Solubility of luteolin in several imidazole-based ionic liquids and extraction from peanut shells using selected ionic liquid as solvent

Separation and Purification Technology 135 (2014) 223–228

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Solubility of luteolin in several imidazole-based ionic liquids and extraction from peanut shells using selected ionic liquid as solvent Li Ge, Fan Xia, Yi Song, Kedi Yang ⇑, Zuzeng Qin, Lishuo Li School of Chemistry & Chemical Engineering, Guangxi University, Nanning 530004, China

a r t i c l e

i n f o

Article history: Received 3 December 2013 Received in revised form 16 August 2014 Accepted 22 August 2014 Available online 29 August 2014 Keywords: Ionic liquids (ILs) Luteolin Extraction Response surface methodology (RSM)

a b s t r a c t Several hydrophilic and hydrophobic 1-alkyl-3-methylimidazole ionic liquids (ILs) were screened for their dissolution ability of luteolin to determine the IL suitable for extraction of luteolin from peanut shells. The experimental data indicated that hydrophilic 1-butyl-3-methyl-imidazolium nitrate ([C4mim]NO3) shows considerably higher dissolution ability of luteolin than commonly used organic solvents and other ILs investigated, which can be used as an efficient substitute of organic solvent for extraction of luteolin. The extraction of luteolin from peanut shells was performed using [C4mim]NO3 aqueous solution as solvent, and extraction conditions were analyzed and optimized by response surface methodology (RSM) with central composite design (CCD). Extraction temperature has significant effect on the yield of luteolin, followed by [C4mim]NO3 concentration. Under the optimal conditions namely liquid–solid ratio of 7.6 mL g1, temperature of 100 °C and [C4mim]NO3 aqueous solution of 49%, determined by RSM, the yield of luteolin was 79.8 ± 1.48%, which was close to the 78.4% predicted by RSM. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Luteolin is an important natural flavonoid widely distributed in plant materials [1], which bioactivities were reported including antioxidant [2], anti-inflammatory [3], vasodilation [4], and cancer prevention [5]. The recent studies have also suggested that luteolin could enter the cellular nuclei and suppress the oxidative damage of DNA [6], reduce IL-6 production in microglia by inhibiting JNK phosphorylation and activation of AP-1 [7] and reverse xylazine/ ketamine-induced anesthesia [8]. Luteolin can be obtained via organic solvent extraction directly from natural herbs, such as Reseda luteola and Angelica keiskei [9,10]. However, most of herbs used in traditional medicine are more or less expensive. Peanut shells are abundant and inexpensive by-products of peanut industry. Every year, the yield of peanut shells reaches as high as 5 million tons in China alone, and most of the peanut shells are either sludged for forage and fuel or abandoned, resulting in an enormous waste of natural resources [11]. Investigations have demonstrated that peanut shells are rich in flavonoid and polyphenol components, such as luteolin, eriodictyol and 5,7-dihydroxychromone, and in which luteolin is the major compound [12,13]. Luteolin occurs in plants as the glycosylated form in most cases, but in peanut shells it is in the form of aglycone. Several studies have reported the solvent extraction of ⇑ Corresponding author. Tel.: +86 18376765663; fax: +86 771 3233718. E-mail address: [email protected] (K. Yang). http://dx.doi.org/10.1016/j.seppur.2014.08.022 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.

luteolin from peanut shells [14–16]. However, with increasing safety considerations for operating personnel and consumers, extraction by organic solvents is a challenge due to the volatility, flammable and toxicity of the solvents. Therefore, the desire to reduce the use of organic solvents in the extraction processes for bioactive substances has led to the development of alternative solvents, such as ionic liquids (ILs). In the past decade, ILs have gained great interests as separation media due to their unique physical and chemical properties of low vapor pressure, high thermal and chemical stability, etc [17,18]. Some of recent studies have revealed the ability of ILs to extract natural organic compounds since Huddleston et al. reported the first successful extraction of substituted-benzene derivatives using ILs [Bmim]PF6 initially [19–24]. In the present work, we investigate the solubility of pure luteolin in several imidazole-based ILs, and report the successful extraction of luteolin from peanut shells using 1-butyl-3-methylimidazolium nitrate, as well as the optimization of extraction conditions by response surface methodology (RSM). 2. Materials and methods 2.1. Materials Luteolin (mass fraction > 98%) was purchased from Shaanxi Sunrun Bio-technology Co., Ltd., (Xi’an, China), and it was recrystallized twice in ethanol, dried in a vacuum oven at T = 378.5 K for 24 h, and then stored in a desiccators before use. The purity of

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Table 1 ILs used in this work.

Table 4 Central composite design arrangement and results.

Ionic liquids

Abbreviation

Water content (%)

1-Butyl-3-methylimidazolium nitrate 1-Butyl-3-methylimidazolium tetrafluoroborate 1-Butyl-3-methylimidazolium hexafluorophosphate 1-Butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide 1-Hexyl-3-methylimidazolium nitrate 1-Hexyl-3-methylimidazolium tetrafluoroborate 1-Hexyl-3-methylimidazolium hexafluorophosphate

[C4mim]NO3 [C4mim]BF4

<0.1 <0.1

[C4mim]PF6

<0.05

[C4mim]NTF2

<0.05

[C6mim]NO3 [C6mim]BF4

<0.1 <0.05

[C6mim]PF6

<0.1

80 60

Factors

Response

Coded variables

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

100

Response

Runs

Actual variables

Y (%)

x1

x2

x3

X1 (mL g1)

X2 (°C)

X3 (%)

0 0 0 0 1 1 0 1 0 1.68 1.68 0 0 1 0 1 1 0 1 1

1.68 0 0 0 1 1 0 1 0 0 0 0 1.68 1 0 1 1 0 1 1

0 1.68 0 1.68 1 1 0 1 0 0 0 0 0 1 0 1 1 0 1 1

7.5 7.5 7.5 7.5 10.0 5.0 7.5 10.0 7.5 3.3 11.7 7.5 7.5 10.0 7.5 5.0 10.0 7.5 5.0 5.0

97.0 55.0 55.0 55.0 80.0 80.0 55.0 30.0 55.0 55.0 55.0 55.0 13.0 80.0 55.0 30.0 30.0 55.0 80.0 30.0

55.0 97.0 55.0 13.0 30.0 30.0 55.0 80.0 55.0 55.0 55.0 55.0 55.0 80.0 55.0 80.0 30.0 55.0 80.0 30.0

77.2 5.7 31.4 5.5 49.8 44.9 35.7 4.5 32.2 28.7 35.0 30.7 5.0 40.1 34.1 6.1 13.3 29.6 47.1 13.8

Sample

40

80 Luteolin standard

20

70 60

0 0

5

10

15

50

20

103 x

time/min Fig. 1. Chromatograms of sample and luteolin standard.

40 30 20

Table 2 Levels of independent variables for CCD.

Min Max

10

Liquid–Solid ratio X1 (mL g1)

Extraction temperature X2 (°C)

IL concentration X3 (%)

5 10

30 80

30 80

1.68 1.00 0.00 1.00 1.68

300

305

310

315

320

325

330

335

340

T/K Fig. 2. Solubility of luteolin in hydrophilic ILs [C4mim]NO3 (}), [C6mim]BF4 (s) and [C4mim]BF4 (4).

Table 3 Coded variables and actual variables for CCD. Coded variables xi (i = 1,2,3)

0 295

Actual variables X1 (mL g1)

X2 (°C)

X3 (%)

3.3 5.0 7.5 10.0 11.7

12.0 30.0 55.0 80.0 97.0

12.0 30.0 55.0 80.0 97.0

the luteolin crystal was more than 99.5% mass fractions, determined on Agilent 1100 HPLC system (Agilent, USA). Eriodictyol (mass fraction P 97%) and 5,7-dihydroxychromone (mass fraction P 98%) were purchased from Shanghai PureOne Biotechnology (Shanghai, China). All of the ILs (purity > 99%, Table 1), were purchased from Lanzhou Greenchem ILS, LICP, CAS (Lanzhou, China). These ILs were distilled in a rotary evaporator for 4 h and dried at 398.5 K under vacuum for 24 h to remove any residual volatile compounds and water prior to use. The HPLC purity of the ILs employed in the experiment were higher than 99.5%, and the water content was determined to be less than 0.1% based on a Karl-Fischer titration using a Metrohm 798 MPT Titrino

(Metrohm Co., Switzerland). Mature peanuts were purchased from a market in Nanning of China. The peanuts were washed and handshelled. All peanut shells were air dried, pulverized in a knife mill (Model FW-100, Taisite Instrument Ltd., Tianjin, China) and passed through a 40-mesh sieve respectively. The content of luteolin, eriodictyol and 5,7-dihydroxychromone in peanut shells were determined as 0.803%, 0.313% and 0.201% by HPLC analysis, respectively. 2.2. Chromatographic conditions The HPLC analysis of luteolin was performed on a Waters symmetry C18 column (250 mm  4.6 mm, 5.0 lm). The mobile phase consisted of methanol and 0.1% phosphoric acid aqueous solution in a 55:45 volume ratio and employed in the separation at a flow rate of 1.0 mL min1 with the detector wavelength at 275 nm. The chromatograms of sample and luteolin standard are shown in Fig. 1. The HPLC analysis of eriodictyol and 5,7-dihydroxychromone was carried out on a Waters symmetry C18 column using the method proposed by Zhang et al. [25] with no modification. 2.3. Extraction of luteolin from peanut shells using IL Two grams of dried samples powder were put into a 20 mL glass vessel with heating jacket and a magnetic stirrer, followed by

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2.5

(%) = (mass of luteolin in extract/mass of luteolin in tested samples)  100%. The optimal conditions of IL concentration, liquid– solid ratio and extraction temperature were determined by RSM.

2.0

2.4. Experiment design and optimization for the extraction conditions

1.5

A three factor central composite design (CCD) was employed to investigate the effects of operational parameters namely liquid– solid ratio (X1, mL g1), extraction temperature (X2, °C) and IL concentration (X3, %) on the extraction yield of luteolin (Y, %) from peanut shells. The actual independent variables (X) and coded independent variables (x) for CCD are shown in Tables 2 and 3. The relationship between xi and Xi can be expressed as [26]:

103 x

3.0

1.0 0.5 0.0 295

305

315

325

335

345

355

xi ¼ X i  ½maxðX i Þ þ minðX i Þ=2=½maxðX i Þ þ minðX i Þ=2

T/K Fig. 3. Solubility of luteolin in hydrophobic ILs [C4mim]PF6 (h), [C4mim]NTF2 (r) and [C6mim]PF6 (s).

70

MeOH 50% [C4 mim]NO3

60

[C4 mim]NO3

Yield/%

A total of 20 experiments (Table 4) by RSM were designed to optimize the extraction conditions, in which 15 were factorial and axial experiments, 5 were zero-point tests used for estimation of a pure error sum of squares. The yield of luteolin, influenced by three independent variables, is set as the desired goal (response) for fitting a second-order polynomial regression model:

Y ¼ b0 þ

50

ð1Þ

3 3 3 X 3 X X X bi xi þ bii x2i þ bij xi xj þ e i¼1

i¼1

ð2Þ

i¼1 j¼iþ1

40 30 20 10 0

0

1

2

3

4

5

6

7

8

9

where Y is the predicted response of yield, b0 is a constant, bi, bii and bij are constant coefficients of linear, quadratic and interactive terms, respectively, and e is a term representing other sources of variability not accounted by the response function. xi, xj are coded independent variables. The Design-Expert V8.0.5 was used for analysis of variance (ANOVA), determination of coefficient as well as estimation of goodness of model fitting.

Time/h Fig. 4. Yield of luteolin via extraction time. (70 °C, liquid–solid ratio: 5 mL g1).

3. Results and discussion 3.1. IL screening

adding a certain volume of IL aqueous solution. Then the vessel jacket was connected to the thermostatic water bath with temperature accuracy of ±0.01 K (Chengdu Instruments Co., Ltd., China) to perform extraction at the specified temperature for 5 h. The IL extract was filtrated through a 0.45 lm filter, and analyzed directly using HPLC to determine the content of luteolin. The filtrate was successively extracted three times with the same volume of ethyl acetate at room temperature, and the combined ethyl acetate solution was evaporated under reduced pressure to remove ethyl acetate and yield a yellow colored residue mainly containing luteolin, by which the IL was recovered and luteolin was obtained. The extraction yield of luteolin was calculated by the formula: Y

In this work, the extraction ability of ILs for luteolin was evaluated by the solubility of pure luteolin in ILs, which was measured using a laser monitoring observation method on purpose-made equipment [27]. The solubility of luteolin in several hydrophilic and hydrophobic ILs were shown in Figs. 2 and 3. It was clear that the hydrophilic imidazole-based ILs exhibited better dissolution of luteolin than hydrophobic imidazole-based ILs. In comparison to the solubility data from the literature [28], at the same temperature, the solubility of luteolin in the hydrophilic ILs [C4mim]NO3, [C6mim]BF4 and [C4mim]BF4, especially in [C4mim]NO3, is considerably higher than that in commonly used

Table 5 Analysis of variance for response surface quadratic model. Source

Sum of squares

Mean square

F-value

p-value

Model x1 x2 x3 x1x2 x1x3 x2x3 x21

6716.58 2.99 5166.41 41.00 0.000 21.13 10.13 0.53

746.29 2.99 5166.41 41.00 0.000 21.13 10.13 0.53

54.12 0.22 374.67 2.97 0.000 1.53 0.73 0.039

<0.0001 0.6512 <0.0001 0.1154 1.0000 0.2441 0.4116 0.8482

x22 x23 Residual Lack of fit Pure error

172.78

172.78

12.53

0.0054

1190.41

1190.41

86.33

<0.0001

137.89 112.43 25.47

13.79 22.49 5.09

4.41

Note: R-squared = 0.98, adjusted R-squared = 0.96, predicted R-squared = 0.90, adequate precision = 27.2.

0.0645

Significant

Not significant

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organic solvents, such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, acetone and hexane. In addition, the experiment results indicated that the solubility of luteolin in ILs increased as the temperature increasing, which is analogous to the dissolution of the solute in organic solvents. However, as for hydrophilic ILs, the solubility of luteolin is largely dependent on the IL anions. Therefore, it was concluded that the hydrophilic interaction between ILs and luteolin has a marked effect on the dissolution of luteolin. Based on the solubility of luteolin, [C4mim]NO3 is a better option as extraction solvent for luteolin from peanut shells in this work. 3.2. Consideration on viscosity of [C4mim]NO3

60 50 40 30 20 10 0

80

As mentioned above, [C4mim]NO3 exhibited high dissolubility of luteolin than conventional organic solvents, and may be employed as potential extractant. However, imidazole-based ILs generally have much higher viscosity than molecular solvents [29], resulting in impairing mixing and transfer properties of extraction process [30]. Fig. 4 showed the yield of luteolin from peanut shells via extraction time in MeOH, pure [C4mim]NO3 and its aqueous solution. As seen in Fig. 4, due to the viscosity of [C4mim]NO3, extraction via MeOH reached solid–liquid equilibrium more easily than that in pure [C4mim]NO3 and its aqueous solution. It was also seen from Fig. 4, using mixtures of [C4mim]NO3 and water as the extraction solvent could significantly reduce the viscosity of extraction system and improve the transfer process in [C4mim]NO3-mediated extraction. In addition, raising the temperature can reduce the viscosity of ILs, but this is not often practical in many extraction processes of bioactive components, due to factors such as the thermal decomposition of components and separation selectivity. Herein, in this work, we used a mixture of [C4mim]NO3 and water instead of pure [C4mim]NO3 for extraction of luteolin from peanut shells below 100 °C.

75

10.0 70

65

9.0 60

T/° C

55

8.0

50

45

-1

.g mL tio/ a r lid -so uid q i L 7.0

40

35

6.0 30

5.0

A 40 30 20 10 0

80

75

70

65

9.0

.g mL tio/ a r 6.0 lid -so 5.0 uid q i L

60

8.0

55 IL 50 coc 45 ent 40 rati 35 30 on/ %

3.3. Response surface optimization for extraction of luteolin from peanut shells

10.0 -1

7.0

B 3.3.1. Response analysis On the basis of the experimental results of CCD (Table 4) and regression analysis, a second-order polynomial equation was established to estimate the relationship between the yield and variables. The model could be expressed as

þ 1:13x2 x3 þ

þ

3:46x22



9:09x23

50 40

Y ¼ 32:23 þ 0:47x1 þ 19:45x2  1:73x3  1:62x1 x2 0:19x21

60

ð3Þ

30 20

The analysis of variance (ANOVA) for the adequacy of the model was shown in Table 5. The F-value of 54.12 and p-value of less than 0.0001 indicated that the quadratic model was significant. Regression analysis showed that the R2 value was 0.98, indicating there was a good agreement between the experimental and the predicted value from the model. Furthermore, results of the error analysis indicated that the lack of fit was not significant with p-value of 0.0645 (>0.05). Hence, the model can be used to navigate the design space and predict the response. In this case, it could be seen that the operational parameter with the largest effect on the yield of luteolin was the linear and quadratic terms of temperature (p < 0.05), followed by the quadratic term of IL concentration (p < 0.05). The liquid–solid ratio and its interactions with temperature and IL concentration had slight effects on the yield of luteolin.

Fig. 5. The three-dimension response surfaces show the correlative effects of liquid–solid ratio and temperature (A), liquid–solid ratio and IL concentration (B), temperature and IL concentration (C) on the yield of luteolin.

3.3.2. Response surfaces In order to well understand the interactions between extraction variables and determine the optimal level of each parameter for the maximum yield of luteolin, based on the quadratic model

discussed above, three-dimensional (3D) response surfaces with contour plots for the yield were constructed in Fig. 5. Fig. 5A and B showed that the yield almost had no change with liquid–solid ratio, suggesting that even more IL solution may be

10 0

80

75

70

65

60

IL coc 55 50 45 ent rati 40 35 on/ 30 %

30

35

40

45

50

55

60

65

70

75

80

C T/°

C

L. Ge et al. / Separation and Purification Technology 135 (2014) 223–228

not beneficial for the extraction of luteolin from peanut shells once its value exceeds 5.0. Fig. 5A and C showed that the extraction temperature had a remarkable effect on the yield. With temperature rising, the yield of luteolin increased dramatically, which is attributed to the solubility increasing of luteolin in IL aqueous solution. As for IL concentration, it could be observed from Fig. 5B and C, when the concentration of [C4mim]NO3 increased from 30% to ca. 55%, the yield of luteolin reached its maximum at first, which was caused by the soluble ability enhancing for luteolin in [C4mim]NO3 aqueous solution, but decreased with the concentration further increased, which was caused by viscosity increasing of [C4mim]NO3 aqueous solution, and so it was more difficult for [C4mim]NO3 to penetrate into the interior of sample matrixes as well as for luteolin to diffuse in [C4mim]NO3 solution. 3.3.3. Determination of optimum conditions As discussed above, in three independent variables, extraction temperature and IL concentration have significant effects on the yield of luteolin. However, temperature exhibits almost linear influence trend for the yield of luteolin, which means an increasing in temperature, is accompanied by an apparently increasing in yield. In this work, in view of the boiling point of most organic solvents and their mixtures used for extraction of bioactive substances are below 100 °C, an optimization function that maximizes the yield of luteolin while maintaining temperature below 100 °C was selected. By employing the Design-Expert software, the optimal extraction conditions were suggested to be liquid–solid ratio of 7.6 mL g1, temperature of 100 °C and [C4mim]NO3 aqueous solution of 49%. The predicted yield of luteolin was 78.4% under the optimal conditions. 3.3.4. Verification tests In order to test the validity of the optimal conditions achieved, five repeated experiments were performed in the optimal conditions determined by RSM. The experimental average yield of luteolin was 79.8 ± 1.48%, which was very close to the 78.4% predicted by RSM. The model fits the experimental data very well under these experimental conditions. In addition, we also measured the extraction yield of eriodictyol and 5,7-dihydroxychromone from peanut shells in the optimal conditions obtained for luteolin. Although the IL suitable for extraction of eriodictyol and 5,7-dihydroxychromone was not determined in this work, the experiments indicated that eriodictyol and 5,7-dihydroxychromone could be effectively extracted simultaneously by [C4mim]NO3 aqueous solution. In the optimal conditions above, the average yield (n = 5) of eriodictyol and 5,7-dihydroxychromone are 56.8% and 69.2%, respectively. 4. Conclusion In this work, the extraction ability of several hydrophilic and hydrophobic imidazole-based ILs for luteolin was evaluated. The ILs screening results indicated hydrophilic [C4mim]NO3 is an efficient extraction solvent for luteolin when compared with commonly used organic solvents and other ILs investigated in this work. Based on the findings, the RSM with central composite design was further employed to investigate the effects of liquid– solid ratio, extraction temperature and [C4mim]NO3 concentration on the yield of luteolin from peanut shells. A second order regression model was used to evaluate the effects of extraction parameters and their interaction towards the optimal conditions. The results showed that temperature has significant effect on the yield, followed by [C4mim]NO3 concentration while the liquid–solid ratio has a relatively smaller influence. In the optimal conditions namely liquid–solid ratio of 7.6 mL g1, temperature of 100 °C and

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[C4mim]NO3 aqueous solution of 49%, determined by RSM, the yield of luteolin was 79.8 ± 1.48%, which were very close to the 78.4% predicted by RSM. As a result, high extraction capability, low consumption of extraction solvent as well as no volatile and flammable organic solvents used can be simultaneously achieved using [C4mim]NO3 aqueous solution as extractant. The above results demonstrated that [C4mim]NO3 is a promising alternative solvent in extracting luteolin from peanut shells. Acknowledgments This work was financially supported by National Natural Science Foundation of China (No. 21166002) and Guangxi Natural Science Foundation (2014GXNSFAA118037). References [1] L.L. Miguel, Distribution and biological activities of the flavonoid luteolin, Mini-Rev. Med. Chem. 9 (2009) 31–59. [2] Y. Lee, L.R. Harvard, B. Villalon, Flavonoids and antioxidant activity of fresh pepper (Capsicum annuum) cultivars, J. Food. Sci. 60 (1995) 473–476. [3] S.H. Kim, K.J. Shin, Y.H. Kim, M.S. Han, T.G. Lee, E. Kim, S.H. Ryu, P.G. Suh, Luteolin inhibits the nuclear factor-jB transcriptional activity in Rat-1 fibroblasts, Biochem. Pharmacol. 66 (2003) 955–963. [4] H. Jiang, Q. Xia, X. Wang, J.F. Song, Luteolin induces vasorelaxion in rat thoracic aorta via calcium and potassium channels, Pharmazie 60 (2005) 444–447. [5] R.P. Samy, P. Gopalakrishnakone, S. Ignacimuthu, Anti-tumor promoting potential of luteolin against 7,12-dimethylbenz(a)anthracene-induced mammary tumors in rats, Chem.-Biol. Interact. 164 (2006) 1–14. [6] K. Kazuki, U. Mari, Y. Hiroaki, H. Takashi, Bioavailable flavonoids to suppress the formation of 8-OHdG in HepG2 cells, Arch. Biochem. Biophys. 455 (2006) 197–203. [7] S. Jang, K.W. Kelley, R.W. Johnson, Luteolin reduces IL-6 production in microglia by inhibiting JNK phosphorylation and activation of AP-1, Proc. Natl. Acad. Sci. U.S.A. 105 (2008) 7534–7539. [8] M.C. Yu, J.H. Chen, C.Y. Lai, C.Y. Han, W.C. Ko, Luteolin, a non-selective competitive inhibitor of phosphodiesterases 1-5, displaced [3H]-rolipram from high-affinity rolipram binding sites and reversed xylazine/ketamine-induced anesthesia, Eur. J. Pharmacol. 627 (2010) 269–275. [9] A. Cerrato, D. De Dantis, H. Moresi, Production of luteolin extracts from Reseda luteola and assessment of their dyeing properties, J. Sci. Food. Agric. 82 (2002) 1189–1199. [10] L. Li, G. Aldini, M. Carini, C.Y.O. Chen, H.K. Chun, S.M. Cho, K.M. Park, C.R. Correa, R. Russell, J. Blumberg, K.J. Yeum, Characterisation, extraction efficiency, stability and antioxidant activity of phytonutrients in Angelica keiskei, Food Chem. 115 (2009) 227–232. [11] C.Y. Liu, X.Q. Sun, The development and utilization of peanut shell in feed industry, Feed Ind. Chin. 31 (2010) 50–52. [12] P.D. Duh, D.B. Yeh, G.C. Yen, Extraction and identification of an antioxidative component from peanut hulls, J. Am. Oil Chem. Soc. 69 (1992) 814–818. [13] J.Y. Qiu, L.L. Chen, Q.J. Zhu, D.J. Wang, W.L. Wang, X. Sun, X.Y. Liu, F.L. Du, Screening natural antioxidants in peanut shell using DPPH-HPLC-DAD-TOF/MS methods, Food Chem. 135 (2012) 2366–2371. [14] A.I. Hussain, S.A.S. Chatha, S.K. Noor, A. Zulifqar, M.U. Arshad, H.A. Rathore, M.Z.A. Sattar, Effect of extraction techniques and solvent systems on the extraction of antioxidant components from peanut (Arachis hypogaea L.) hulls, Food Anal. Method. 5 (2012) 890–896. [15] G.W. Zhang, M.M. Hu, L. He, P. Fu, L. Wang, J. Zhou, Optimization of microwave-assisted enzymatic extraction of polyphenols from waste peanut shells and evaluation of its antioxidant and antibacterial activities in vitro, Food Bioprod. Process. 91 (2013) 158–168. [16] Y. Yu, S. Zhang, F. Gao, Comparation of the polyphenol yield and antioxidant activity of peanut shell extract made by different solvent extraction methods, J. Biotech. (Supplement) 136 (2008) S508. [17] A. Berthod, A.M. Ruiz, B.S. Carda, Ionic liquids in separation techniques, J. Chromatogr. A 1184 (2008) 6–18. [18] S. Werner, M. Haumann, P. Wasserscheid, Ionic liquids in chemical engineering, Annul. Rev. Chem. Biomol. Eng. 1 (2010) 203–230. [19] J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Room temperature ionic liquids as novel media for ‘‘clean’’ liquid–liquid extraction, Chem. Commun. 16 (1998) 1765–1766. [20] Y.L. Gu, F. Shi, H.Z. Yang, Y.Q. Deng, Leaching separation of taurine and sodium sulfate solid mixture using ionic liquids, Sep. Purif. Technol. 35 (2004) 153– 159. [21] F.Y. Du, X.H. Xiao, X.J. Luo, G.K. Li, Application of ionic liquids in the microwave-assisted extraction of polyphenolic compounds from medicinal plants, Talanta 78 (2009) 1177–1184. [22] H. Zeng, Y.Z. Wang, J.H. Kong, C. Nie, Y. Yuan, Ionic liquid-based microwaveassisted extraction of rutin from Chinese medicinal plants, Talanta 83 (2010) 582–590.

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