Hydrotropic Extraction of Citral from Cymbopogon ... - ACS Publications

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Hydrotropic Extraction of Citral from Cymbopogon flexuosus (Steud.) Wats. Meghal A. Desai and Jigisha Parikh* Chemical Engineering Department, Sardar Vallabhbhai National Institute of Technology, Surat-395007, Gujarat, India S Supporting Information *

ABSTRACT: A novel technique for the extraction of citral from the leaves of Cymbopogon flexuosus (Steud.) Wats. using hydrotropic solutions (sodium salicylate and sodium cumene sulfonate) was investigated. The yield of citral was dependent on the concentration of hydrotrope, solid loading, temperature, and size of the plant material. Using the Taguchi method, the extraction was optimized, and both the hydrotropes gave the highest yield of citral at a concentration of 1.75 M, 5% solid loading, a temperature of 30 °C, and a size of 0.25 mm of the plant material. Sodium salicylate gave a better extraction yield of citral than sodium cumene sulfonate. The microscopic analysis of plant leaves provided insight into the extraction mechanism. A kinetic study was carried out to check the extraction efficiency for both the hydrotropes. The hydrotropic solution was successfully recycled. Using hydrotropic extraction, citral could be extracted under normal operating conditions, and the use of traditional organic solvents could be eliminated. As a simple and environmentally friendly technique, hydrotropic extraction could be utilized for the extraction of bioactive compounds from plants.

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INTRODUCTION Cymbopogon flexuosus (Steud.) Wats., commonly known as lemongrass, has a major role in the medical and perfume industries. Essential oil extracted from the leaves of lemongrass possesses prominent biological properties such as anticancer activity1 and antimicrobial, antifungal, and antibacterial activities.2−5 The natural oil has demonstrated improved antifungal activities compared to the synthetic version in some studies.4 Also, it is an anxiolytic with proven sedative or anticonvulsive effects.6 The oil inhibits β-glucuronidase activity and the growth of pathogens found in humans, plants, and animals.7 The lemongrass oil was found effective in the treatment of various orthopedic ailments and muscular and skin problems.2 The oil is widely used in the flavor and fragrance industries.8 Citral (3,7-dimethyl-2,6-octadienal), an aliphatic aldehyde, is the major compound present in the oil extracted from lemongrass leaves. Citral is the name given to the mixture of two isomers, neral and geranial, and is used for the preparation of valuable compounds like ionone, vitamin A, and β-carotene.2,9,10 Interest has also been shown in the synthesis of menthol using citral.11 Citral displays sedative, antidepressant,12 antiviral, antifungal, and antitumor properties. Because of its strong lemon odor, citral is used extensively as a lemon flavoring and scenting agent in food, beverages and candies, perfumes, and other toiletries.13 Considering the potential of lemongrass oil (mainly citral), efforts were made to extract the oil using conventional techniques like solvent extraction, steam-distillation, and hydrodistillation.14−17 Solvent extraction suffers from the major drawbacks of the consumption of large amounts of volatile organic solvents and residual solvent issues in the end products. The involvement of organic solvents therefore makes the process labor-intensive, time-consuming, and, more importantly, environmentally hazardous. The distillation process involves the utilization of intense thermal energy, which may lead to the © 2012 American Chemical Society

loss of volatile compounds and degradation of heat-sensitive materials. The time-consuming distillation process requires high energy, which leads to the necessity for more energy production and higher carbon dioxide discharge into the environment. Novel techniques like supercritical fluid extraction have gained prominence as a green technique for the extraction of lemongrass oil using supercritical2,14,15 and dense10 carbon dioxide. The fractionation of citral from lemon oil using supercritical fluid extraction has also been reported.18 However, the major limitation of this technique lies in its operating pressure, creating safety issues, and rendering the process uneconomical. In recent years, hydrotropes have found extensive use in the separation of isomeric−nonisomeric mixtures that have similar boiling points and protein mixtures.19−21 In reaction engineering, enhancement of the rate of reactions such as alkaline hydrolysis of esters, oximation of cyclodecanone, etc.22,23 and synthesis of chalcones and dihydropyridines24,25 were successfully achieved with the help of hydrotropes. Hydrotropes also have uses in vesicle preparation, as oil/water microemulsion stabilizers, viscosity modifiers, cleaning agents in cloudy detergent formulations, and solubilizers for drugs.26 Hydrotropes are highly water-soluble organic salts and thermally stable nonvolatile compounds. The phenomenon of increasing the solubility of normally insoluble or sparingly soluble organic substances in water, brought about by the hydrotropes, is known as hydrotropy. The wide acceptance of hydrotropes in solubilization is particularly attractive because of various factors like high selectivity, avoidance of emulsification, and easy recovery of the solute by mere dilution with water.26,27 Moreover, the diluted hydrotropic solution, after recovery of the solute, can be Received: Revised: Accepted: Published: 3750

September 5, 2011 January 5, 2012 January 18, 2012 January 18, 2012 dx.doi.org/10.1021/ie202025b | Ind. Eng.Chem. Res. 2012, 51, 3750−3757

Industrial & Engineering Chemistry Research

Article

concentration, solid loading, and temperature and size of the plant material. To select the range of these parameters, a preliminary study was carried out. The solubility study showed optimum solubility in the concentration range of 1.5−2.0 M for both the hydrotropes. The solid loading was varied in the range of 2−8% (w/v). When solid loading exceeded 8% (w/v), the suspension became extremely viscous because of swollen starchy grains, making agitation and filtration difficult. The temperature of the system was maintained in the range of 30−50 °C. Temperatures above 50 °C were avoided to minimize water vaporization, thus maintaining the solution concentration at a constant value. The size of the plant material was varied from 25 mm to 0.25 mm. The extraction was carried out for 8 h, to ensure the equilibrium of the process. Experiments were performed in triplicate. The details of factors and their levels are given in Table 1. The results were analyzed using the Minitab (version 16.0) software (Minitab Inc., PA, U.S.A.).

concentrated by evaporation and can be recycled for extraction purposes. Keeping these benefits in mind, hydrotropes were effectively used in the extraction of various phytochemicals like curcuminoids,27 limonin,28 and piperine29 from plant matrices. To date, no work is reported in the literature concerning the extraction of citral using hydrotropic solutions. To eliminate the use of organic solvents and to carry out extraction under safer operating conditions, hydrotropic extraction was employed to isolate citral from the leaves of Cymbopogon flexuosus (Steud.) Wats. in the present research work. Two hydrotropes (sodium salicylate (NaSal) and sodium cumene sulfonate (NaCuS)) were used for this purpose. The hydrotropic extraction was optimized using the Taguchi method by studying four factors: hydrotrope concentration, solid loading, temperature, and size of the plant material at three levels. An analysis of variance (ANOVA) was performed to find the most significant parameter affecting the process. Experimental results were assessed by performing a kinetic study, and first order rate constants were found. A scanning electron microscopic study was performed to examine the structural changes and understand the extraction phenomena. This work will be helpful in developing a green technique for the selective extraction of natural products from various plant matrices.

Table 1. Factors and Their Levels in Experimental Design level

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MATERIALS AND METHODS Materials. Leaves of Cymbopogon flexuosus (Steud.) Wats. (lemongrass) were collected from Navsari Agriculture University, Navsari, Gujarat, India. Leaves were dried under a shed for 48 h at room temperature and subsequently stored in a moisture-free environment at room temperature. NaSal was purchased from S. D. Fine Chemicals (Mumbai, India) with a purity of 99% (by mol). NaCuS was obtained from National Chemicals (Baroda, India) and purified by recrystallization. For the preparation of aqueous solutions, deionized water from a Millipore Elix 3 Century Distillation System (Millipore, Billerica, MA, U.S.A.) was used. Citral (97% pure) was purchased from Spectrochem Pvt. Ltd. (Mumbai, India). Dichloromethane (DCM, 99% pure) was procured from Merck Ltd. (Mumbai, India). Methods. Hydrotropic Extraction. The solubility of citral was determined by adding citral to hydrotropic solutions of varying concentrations. The mixture was homogenized at 6000 rpm for 30 min in a constant temperature water bath maintained at 30 ± 0.5 °C. This study was helpful for the selection of the concentration range of the hydrotropes and, more importantly, to evaluate the effectiveness of the hydrotropes. A fully baffled cylindrical vessel (internal diameter 10 cm and height 10 cm, equipped with a four-blade turbine impeller of 3 cm in length) was used to extract citral from the leaves of lemongrass. The vessel was kept in a constant temperature water bath, which was able to maintain the temperature of the system within a range of ±0.5 °C. The plant material was suspended in 250 mL of hydrotropic solution, and extraction was carried out in batch mode with stirring under different operating conditions. Upon the completion of extraction, the solution was separated from the solid residue by filtration. The extract was then diluted using water to bring the mixture below the minimum hydrotrope concentration (MHC) of the hydrotrope to recover citral. The recovered citral was then dissolved in DCM for analysis. Design of Experiment. Well-defined sets of experiments were planned and performed according to the guidance of the Taguchi method.30−34 The parameters studied were hydrotrope

factors

1

2

3

A (hydrotrope concentration, M) B (solid loading, % w/v) C (temperature, °C) D (size of the plant material, mm)

1.50 2 30 25

1.75 5 40 10

2.00 8 50 0.25

Analysis by Gas Chromatography (GC). All of the samples were analyzed by Clarus 500 GC (Perkin-Elmer, MA, USA) equipped with FID and using a fused silica Stabilwax column (50 m × 0.32 mm × 0.5 μm film thickness). The conditions maintained were as follows: carrier gas (N2) with a flow rate of 1.2 mL·min−1; split ratio, 1:50; injection volume, 0.4 μL; injector and detector temperatures at 220 and 250 °C, respectively; oven temperature, 50 °C for 5 min and progressing from 50 to 220 °C at a rate of 10 °C·min−1 and then held for 8 min at 220 °C. The quantification was performed using a calibration curve with an R2 value of 0.9995. Analysis by Scanning Electron Micrographs (SEM). The surface morphology of lemongrass leaves, before extraction and after extraction, was studied using an S-3400 SEM, Hitachi (Tokyo, Japan) with a resolution of 4.0 nm in the high vacuum mode and a high sensitivity semiconductor backscattered electron detector. The examination was carried out with an accelerating voltage of 10.0 kV and a working distance of 20.6 mm.

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RESULTS AND DISCUSSION

Solubility and Recovery Studies. Solubility studies were performed to find the appropriate concentration range of hydrotropes for the extraction of citral. An attractive interaction between citral and the hydrotrope is necessary for better solubilization. In both the hydrotropic solutions, solubility was found to increase with increased concentration, as shown in Figure 1. A rapid increase in solubilization was observed for concentrations above 1 M, indicating better aggregation of the hydrotrope with citral. In comparison with NaCuS, NaSal showed better dissolution of the citral. The effectiveness of a hydrotrope can be measured by the Setschenow constant, Ks, using the following equation where the exponential part of the solubility curve is used for the determination of Ks.19,35

ln (S /Sm) = K s(Cs − C m) 3751

(1)

dx.doi.org/10.1021/ie202025b | Ind. Eng.Chem. Res. 2012, 51, 3750−3757


Article

the “higher is better” case was selected and subsequently calculated from the yield of citral. The total S/N ratio for each factor at each level was calculated for both the solutions, as shown in Table 3, to determine the optimum conditions. The level showing the highest S/N ratio corresponds to the optimal condition. To observe the effect of various parameters on the S/N ratio and to understand the mechanism for achieving the optimal conditions, plots were constructed as shown in Figures 2−5. Figure 2 depicts the variation in the S/N ratio with respect to the change in the concentration of the hydrotropes. For the first two levels (1.5 and 1.75 M), the response increased with an increase in concentration, and then it dropped drastically. With an increase in hydrotrope concentration, an increase in solubility of citral was observed (Figure 1). This inconsistent behavior can be attributed to the highly cooperative aggregation between the aromatic ring of the hydrotropes and the hydrocarbon structure of citral. The aggregation probably formed an inclusion complex. During extraction, a larger amount of citral might be extracted because of the strong interaction between citral and the hydrotrope. However, complete recovery of citral could not be achieved because of the formation of the complex. Similar observations were reported for curcuminoids extraction using NaCuS27 and bromelain extraction using reverse micelle.36 In addition, the decrease in yield might be related to the presence of high amounts of hydrotrope, leading to an increase in the volume of the hydrotrope-rich phase.37,38 Figure 3 shows the effect of solid loading on the response. Less variation was observed initially with a change in solid loading. Then, a drop in the S/N ratio was observed at a higher solid loading (8%, w/v). The increased viscosity of the suspension at the higher solid loading (8%) might have led to poor mixing, which retarded the proper penetration of the hydrotrope solution through the plant matrix and thereby decreased the yield of citral. At the higher solid loading, the amount of citral present in the suspension would be higher, leading to an increase in the equilibrium concentration in both the solid and liquid phases and ultimately lowering the extraction yield. The effect of temperature on the extraction is shown in Figure 4. With an increase in temperature, a decrease in the amount of citral extracted was observed. Because of the reduced viscosity and higher solubility at the higher temperature, the extraction of citral was expected to be more efficient. However, a reverse trend was observed. The efficacy of adsorption of the hydrotrope at the liquid−liquid or gas−liquid interface is expressed by the surface excess concentration (Γmax), which depends on the minimum area occupied by the hydrotrope molecule at the air−water interface (Amin). The relationship

Figure 1. Solubility of citral in different hydrotropes at 30 °C.

where S and Sm are the solubility of citral in hydrotropic solution (g·L−1) at any concentration Cs and at the minimum concentration Cm (M), respectively. The minimum concentration of both the hydrotropes was 0.5 M, and at this concentration, the solubility of citral was 0.776 and 0.211 g·L−1 in NaSal and NaCuS, respectively. The value of the Setschenow constant was found to be 2.523 and 1.772 M−1 for NaSal and NaCuS, respectively, signifying a better interaction of citral with NaSal than with NaCuS. Two different methods were examined to achieve the efficient recovery of citral from the hydrotropic solutions. In the first method, the extract was diluted below MHC using water. Rapid recovery of citral was observed using this method with NaCuS. However, for NaSal, no traces of citral were found by the dilution method. The diluted solution was then cooled to 4 °C for 48 h, and citral was finally recovered. The difficulty in recovery observed with NaSal may have been due to the cooperative and strong aggregation between the NaSal molecule and citral. Similar results were reported for the recovery of limonin from NaCuS.28 In the second method, partitioning of the extract with DCM (1:1, v/v) was carried out. Partitioning twice was found to be sufficient for complete extraction of citral from both of the solutions. Optimization of Hydrotropic Extraction Using NaSal and NaCuS. Experimental planning was done using the Taguchi method (L9 array), and experiments were performed accordingly. The data are reported in Table 2. A higher amount of citral was extracted using NaSal compared to NaCuS. To maximize the yield of citral, the signal-to-noise (S/N) ratio for

Table 2. L9 Array, Yield, and S/N Ratio for Extraction Using NaSal and NaCuS extraction by NaSal

extraction by NaCuS

yield (mg·g−1)

yield (mg·g−1)

exp. no.

A (M)

B (%, w/v)

C (°C)

D (mm)

y1

y2

y3

S/N ratio

y1

y2

y3

S/N ratio

1 2 3 4 5 6 7 8 9

1 (1.50) 1 1 2 (1.75) 2 2 3 (2.00) 3 3

1 (2) 2 (5) 3 (8) 1 2 3 1 2 3

1 (30) 2 (40) 3 (50) 2 3 1 3 1 2

1 (25) 2 (10) 3 (0.25) 3 1 2 2 3 1

14.48 7.47 4.22 10.85 11.32 8.01 5.18 11.89 2.67

14.34 7.79 4.46 10.72 11.54 8.10 5.30 11.78 2.58

14.64 7.67 4.32 10.96 11.46 8.15 5.39 11.98 2.75

23.22 17.66 12.73 20.70 21.17 18.15 14.46 21.50 8.51

11.53 6.94 2.74 9.78 9.23 3.98 5.42 10.81 2.03

11.65 7.07 2.90 9.94 9.42 4.13 5.51 10.90 1.90

11.46 7.12 2.83 9.94 9.31 4.16 5.26 11.02 1.95

21.25 16.96 9.00 19.90 19.39 12.23 14.64 20.76 5.85

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Table 3. Total S/N Ratio for Each Factor for Determination of Optimum Conditions extraction using NaSal

extraction using NaCuS

level

A (hydrotrope concentration)

B (solid loading)

C (temperature)

D (plant size)

A (concentration)

B (solid loading)

C (temperature)

D (plant size)

1 2 3

53.61 60.02 44.47

58.38 60.33 39.39

62.87 46.88 48.36

52.90 50.28 54.93

47.21 51.52 41.24

55.78 57.10 27.08

54.23 42.70 43.03

46.48 43.83 49.66

S/N ratio

Figure 2. Effect of the concentration of hydrotropes on the S/N ratio (level 1, 1.5 M; level 2, 1.75 M; level 3, 2 M). Figure 5. Effect of the size of the plant material on the S/N ratio (level 1, 25 mm; level 2, 10 mm; level 3, 0.25 mm).

where Amin is the area occupied by the hydrotrope molecule at the air−water interface (Å2), Γmax is the surface excess concentration (mol·m−2), and NA is Avogadro’s number. An increase in temperature was reported to cause increased molecular motion at the air−water interface, leading to loosely packed hydrotrope molecules at the interface, which eventually increased the area per molecule.40 This packing of the hydrotrope molecules may affect the selectivity of the solution. With an increase in temperature, the effectiveness of the hydrotrope decreases. A lower temperature is therefore favorable for extraction. Similar results were obtained for piperine extraction using aromatic sulfonate, where a slightly higher rate of extraction was achieved at the cost of the purity of the product.29 As shown in Figure 5, a smaller particle size (0.25 mm) gave a better response. With a decrease in particle size, the surface area increases, leading to rapid and easy accessibility of the hydrotropic solution to the cell matrix and thereby enhancing the rate of extraction and the yield. In the case of curcuminoids extraction, a smaller sized particle substantially increased the extraction of the products but with marginally decreased purity.27 As observed from Table 3 and Figures 2−5, the parameters to achieve the maximum yield of citral can be selected as A2B2C1D3, i.e., a hydrotrope concentration of 1.75 M, solid loading of 5% (w/v), 30 °C temperature, and plant size of 0.25 mm for extraction using NaSal as well as NaCuS. In both of the hydrotropes, the trend in the variation of the response with respect to the change in operating conditions was found to be similar. Any dissimilarity in response may result from the different degrees of interaction of the hydrotrope molecules with citral. Analysis of Variance. ANOVA is very important for analyzing the influence of each parameter on the yield and on the process (Table 4 and 5). In the present study, all of the degrees of freedom were consumed by the factors, leading to a lack of available information for error calculation. To facilitate further calculations, the factor having minimum variance was considered an error, and the information related to this factor

Figure 3. Effect of solid loading on the S/N ratio (level 1, 2%; level 2, 5%; level 3, 8%).

Figure 4. Effect of the temperature on the S/N ratio (level 1, 30 °C; level 2, 40 °C; level 3, 50 °C).

between these terms is given in eq 2.39

A min =

1020 NA Γmax

(2) 3753



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Table 4. ANOVA Table for Extraction Using NaSala factors A (hydrotrope concentration) B (solid loading) C (temperature) D (size of the plant material) error total pooled error a

S

D

V

F

% contribution

40.72

2

20.36

11.25

21.94

89.17 52.05 3.62

2 2 2

44.59 26.03 1.81

0.00 185.57 3.62

2 8 2

0.00 23.20 1.81

24.63 14.38 1.00

Table 6. Confirmation Experiment and Its Comparison with the Maximum Yield of L9 Array

factors

48.05 28.05 1.95

A (concentration, M) B (solid loading, % w/v) C (temperature, °C) D (size of the plant material, mm) citral (mg·g−1)

100.00

S = individual variation; D = degree of freedom; V = variance. S/N ratio

Table 5. ANOVA Table for Extraction Using NaCuSa factors A (hydrotrope concentration) B (solid loading) C (temperature) D (size of the plant material) error total pooled error a

D

V

F

17.76

2

8.88

3.13

7.28

191.87 28.73 5.68

2 2 2

95.94 14.36 2.84

33.78 5.06 1.00

78.62 11.77 2.33

0.00 244.04 5.68

2 8 2

0.00 30.50 2.84

extraction using NaCuS

exp 1 of L9 array

confirmation experiment

exp 1 of L9 array

1.75 5 30 0.25

1.5 2 30 25

1.75 5 30 0.25

1.5 2 30 25

15.78 15.91 15.64 23.96

14.48 14.34 14.64 23.22

14.68 14.39 14.58 23.26

11.53 11.65 11.46 21.25

Essential oil, consisting mainly of citral, is accumulated in a compartmentalized cell surrounded by a lignified cell wall, which acts as an impermeable barrier. This cell wall maintains the balance between solution residing in neighboring tissues and the solution within the cell.42 The extraction of citral requires two steps: (a) penetration of hydrotropic solution into the cell wall and destabilization or rupture of the cell structure and (b) dissolution of citral and back transfer to the bulk solution. The addition of hydrotropes in water reduces the surface tension at the interface, which should improve the wettability of the cell wall, thereby facilitating penetration. Hydrotropes were reported to have the ability to disorganize the lamellar crystal structure of surfactant,43 which would help in disturbing the cell wall and likely cause the cell to rupture. Finally, selective solubilization by the hydrotropic solution would lead to the extraction of citral from the cells. A periodic arrangement of well-shaped cells was observed in the case of untreated leaves, and cells were observed to be filled (Figure 6a). Upon exposure to the hydrotropic solutions (Figure 6b and c), the cell walls were found to be vigorously disturbed. Large shape irregularities were observed, and cells were found to be shrunken and shattered. Moreover, complete washout of the cell was noticed. No rupture was observed in either case. However, the hydrotropic solution might have attacked the cell wall from all directions, changed the permeability of the wall, and penetrated into the cell, leading to the extraction of citral. Though the hydrotropes used were of different natures, a similar effect on the cell structure was found, possibly due to the ability of hydrotropes to adsorb onto the cell wall and penetrate through the cell wall according to the parameter, ΔGad°/Amin (mJ·m−2), which can be found with eq 3. A higher magnitude of ΔGad°/Amin indicates better adsorption and penetration of hydrotropes.44

% contribution

S

extraction using NaSal confirmation experiment

100.00

S = individual variation; D = degree of freedom; V = variance.

was pooled. In the present study, the size of the plant material had minimum variance, so this parameter was used further. The F test gives a qualitative analysis to determine the most important factor, while the percentage contribution provides information on a quantitative basis.41 In the case of the extraction of citral using NaSal (Table 4), the F value for the solid loading was found to be the highest. Solid loading is therefore affecting the process to a greater extent followed by the temperature. The percentage contribution of solid loading was 48.05, whereas the percentage contribution for the size of the plant material was 1.95. Solid loading is therefore the most significant factor affecting the process, and the size of the plant material is an insignificant factor. The order of the factors according to their level of influence can be presented as follows: solid loading > temperature > hydrotrope concentration > size of the plant material. For extraction using NaCuS (Table 5), the F value for solid loading (78.62% contribution for influencing the process) was found to be substantially higher than the F value for other factors involved. Other factors influence the process to a lesser extent, and the order of influence is as follows: solid loading > temperature > hydrotrope concentration > size of the plant material. Confirmation Experiment. The confirmation experiment is essential for testing the validity of the Taguchi method. The confirmation experiment was performed in triplicate using the optimum conditions obtained by the Taguchi method, and the results are reported in Table 6. The yield and S/N ratio for extraction under optimum conditions are higher than the maximum yield and the highest S/N ratio obtained in the L9 array. These results confirmed the validity of the Taguchi method for the extraction of citral from the leaves of lemongrass using NaSal and NaCuS. SEM Analysis. To understand the mechanism of extraction using hydrotropes, plant materials before extraction and after extraction were studied using SEM, as shown in Figure 6.

° ΔGad = − (947.8/A min )(pC20 + 1.74 + 0.0211A min ) A min

(3)

where ΔGad° is the standard free energy of adsorption of the hydrotrope at the interface, Amin is the minimum cross-sectional area of the hydrotrope at the interface, and pC20 is the negative logarithm concentration of the hydrotrope, C20, required to reduce the surface tension of water by 20 dyn·cm−1. The values were found to be −61.50 and −66.45 mJ·m−2 for NaSal and NaCuS, respectively, using the data reported,45 with no large difference between the solutions. These values can be correlated widely to the observations made using SEM that both of the hydrotropes might have a similar effect on the 3754



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Figure 6. Scanning electron micrographs of lemongrass leaves (a) prior to extraction, (b) after extraction using NaSal, and (c) after extraction using NaCuS.

For extractions carried out at a 2.0 M concentration of hydrotrope, 8% solid loading, 30 °C temperature, and 25 mm size of the plant material, lower rate constants (0.4465 h−1 for NaSal and 0.4011 h−1 for NaCuS) were obtained, possibly due to a higher concentration of the hydrotrope, higher solid loading, and larger size of the plant material. A higher value of rate constant was found for NaSal extraction because of the higher solubilization capacity (0.4465 vs 0.4011 h−1 and 0.6215 vs 0.5415 h−1); hence, better extraction of citral was possible using NaSal. Reusability of Hydrotropes. The usability of the hydrotropic solution must be checked after extraction. NaSal and NaCuS, used under optimized conditions, were considered for testing the recyclability of the hydrotropic solution. The hydrotrope solutions were recycled twice for this purpose. Hydrotropic solutions were diluted for the recovery of citral extracted from the leaves of lemongrass. The diluted solutions were then concentrated to 1.75 M by simple evaporation. During extraction and filtration, about a 5% loss of hydrotropic solution was observed. Make-up hydrotropic solution was added to both the solutions, and experiments were again conducted under optimum conditions (1.75 M concentration of hydrotrope, 5% solid loading, temperature of 30 °C, and 0.25 mm size of the leaves). The amount of citral extracted was found to be almost constant in both the extractions, and the average yields were found to be 15.76 mg·g−1 for extraction using NaSal and 14.48 mg·g−1 for extraction using NaCuS. This study reveals that the hydrotropes can be reused for extraction by concentrating the dilute solution. The recovery step was very easy because hydrotropes are stable at higher temperatures, their

cell structure of lemongrass leaves. Thus, the microscopic investigations were useful in understanding the extraction mechanism. Kinetic Study. Data obtained for the extraction of citral under optimized conditions were fitted to a first-order kinetic equation (eq 4) to estimate the rate constant, k, relating to the extraction efficiency. The results initially obtained from deciding the time for the extraction were taken into consideration for fitting the data. These experiments were carried out at a 2.0 M concentration of hydrotrope, 8% solid loading, 30 °C temperature, and 25 mm size of the plant material using NaSal and NaCuS.

u(t ) = 1 − e−kt

(4)

where u(t) is the fraction of citral extracted after time t, and k is the first order rate constant describing the extraction (h−1). Using the MATLAB program (Mathworks, MA, U.S.A.), rate constants were determined by minimizing the error between the experimental value and the predicted value. Figure 7 shows the model fit for the extraction of citral using a hydrotropic solution. For extractions performed under optimized conditions, the extraction using NaSal was complete in 4.5 h, whereas 5 h was required in the case of extraction by NaCuS. On the basis of these observations, the optimum time for extraction can be considered to be 4.5 and 5 h for NaSal and NaCuS, respectively. For other experiments, the time required to complete the extraction was found to be 6 h for both of the hydrotropes, and a plateau was then observed, justifying the decision to keep an extraction time of 8 h for all experiments. 3755



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Figure 7. First order kinetic fit (operating conditions for NaSal 1 and NaCuS 1: 2.0 M concentration, 8% solid loading, 30 °C temperature, and 25 mm size of the plant material; operating conditions for NaSal 2 and NaCuS 2: 1.75 M concentration, 5% solid loading, 30 °C temperature, and 0.25 mm size of the plant material).

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melting points are high, and they do not produce any toxic effects during evaporation. The use of hydrotropes may prove viable on an industrial scale.

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ASSOCIATED CONTENT

S Supporting Information *

Importance and calculation procedure of Taguchi method. This information is available free of charge via the Internet at http:// pubs.acs.org.

CONCLUSION

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The extraction of citral from the leaves of Cymbopogon flexuosus (Steud.) Wats. using hydrotropic solutions was successfully optimized using the Taguchi method. The optimum conditions were found to be a hydrotrope concentration of 1.75 M, solid loading of 5% (w/v), a temperature of 30 °C , a plant size of 0.25 mm for both the hydrotropes, and extraction times of 4.5 h (NaSal) and 5 h (for NaCuS). The yield of citral was comparable for both of the hydrotropes. Solid loading was found to be the most significant parameter affecting the yield of citral. In the case of NaSal extraction, the contribution of solid loading was 48.05%, while the contribution of solid loading was found to be 78.62% for extraction using NaCuS. Solid loading therefore requires greater attention, or it may affect the yield of citral. A higher rate constant was obtained in the case of NaSal extraction; thus better mass transfer was possible using NaSal. The results obtained indicate the importance of optimization and kinetic studies, which may prove helpful for the further development of the process. The extraction could be performed under ambient conditions, which may eliminate intensive energy requirements. The exclusion of organic solvent made the process free of contamination. Citral could be recovered by the mere dilution of the hydrotropic solution. The reusability of the hydrotropic solution was found to be feasible by using simple evaporation. All of these advantages will possibly lead to the development of an economical extraction technique. On the basis of the present studies, hydrotropic extraction may provide a better alternative for the extraction of natural products in terms of simplicity, economics, ease of operation, and environmental friendliness.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected].

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REFERENCES

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Hydrotropic Extraction of Citral from Cymbopogon ... - ACS Publications

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