Optimization, Kinetics, and Thermodynamics in the Extraction Process

Apr 27, 2012 - In this work, DIE was employed to extract puerarin from puerarin lobata (Willd.) ... Experiment results showed that the extraction yiel...
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Optimization, Kinetics, and Thermodynamics in the Extraction Process of Puerarin by Decompressing Inner Ebullition Xiaoguang Chen,†,‡ Tengyou Wei,†,‡ Mengwei Peng,†,‡ and Zhangfa Tong*,†,‡ †

School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Nanning 530004, China

‡

ABSTRACT: Decompressing inner ebullition (DIE) is a novel method and it can achieve a rapid extraction at lower temperature for active constituents of medical plants. In this work, DIE was employed to extract puerarin from puerarin lobata (Willd.) Ohwi (PLO). The extraction conditions were investigated according single factor analysis. Experiment results showed that the extraction yield of puerarin reached a maximum value when the ethanol mass concentration in the aqueous solution immersing the PLO was 60%, the amount of ethanol solution for premaceration and of water for extraction were 1.6 and 10 mL·g−1 of PLO, respectively, the operation gauge pressure was −0.084 MPa, and the extraction temperature was 50 °C. Under the optimal conditions, the extraction kinetics of DIE was studied. The mass transfer coefficient of DIE ranged between 0.09 and 0.13 s−1, which is 183 times greater than that of ultrasonic assistant extraction under the same conditions, and its extraction activation energy ΔE is 10.86 kJ·mol−1. Also, the thermodynamics properties in the extraction process of DIE at 50 °C were also calculated, the enthalpy change ΔH is 58.76 kJ·mol−1, the entropy change ΔS is 224.50 J·mol−1·K−1, and Gibbs free energy ΔG is −13.73 kJ·mol−1, respectively. The extraction of DIE belongs to a spontaneous process with endothermic and entropy increasing.

1. INTRODUCTION Puerarin lobata (Willd.) Ohwi (PLO) is a famous traditional medicinal herb in China, which is also widely accepted by consumers over the world for its healthy effects. As a principal and major isoflavone, puerarin can be extracted from PLO,1−3 and it has been proven to have notable pharmacological functions including dilating coronary arteries, decreasing myocardial oxygen consumption, and improving microcirculation in both animals and human patients suffering from cardiovascular disease.4−6 During the extraction process of puerarin from PLO, the effect of starch gelatinization must be considered because that starch is also a main constituent with 15%∼34.2% content.7 When the temperature is higher than 60 °C, the starch gelatinizes and blocks the catheters within the plant materials, which not only greatly reduces the yield of puerarin, but also diminishes the extraction efficiency. To avoid the adverse effects of starch gelatinization,8,9 conventional methods for extraction of puerarin include organic solvent extraction with aqueous methanol or ethanol using heating reflux or leaching10 techniques for a few to several hours. These procedures have important drawbacks such as long extraction times and consumption of large quantities of solvents. Recently, several modern extraction methods such as microwave assisted extraction (MAE)11,12 and ultrasonic assistant extraction (UAE)13,14 have been used as an alternative to conventional methods. MAE and UAE can accelerate the extraction rate and also can be carried out at lower temperatures below that of starch gelatinization. However, these methods require a large investment in equipment and a huge amount of energy consumption. Thus, they are difficult to be applied for largescale industry. Decompressing inner ebullition (DIE) is a rapid extraction method established by Wei et al.15 Generally speaking, DIE © 2012 American Chemical Society

consists of a premaceration and an extraction process under decompressed pressure. During the premaceration, a small amount of ethanol solution is used to immerse the raw material, and the inner active constituents in the cells of PLO are dissolved or moistened. Afterward the extraction pressure is decompressed and the extraction process is strengthened by the vaporization effect of the inner ethanol. The extraction process can be completed in a short time at lower temperatures between 40 and 60 °C. Good results with respect to extraction efficiency and yield were obtained when the active constituents of medical plants were extracted by DIE at lower temperatures below that of starch gelatinization.16 Therefore, DIE was employed to extract puerarin in this study. After a premaceration with aqueous ethanol, water was used as the extraction solution and the extraction conditions were optimized. Moreover, the extraction kinetics and thermodynamics of DIE were investigated. The results provide useful information for extraction of the effective constituents from herbal medicine with the higher content of starch.

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials.17 The roots of PLO were obtained from Xi’an, China. The reference substance of puerarin (95.6%, lot no. 110752-200511) was purchased from The National Institute for the Control of Pharmaceutical and Biological Products, Beijing, China. The aqueous ethanol solutions were prepared by analytical grade ethanol, which were used to immerse PLO. Analytical grade methanol was used to dissolve the extract samples for HPLC analysis, and the Received: Revised: Accepted: Published: 6841

September 5, 2011 April 20, 2012 April 27, 2012 April 27, 2012 dx.doi.org/10.1021/ie2020153 | Ind. Eng. Chem. Res. 2012, 51, 6841−6846

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residual of PLO by vacuum filtration. After determining the weight and volume of the extract, 1−1.5 mL samples were taken by a sampler into a volumetric flask and mixed with chromatographic grade methanol to a metered volume of 25 mL. Then, the sample solution was centrifuged for 10 min at 10 000 rpm (12 000g). The supernatants were filtered through a 0.45 μm filter membrane and analyzed by HPLC. 2.2.2. Optimization of Extraction Conditions. Several factors can impact the yield of puerarin in the process of extraction by DIE. For increasing the extraction yield of puerarin as much as possible, the extraction conditions were optimized. The extraction conditions of DIE at different levels are given in Table 1. According to the single factor analysis, the mass concentration of aqueous ethanol solution immersing PLO was first optimized when the amount of ethanol solution and ratio of water to PLO were set as 1.6 and 10 (v/w), and the extraction temperature and gauge pressure were selected as 50 °C and −0.084 MPa, respectively. Similarly, the ratio of ethanol solution to PLO, the extraction gauge pressure, and the ratio of water to PLO were optimized one by one. 2.2.3. Derivation and Determination of Extraction Kinetics Equation. During the extraction process by DIE, a large amount of bubbles generated when the extraction pressure was decompressed. It caused the turbulence for particles of PLO in the solvent. The particles of PLO were contacted ceaselessly with the solvent, and the puerarin was extracted. This process belongs to an unsteady-state diffusion process, and it shows good agreement with the hypothesis of film theory. Therefore, eq 1 was selected, and it was described as follows:19

HPLC grade methanol was used as the mobile phase. Both of them were purchased from West Chemical, Shantou, China. 2.2. DIE. 2.2.1. Experimental Apparatus and Process.18 A schematic diagram of the DIE extraction apparatus was given in Figure 1. A pressure gauge with a precision of 0.25% MPa and a

Figure 1. Schematic diagram of the DIE extraction apparatus.

vent valve with circular holes were selected to adjust the extraction pressure. A 1 L flask was used as buffer bottle to reduce the deviation caused by the pressure regulator. The extraction process was as follows: The dried sample powders were first ground and then sieved through 100-mesh prior to use. The experiments were carried out by placing 10 g of sample powder in a certain amount of aqueous ethanol solutions in a 250 mL extraction flask (marked no. 3) and immersing it for 30 min. Then, some amount of deionized water with a desired temperature was added to the no. 3 flask. Sequentially, the pipelines between buffer flask and extraction flask were connected, and the vent valve (marked no. 4 in Figure 1) was used to regulate the extraction pressure. Extracting for a desired time, the extraction pressure was relieved and the extraction solution was separated from the

(q∞ − q)/q∞ = (1 − b)e−kt

(1)

where t (s) is time of convective mass transfer; q (g) is the amount of the puerarin extracted at time t (then, q = q∞ when t = ∞); b is the washing coefficient, which represents the extraction yield of puerarin recovered instantaneously when

Table 1. Extraction Conditions of DIE at Different Levels by Single Factor Analysis extraction conditions for optimization

levels

other conditions for extraction

mass concentrations of aqueous ethanol solutions for immersing PLO (%)

40 50 60 70 80

amount of aqueous ethanol solutions immersing PLO, 1.6 mL·g−1; operating gauge pressure, −0.084 MPa; ratio of water to PLO, 10 mL·g−1; extraction temperature, 50 °C

amount of aqueous ethanol solutions for immersing PLO (mL·g−1)

1.2 1.4 1.6 1.8 2.0

mass concentrations of aqueous ethanol solutions for immersing PLO, 60%; operating gauge pressure, −0.084 MPa; ratio of water to PLO, 10 mL·g−1; extraction temperature, 50 °C

operating gauge pressure/MPa

−0.086 −0.084 −0.082 −0.080 −0.078

amount of aqueous ethanol solutions immersing PLO, 1.6 mL·g−1; mass concentrations of aqueous ethanol solutions for immersing PLO, 60%; ratio of water to PLO, 10 mL·g−1; extraction temperature, 50 °C

ratio of water to PLO (mL·g−1)

6 8 10 12 14

amount of aqueous ethanol solutions immersing PLO, 1.6 mL·g−1; mass concentrations of aqueous ethanol solutions for immersing PLO, 60%; operating gauge pressure, −0.084 MPa; extraction temperature, 50 °C.

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PLO is submersed into the solvent and k (s−1) is the mass transfer coefficient. Taking the logarithm of both sides of eq 1 and defining A′ = ln (1 − b) + ln q∞, eq 2 can be obtained as follows: ln(q∞ − q) = −kt + A′

membrane to a 25 mL volumetric flask. The concentration of puerarin within the sample was measured by HPLC, and the extraction kinetic curve was determined. During the entire extraction process, the temperature of water was controlled at 50 °C. 2.4. HPLC Analysis and Calculation Method.24 A HPLC analysis was performed on an Agilent 1200 series HPLC system (Palo Alto, CA, USA) with a E1818009-C18 column (250 mm × 4.6 mm, i.d., 5 μm, Hypersil ODS2). The column temperature was set at 25 °C, and the analysis was performed using methanol−water solution (25:75, v/v) as a mobile phase. The flow rate was at 0.8 mL·min−1. The detection wavelength was set at 250 nm. The extraction yield of puerarin can be calculated with mass of the extracted puerarin divided by that of PLO.

(2)

If ln M is subtracted from both sides of the eq 2, where M (g) is the mass of the extracting solution; eq 2 can be simplified to eq 3: ln[(q∞ − q)/M ] = −kt + A′ − ln M

(3)

Let C = q/M and A = A′ − ln M, where C (%) is the concentration of the puerarin in the extraction solution at time t (then, C = C∞ = q∞/M when t = ∞); the extraction kinetics equation is given by ln(C∞ − C) = −kt + A

3. RESULTS AND DISCUSSION 3.1. Optimization of Extraction Conditions. 3.1.1. Concentration and Amount of the Aqueous Ethanol Solutions Immersing PLO. The effects of the concentration and amount of aqueous ethanol solutions immersing PLO were given respectively in Figure 2 and Figure 3. It can be seen from Figure

(4)

To avoid starch gelatinization, three extraction temperatures for determining the mass transfer coefficient of DIE were chosen as 40, 50, and 60 °C, respectively. When the extraction temperature was 50 °C, the extraction kinetic curves of puerarin were measured under the optimum extraction conditions as follows: the mass concentration of aqueous ethanol solutions immersing the PLO was 60%, the ratios of ethanol solution and water to PLO were set as 1.6 and 10 (v/w), the operation gauge pressure was −0.084 MPa. Additionally, the extraction kinetic curves of puerarin at 40 and 60 °C under the operating gauge pressure of −0.086 and −0.082 MPa, respectively, were also done, and the other extraction conditions were same as that at 50 °C. 2.2.4. Calculation of Thermodynamics Properties. The thermodynamic quantities such as change in free energy (ΔG), change in enthalpy of extraction (ΔH), and change in entropy (ΔS) may give an insight into the nature and mechanism of DIE. The values of ΔH and ΔS were determined from the Van’t Hoff equation as given as follows:20 ln K = −(ΔH /RT ) + (ΔS /R)

Figure 2. Effect of the mass concentration of aqueous ethanol solutions immersing PLO on the yield of puerarin by DIE. Other conditions for extraction were as follows: amount of aqueous ethanol solutions immersing PLO, 1.6 mL·g−1; operating gauge pressure, −0.084 MPa; ratio of water to PLO, 10 mL·g−1; extraction temperature, 50 °C (±standard deviations, n = 3).

(5)

where R is the gas constant, T is the temperature (in Kelvin), and K is distribution coefficient and is determined as21 K = YEP/YRP = YEP/(Y0 − YEP)

(6)

2 that the yield of puerarin has a maximum value with the change of the mass concentrations of aqueous ethanol solution.

where YEP (%) is the yield of puerarin when the extraction of DIE achieves equilibrium; YRP (%) is the percent of puerarin in PLO; Y0 (%) is the total content of puerarin in PLO before extraction, and it is considered as the sum of puerarin yields when PLO was extracted by DIE for three times. 2.3. UAE.22 Ultrasonic power has an important influence on the extraction rate of UAE. To compare with DIE, the extraction temperature of UAE was selected as 50 °C. It was presented by Zhou23 that a high yield of puerarin extracted by UAE at 50 °C was obtained when the ultrasonic power was up 200 W. To compare with DIE, an experiment was designed and carried out applying an ultrasonic bath of 40 kHz and 200 W (SK3300LH, kudos Ultrasonic Co. Ltd., Shanghai, China). The extraction process was as follows: 30.0 g of dried powders and 48 mL of 60% aqueous ethanol solutions were added into a flask. (To decrease the effect of sampling, the experimental scale was enlarged for three times). After 30 min of sample immersion, 300 mL of 50 °C water was added, the mixture was treated with ultrasound simultaneously. At a given duration, 0.5∼1.0 mL of extract was removed by a sampler with filter

Figure 3. Effect of the amount of aqueous ethanol solutions immersing PLO on the yield of puerarin by DIE. Other conditions for extraction were as follows: mass concentrations of aqueous ethanol solutions for immersing PLO, 60%; operating gauge pressure, −0.084 MPa; ratio of water to PLO, 10 mL·g−1; extraction temperature, 50 °C (±standard deviations, n = 3). 6843

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As we know, the polarities of aqueous ethanol solution are related to their concentrations. When the polarity of aqueous ethanol solution at certain concentration approaches to that of PLO, the maximum amount of the solvent would permeate into the cells containing puerarin. Good desorption effects will increase the transmitted amount of puerarin during the vaporization process of aqueous ethanol solutions as the extraction pressure is decreased. In this work, the optimal mass concentration of aqueous ethanol solutions for immersing PLO was selected as 60%. The optimal amount of aqueous ethanol solutions used for immersing PLO is given in Figure 3. A less amount of aqueous ethanol solutions cannot maintain a longer time for convection of DIE, while a larger amount of aqueous ethanol solutions will cause waste. In this process, 1.6 times to the PLO of aqueous ethanol solutions was the optimal value. 3.1.2. Operation Gauge Pressure and Ratio of Water to PLO (v/w). To achieve the inner ebullition of the extraction process, a proper operation gauge pressure was indispensable under a certain temperature. Figure 4 is the plot of puerarin

Figure 5. Effect of the ratio of water to PLO (v/w) on the yield of puerarin by DIE. Other conditions for extraction were as follows: amount of aqueous ethanol solutions immersing PLO, 1.6 mL·g−1; mass concentrations of aqueous ethanol solutions for immersing PLO, 60%; operating gauge pressure, −0.084 MPa; extraction temperature, 50 °C. (±standard deviations, n = 3).

Figure 4. Effect of the operating gauge pressure on the yield of puerarin by DIE. Other conditions for extraction were as follows: amount of aqueous ethanol solutions immersing PLO, 1.6 mL·g−1; mass concentrations of aqueous ethanol solutions for immersing PLO, 60%; ratio of water to PLO, 10 mL·g−1; extraction temperature, 50 °C. (±standard deviations, n = 3).

Figure 6. Relationship between extraction time and the concentration of puerarin in extracts (±standard deviations, n = 3) (I, DIE; II, UAE).

However, 5400 s are needed to reach the equilibrium comparing with DIE. The total extraction time of DIE is less than half time of UAE. Especially for external diffusion rate of puerarin in the extraction process, DIE is 135 times faster than that of UAE, which is intensified enormously by improving convective mass transfer. Also, due to its faster rate of mass transfer, DIE performs with high efficiency over the traditional methods. For example, it will take 100 h to leach out the puerarin completely with the traditional immersing method at room temperature, and 5 h is needed for the method of heating reflux with ethanol solvent to extract puerarin.10 Table 2 shows the fitting results of extraction kinetics for both DIE and the comparison experiment as obtained by eq 4. It can be seen that all fitting coefficients are larger than 0.99. These results indicate that the extraction kinetics of DIE and

yields under different extraction gauge pressures at 50 °C. It shows that the yield of puerarin achieved a maximum value when the operating gauge pressure was −0.084 MPa. If the extraction pressure decompresses too little, either the inner aqueous ethanol solutions cannot boil or the intensity of ebullition is insufficient. On the other extreme, if the extraction pressure decompresses too much, the vaporization rate of the inner aqueous ethanol solutions accelerates greatly, which shortens the duration for DIE. In this case, the extraction yield of puerarin is reduced. Thus, the optimal operation gauge pressure was selected at −0.084 MPa. The relationship between yields and the ratio of water to PLO (v/w) is shown in Figure 5. According to the equilibrium principle, the more the solvent used, the greater the yield of puerarin should be. However, it was shown in experiment that volumes of water to PLO up 10:1 (v/w) did not obviously affect the yield of puerarin. 3.2. Kinetics of Puerarin Extraction by DIE. The extraction kinetic curves of DIE were given in Figure 6-I. The extraction kinetic curves of the UAE at 50 °C were shown in Figure 6-II. It can be seen easily from Figure 6, that the extraction equilibrium of DIE is reached in 40 s, even though the extraction rates differ slightly at different temperatures.

Table 2. Comparison of Parameters and Correlation Coefficients of Kinetic Equation between DIE and UAE parameters of the kinetics equations method

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temperature (°C)

k (s−1)

A

R2

−2

DIE

40 50 60

9.48 × 10 1.08 × 10−1 1.22 × 10−1

−5.65 −5.68 −5.72

0.9915 0.9922 0.9918

UAE

50

5.91 × 10−4

−5.72

0.9920

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the comparison experiment agree with eq 4, which corresponds with a diffusion model. The mass transfer coefficient k of DIE increases slightly with increased temperature. The values are all in the range of 0.09∼0.13 s−1. It is affected a little by temperature, which is due to characteristic of the convective mass transfer. However, the mass transfer coefficient of UAE is 5.91 × 10−4 s−1, which is just 1/183 times to that of DIE at the same temperature. DIE can accelerate the extraction rate in a short time mainly owing to the manner change of mass transfer. According to k = k0 exp(−ΔE/RT), the equation ln k = ln k0 − ΔE/RT can be obtained easily. Figure 7 shows the

Figure 8. Relationship between ln K and 1/T in the extraction process of DIE.

puerarin at a lower temperature and greatly reduces the requirement of aqueous ethanol solutions. Due to the mass transfer manner of convection, DIE cuts down the energy barrier in the extraction process and enormously accelerates the extraction rate of puerarin. DIE shows a bright prospect for extraction of the effective constituents from herbal medicine with the higher content of starch.

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Figure 7. Relationship between ln k and 1/T during the extraction process of puerarin by DIE.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-771-323-9697. Fax: +86771-323-3718.

relationship between ln k and 1/T by DIE. The calculation result of the active energy is 10.86 kJ·mol−1, which is 34% of the process extracted by MAE.25 Therefore, DIE cuts down the energy barrier in the extraction process and accelerates the extraction rate of puerarin. 3.3. Calculation of Thermodynamics Properties for the Extraction Process. Factually, convection mass transfer is dominant in DIE process, and the extraction process was just affected a little by temperature. Therefore, eq 5 can be fitted by the determination of the distribution coefficient at 40, 50, 60 °C, and the thermodynamics properties in extraction process of DIE were calculated. The distribution coefficients at different temperatures and the fitting relationship are shown in Table 3 and Figure 8.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Partial support for this work provided by the Scientific and Technological Project of Guangxi (0718002-4-4) is gratefully acknowledged. Partial supports for this work provided by Scientific and Technological Project Of Guangxi (0718002-4-4) and the Innovation Talent Team Project in Guangxi Universities (Guijiaoren 2010-38) are gratefully acknowledged.

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Table 3. Distribution Coefficients of Puerarin at Different Temperatures ln K

40 °C

50 °C

60 °C

4.43

5.10

5.79

REFERENCES

According to the eq 5, the calculation results of the thermodynamics properties in the extraction process of DIE at 50 °C were shown as follows: ΔH is 58.76 kJ·mol−1, ΔS is 224.50 J·mol−1·K−1, and ΔG is −13.73 kJ·mol−1, which was calculated with the equation of ΔG = ΔH − TΔS. The calculated results indicate that the extraction of DIE belongs to a spontaneous process with endothermic and entropy increasing.

4. CONCLUSIONS In this work, a highly effective and green process, DIE, was presented for extracting puerarin. It achieves a fast extraction of 6845

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