) were evaluated using the desirability function from Statistica 6.0 to maximize saponin and phenolic compounds recoveries, as shown in Figure 4. The Pareto diagrams are given Figure S1 and S2 of the Supporting Information. In the extraction of mate and tea saponins, the linear term wt % H2O had the largest influence in saponin extraction, while the linear term of IL/S was mostly important in the phenolics compounds. The experimental data correlation for mate and tea phenolics compounds presented higher correlation coefficients R2 = 0.969 and 0.982, respectively, than those obtained for saponins, R2 = 0.705 and 0.854, respectively. The optimal conditions were similar for both raw materials: for mate, 70.5% water and 29.5% [Ch]Cl, with a ratio solvent/raw material 44.94; for tea, 71.2% water and 28.8% [Ch]Cl, with a ratio solvent/raw material 46.21. Using these optimized conditions, the influence of temperature and contact time was evaluated in the extraction of mate and tea saponins using an aqueous solution of [Ch]Cl, and the obtained result was compared to that of aqueous solution of ethanol 30%. As it can be seen in Figure 5, the temperature has

little influence in the saponin extraction for both ethanol and [Ch]Cl aqueous solution.

Figure 5. Influence of the temperature in the saponin extraction from mate (1) and tea (2).

Since temperature has low influence on the extraction of saponins and in order to avoid any saponin degradation, the evaluation of the influence of the extraction time was accomplished at 40 °C, and the results are plotted in Figure 6 for mate and tea. These tests could have been carried out at ambient temperature, thus enhancing the sustainability of the process. In the extraction of mate saponins, [Ch]Cl obtained extraction efficiencies near 70% in 4 h, while for ethanol only 60% of saponins were extracted in the same time period. In the case of tea saponins, the extraction efficiencies were lower than those obtained for mate: in 30 min, a maximum of extraction efficiency near 35% was obtained, while for ethanol the extraction efficiency started at 25% at 30 min and increased to 39% in 4 h. 12150



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in Table 3 and indicate that it is possible to concentrate the saponins using this technique. Again, different results were Table 3. Partition of Tea and Mate Saponins in Biphasic [Ch]Cl, [Ch][NTf2], Inorganic Salt, and Water System materials tea mate

α

K

metabolites saponins polyphenols saponins polyphenols

6.36 8.08 20.87 7.16

± ± ± ±

0.30 0.45 1.40 0.50

0.75 0.86 0.87 0.62

± ± ± ±

0.05 0.09 0.10 0.07

obtained for the two saponins: for tea saponins, the K value is 1 order of magnitude smaller than that for mate. Nevertheless, the concentration factor is similar for both matrixes and smaller than the unity. The removal of the hydrophilic ionic liquid from the aqueous phase was tested by using a chloride anion electrode and is almost complete, since only 1.5 wt % of [Ch]Cl is present in the aqueous phase for both raw materials. In this way, the aqueous phase consists almost exclusively of saponins and phenolic compounds. Figure 6. Influence of time in the saponin extraction from mate (1) and tea (2).

4. CONCLUSIONS In this work, the extraction of saponins and polyphenols was accomplished using ionic liquids as extractants. The obtained results indicated that it is possible to tune the ionic liquid affinity for a specific solvent and considerably increase the extraction efficiency of saponins and phenols. Cholinium-based ionic liquids, in particular [Ch]Cl salt, were favored instead of imidazolium-based ionic liquids, since the former has the advantages of being biocompatible, biodegradable, less toxic, agro-based resources, and at least 10 times cheaper. The results obtained indicate that high concentration of saponins could be achieved using [Ch]Cl and K3PO4 as well as their recovery in a water phase, with only a very small amount of IL present. In conclusion, ionic liquids are clear alternatives to organic solvents in extraction of plant metabolites, as such saponins.

3.2. Recovery of Saponins using Aqueous Biphasic Systems. According to the optimized conditions obtained from central composite experimental design, a 30 wt % aqueous solution of [Ch]Cl was used to extract saponins and polyphenols from the two matrixes. Then, two inorganic salts, K3PO4 and Na2CO3, were rehearsed to promote the formation of the two aqueous immiscible phases. Only K3PO4 was able to render phase separation. The traditional method, the extraction with an aqueous solution of 30 wt % of ethanol was also tested for aqueous biphasic system formation, which was achieved using both inorganic salts. Table 2 summarizes the extraction results obtained using [Ch]Cl and ethanol-based aqueous biphasic systems. For tea metabolites, 2 orders of magnitude higher K values were obtained for both saponins and phenolic compounds using [Ch]Cl compared to the ethanolic solutions. However, the concentration factors of tea saponins were bellow the unit for all the aqueous biphasic system used. For mate metabolites, similar results, in terms of K values and saponin recovery, were obtained for the 3 ABS used. Nevertheless, the concentration factors for mate saponins are higher than those obtained for tea saponins. In order to recover the saponins from the ionic liquid rich phase of the aqueous biphasic system, a more hydrophobic IL, [Ch][NTf2], was added in molar ratio 1/2.40 The objective is to remove the maximum quantity of the hydrophilic IL, so that an aqueous concentrated solution of saponins and phenolic compounds could be obtained. The results obtained are listed

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information about the extraction efficiencies of saponins and polyphenols from mate and tea leaves as well as the pareto charts of experimental design. This information is available free of charge via the Internet at http://pubs.acs.org/

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +351 214469414. Fax: +351 214411277. E-mail: dias. [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

Table 2. Partition Constants and Concentration Factors of Tea and Mate Metabolites in the ABS Used raw materials

solvent

inorganic salt

tea tea tea mate mate mate

ethanol ethanol [Ch]Cl ethanol ethanol [Ch]Cl

Na2CO3 K3PO4 K3PO4 Na2CO3 K3PO4 K3PO4

KSAP 0.12 0.13 46.28 5.58 2.32 1.49 12151

± ± ± ± ± ±

α

KPHE 0.03 0.03 2.70 0.42 0.18 0.10

0.69 0.40 2.87 1.47 1.02 2.24

± ± ± ± ± ±

0.05 0.03 0.32 0.15 0.08 0.25

0.76 0.68 0.60 7.77 6.61 3.34

± ± ± ± ± ±

0.06 0.07 0.05 0.65 0.58 0.25



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ACKNOWLEDGMENTS Authors are grateful for the financial support given by the Fundaçaõ de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ), Coordenaçaõ de Aperfeiçoamento de Pessoal de ́ Superior (CAPES), Conselho Nacional de DesenvolviNivel ́ mento Cientifico e Tecnológico (CNPq), and Fundaçaõ para a Ciência e a Tecnologia (FCT) through the project PTDC/ EQU-EPR/104554/2008. I.M.M. acknowledges FCT/MCTES (Portugal) for a contract under Programa Ciência 2007.

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Ionic Liquids as Additives for Extraction of Saponins and Polyphenols

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