Microwave assisted extraction for phytoconstituents – An overview

 

Tripti Jain^1*, V Jain², R Pandey², A Vyas², S Gandhi¹, SS Shukla²

¹Rungta College of Pharmaceutical Sciences and Research, Bhilai  

²Institute of Pharmacy, Pt. Ravishankar Shukla University, Raipur

ABSTRACT

At the present time, there are a number of non-conventional extraction methods in use that are all, in principle, solid-liquid extractions (SLE) but which introduce some form of additional energy to the process in order to facilitate the transfer of analytes from sample to solvent. These methods include fairly inert, insoluble, and often polymeric material, such as cellulose of plants or fungi and the microbial cell wall. The first step of the extraction is therefore to release and solubilize the smaller secondary metabolites in the matrix, resulting in the initial extract. Forced-flow solid-liquid extraction (FFSLE) techniques, such as medium-pressure solid-liquid extraction (MPSLE) and rotation planar extraction (RPE), in these methods the extraction solvent is forced through the sample bed either by means of pressure or by centrifugal force, thus increasing the efficiency of the extraction process. Even extraction by electrical energy has been studied. The main advantage of these non- conventional methods compared to conventional SLE methods is the increase extraction efficiency, which leads to increased yields and/or shorter extraction times. Indigenous cultures have learnt to exploit the properties of secondary metabolites in many ways, e.g. specific plants or parts of them have been used as poisons, analgesics, stimulants, preservatives, colorants, tanning agents for tanning leather etc. As our understanding of chemistry and other natural sciences has increased, the active chemical compounds of these traditionally used plants have been successfully isolated and identified. There is an increasing trend of using pure compounds instead of crude extracts prepared from plant material, irrespective of their intended use.

 

 

INTRODUCTION:

Microwaves are electromagnetic fields in the frequency range 300 MHz to 300 GHz or between wavelengths of 1 cm and 1m. These electromagnetic waves made up of two oscillating perpendicular fields: electrical field and magnetic field. Microwaves are used as information carriers or as energy vectors. This second application is the direct action of waves on material which is able to absorb a part of electromagnetic energy and to transform it into heat. The most commonly used frequency for commercial microwave instruments is 2450 MHz, which corresponds to an energy output of 600- 700 Watts.¹

At this frequency, the electric field swings the orientation of water molecules 2.45 x 109 times every second and the chaos inherent to the system opposes the synchrony of the oscillation with that of the field. Thus creating an intense heat that can escalate as quickly as several degrees per second (estimated as 100C/s at 4.9 GHz).²

Most of the bulk of the biomass, irrespective of whether it is plants or microbes, exists as pressurized liquid extraction, vertical (turbo) extraction, supercritical fluid extraction.^3,4

The first step in the process of obtaining secondary metabolites from biogenic materials is to release them from the matrix by means of extraction.⁵ Due to the often very complex composition of the material and the minute amounts of some of the constituents present, the choice of extraction method is of great importance.

 

The heat phenomenon is based on the interaction of the electrical field with compounds of a material. The transformation of electromagnetic energy in calorific energy occurs by two mechanisms: ionic conduction and dipole rotation. The ionic conduction generates heat due to the resistance of medium to ion flow. The migration of dissolved ions causes collisions between molecules because the direction of ions changes as many times as the field changes sign. The dipole rotation is related to alternative movement of polar molecules, which try to line up with the electric field. Multiple collisions from this agitation of molecules generate energy release and therefore a temperature increase.⁶

The main advantages of microwave assisted extraction over the conventional extraction techniques are reduced solvent consumption, shorter operational times, moderately high recoveries, good reproducibility and minimal sample manipulation for extraction process.^{7, 8}

The Microwave Assisted Extraction Process is a high-speed method used to selectively extract target compounds from various raw materials. Microwave assisted extraction uses energy of microwave radiation to heat solvents quickly and efficiently. By using a closed system, extraction can be performed at higher temperatures and extraction times can be reduced drastically. It is an innovative solvent-extraction technology, offers a superior alternative to several thermal applications owing to its efficient volumetric heat production and has many advantages over conventional solid liquid extraction methods. Applications include the extraction of high-value compounds from natural sources including phytonutrients, nutraceutical and functional food ingredients, and pharma actives from biomass. In the present article basis of microwave extraction technology and their advantages are discussed. The most widely used extraction processes have traditionally been based either on different liquid extraction methods or on vapor-phase extraction methods.⁹

Two parameters define the dielectric properties of materials. The first, is ε’, the dielectric constant which, describes the polarizability of the molecule in an electric field. The dielectric loss factor; ε’’, measures the efficiency with which the absorbed microwave energy can be converted into heat. The ratio of the two terms is the dissipation factor, δ, ultrasonic extraction,

δ=.ε"/ε',.................. eq.1

 

Use of microwave irradiation is another way of increasing the efficiency of conventional extraction methods. Microwave- assisted extraction consists of heating the solvent in contact with the sample by means of microwave energy. The process involves disruption of hydrogen bonds, as a result of microwave-induced dipole rotation of molecules, and migration of the ions, which enhance penetration of the solvent into the matrix, allowing dissolution of the components to be extracted.¹⁰

When choosing parameters for microwave extraction physical parameters like solubility, dielectric constant, and the dissipation factor must be considered (δ). The first factor is to choose a solvent, with a high extracting power in which the target analyte is soluble. Usually higher dielectric constant the higher degree of microwave absorption. Water has the highest dielectric constant of common solvents. However, the dissipation factor is significantly lower than other solvents. So, the rate at which water absorbs microwave energy is higher than the rate at which the system can dissipated the heat. This phenomena account for the “superheating” effects, which occur when water is present in the matrix. Localized superheating can have positive or negative effects, depending on the matrix. In some cases it can increase the diffusivity of analyze in the matrix. In other cases, the intense heating can

cause degradation of analyze and/or “explosion” of the solvent. To get maximum heat distributed through the matrix, it is best to choose a solvent that has a high dielectric constant as well as a high dissipation factor.

 

TABLE-1 Physical Constants for commonly used solvents¹¹

 

Solvent	Dielectric (ε’) F/m	Dielectric                            Loss Factor (ε’’) F/m	....□.x 104
Water	80	12	1500
Acetone	20.7	11.5	5555
Methanol	23.9	15.2	6400
Ethanol	7	1.6	2286
Hexane	1.88	0.00019	0.10
Ethyl Acetate	6.02	3.2	5316

 

Extraction of bioactive through microwaves

During MAE, a polar solvent with a high dielectric constant surrounds the matrix. Microwaves generated in a magnetron are applied in a pulsed fashion. The solvent molecules absorb the microwave energy and become polarized. When the microwave field is removed thermally induced disorder is restored (Fig. 1). This process heats the bulk solution and may cause localized superheating effects (only in matrices that contain water). Thermal equilibrium is eventually established within the system because the heat is transferred from the bulk solution and the “pockets affected by superheating effects” via collisions so that the energy is distributed uniformly throughout the system. The final temperature of the extraction is proportional to the power (watts), time, and initial temperature; it is inversely proportional to the heat capacity of the solvent, and the mass of sample in grams (eq. 2)¹². The heat produced by the interaction of the microwaves with the solvent subsequently increases the diffusivity of the solvent and hopefully that of the analyte. The solvent is then able to diffuse into the matrix and extract the analytes; and then diffuse out of the matrix carry along the soluble components.

T_f = T _i + (Pabs t/ K Cp m) – loss….eq.2

Where, K is the conversion factor of calories to Joules, Cp is the heat capacity of the solvent, m represents the mass of the matrix, Pabs is the power absorbed, t is the time that the microwave field is applied, and Ti is the initial temperature.

 

The dominate factors that govern the extraction of an analyte from a matrix by MAE are the solubility of the analyte in the solvent, the mass transfer kinetics of the analyte from the matrix to the solution phase, and the strength of analyte/matrix interactions. The first factor is obvious. For samples, with a homogenous composition and limited porosity, the rate of extraction is determined by the diffusion of the analyte to the surface of the matrix particle. Higher temperatures and swelling of the matrix increase the rate of diffusion and promote faster

extraction kinetics. For wood samples the rate of extraction is dependant on the diffusion of the analyte out of the pores, migration from one adsorption site to another, and displacement of the analyte molecules on adsorption sites by the solvent.¹³

 

Figure 1: Effects of Microwaves on Water

 

Microwave Ovens

Microwave ovens come in a variety of designs. However, the underlying principles of operation are very much the same. Microwaves are generated inside an oven by the alternating current from domestic power lines at frequency of 60 Hz and stepped up to 2450 million Hz. This is accomplished by a device called magnetron, which operates at 4000 to 6000 volts inside microwave ovens. The step up transformer that powers the magnetron accounts for more than half the weight and value of the domestic microwave ovens. A waveguide channels the electromagnetic waves through a conduit called waveguide, into the cavity that holds samples/substrates for heating.

 

Figure. 2a and 2b Monomode and multimode ovens.

 

Microwave ovens can have monomode or multimode cavity. The monomode cavity (Fig. 2a) can generate a frequency, which excites only one mode of resonance. The sample can be

placed on the maximum of the electrical field as the distribution of the field is known. The multimode cavity is large (Fig2b) and the incident wave is able to affect several modes of resonance. This superimposition of modes allows the homogenization of field. Homogenization systems such as rotating plate are added.

 

Following are the few commercially available microwave extraction models:

a)  CEM solvent extractor¹⁴

b)  microwave assisted extractor¹⁵

c)   microwave reflux¹⁶

d)  Sub-500 W microwave extractor¹⁷

e)   drydist model of milestone¹⁸

f)  solvent free extractor¹⁹

g)        Monolithic     equipment    for     microwave    assisted extraction²⁰

h)  Closed vessel mono-model of CEM Co¹⁷

Advantages:

1)    Extremely short extraction times, 15 to 30 minutes

2)      High sample throughput, 8 to 16 samples in one extraction run

3)    Very pure vessel materials

4)      Exact reaction control by temperature and pressure sensors

5)    Possibility of automation

6)    Documentation for GLP requirements

7)      The volumetric heating or heating of the bulk as opposed to transferring heat from the surface, inwards, is more efficient, uniform and less prone to overkill or supererogation.

8)           In processing applications, the ability to instantaneously shut the heat source makes enormous difference to the product quality and hence the production economics.

9)    The very nature of heating through the involvement of the raw material under processing brings about quality consistency as well as positive environmental impact.

10)       Microwaves allow simple, rapid and low solvent consuming processes.

 

Superiority over other methods:

Traditionally, plant materials were subjected to mechanical shear to release the volatiles in virgin state. There are today various closely controlled sophisticated methods of extraction from distillation, through leaching to super- or sub-critical solvent extraction. Among the various available methods, microwave assisted extractions are also well reported and show the highest promise.²¹

The various methods of extraction, such as sonication, super/sub-critical fluid extractions, etc have extraction capabilities and limitations owing to their inherent and unique features. MAHD (microwave assisted hydro- distillation) method has been compared in Table 3.4 with other extraction methods to show similarities in results for rosemary (Presti 2005). The methods compared are solvent extraction (SE), microwave assisted hydro- distillation (MAHD), hydro-distillation (HD), supercritical fluid extraction (SFE). If the benefits of using microwaves in terms of time, manpower training and environmental impact were included, the MAHD would stand apart as a more desirable as SE, SFE, and HD.

 

 

TABLE-2 Comparison of microwave assisted extraction with other methods

Parameters	Soxhlet	Sonication	Microwave	Supercritical Fluid
Sample* Weight (gram)	5.00-10.00	5.00-30.00	0.50-1.00	1.00-10.00
Solvent	**	**	Hexane/Ethanol	CO2
Solvent Volume (mL)	>300.00	300.00	10.00-20.00	5.00-25.00
Vessel Volume (mL)	500.00-1000.00	500.00	<100.00	5.00-25.00
Temperature (Degrees C)	Boiling Point	Room Temp.	40, 70, 100	50, 200
Time	16 hours	30 minutes	30-45 seconds	30-60 minutes
Pressure (atm.)	Ambient Atm.	Ambient Atm.	1.0-5.0	150.0-650.0
Relative Energy Consumption	1.00	0.05	0.05	0.25

*Dependent on the concentration and type of sample; ** Dichloromethane, acetone, hexane, cyclohexane, toluene, etc.

 

TABLE-3 Comparison of various techniques for essential oil extraction.

Compound	Literature*	SE*	MAHD*	HD*	SFE*
Alpha pinene	9.0-14.0	12.1	8.1	8.6	2.3
camphene	2.5-6.0	3.5	2.7	2.6	1.1
Beta- pinene	4.0-9.0	4.5	6.3	4.8	2.3
Myrcene	1.2-2.0	1.9	2.1	21.1
p-cymene	0.8-2.5	-	0.5	1.6	0.7
1,8 ceneole	38.0-55.0	50.8	45.8	56.9	35.6
Camphor	5.0-15.0	2.5	5.9	5.9	3.7
Borneol	1.5-5.0	2.8	2.9	2.5	6.8
Alpha terpeneol	1.0-2.63.3	3.1	2.3	5.9
Bornyl acetate	0.5-1.5	0.6	1.1	0.5	1.1

SE: solvent extraction; MAHD: microwave assisted hydro- distillation; HD: hydro-distillation; SFE: supercritical fluid extraction. * values representing comparative ratios.

 

TABLE-4 Applications of Microwave assisted extraction

A.   EXTRACTION OF PLANT CONSTITUENTS

1.   Polyhenols from Green Tea. 2.Carotenoids from Paprika powders²⁷

3.   glycyrrhizic acid from liquorice root²⁸

4.   Saikosaponins from Bupleurum falcatum roots²⁹

5.   Cocaine and benzoylecgonine from coca leaves³⁰

6.   Carnosic acid from Rosemary.

7.   Canola oil from Canola.

8.   Oil from evening primrose and borage seeds.

9.   alkamides from Echinacea purpurea L. roots ¹⁰

10.   Pierine from black pepper

11.    Oil from olive seeds^{7, 8}

12.    lipids from several oleaginous seeds^{7, 8}

13.  Essential oil from Mint Leaves³¹

14.       Essential oil from Cuminum cyminum and Zanthoxylum bungeanum ³²

15.    Camptothecin from Nothapodytes foetida³³

16.    Essential oil from Lippia alba³⁴

17.    Sanguinarine and chelerythrine from Macleaya cordata³⁵

18.    Diterpenes like tanshinones from Salvia miltiorrhiza³⁶

19.       Geniposidic acid and chlorogenic acid from Eucommia ulmodies³⁷

20.    Solanesol from Tobacco leaves³⁸

21.    Embelin from Embelia ribes³⁹

22.    Artemisnin from Artemisia annua L.⁴⁰

25.    Furanocoumarins from Pastinaca sativa⁴¹

26.    Coumarin and melilotic acid from Melilotus officinalis L.⁴²

27.     Phenolic compound and Hypericum perforatum and Thymus vulgaris⁴³

28.    Pigments from Capsicum annum⁴⁴

29.    Saponin from Panax ginseng⁴⁵

30.    Paclitaxel from Taxus baccata⁴⁶

31.    Oleuropein and related biophenols Olea europeaea⁴⁷

32.    Anthraquinones from Morinda citrifolia⁴⁸

B.       EXTRACTION OF OTHER ORGANIC COMPOUNDS FROM SOIL AND SEDIMENTS

1.   organochlorine pesticides, PCBs from soil^49,50

2.   phenolic compounds from soil^51,52

3.    PAHs, PCBs, neutral and basic compounds, phenols, pesticides from soil and  sediment⁵³

4.   triazines from soil^54,55

5.f ungicides from soil⁵⁶

6.   herbicides from soil⁵⁷

7.     PAHs, phenols, organochlorine pesticides, neutral and basic compounds soil and sediment^31,58

8.   PCDDs/PCDFs from soil⁵⁹

9.   PAHs from soil^60,61

10.    organochlorine pesticides from soil and sediment⁶²

11.    organochlorine and organophosphorus pesticides, arochlors from soil⁶³

12.    TPH from soil⁶⁴

13.    PAHs from coal⁶⁵

14.    dioxins and furans from soil⁶⁶

C.   MISCELLANEOUS 1.packaging^67,68 2.defrosting^69,70,71 3.browning^72,73,74 4.microbial inactivation⁷⁵ 5.tempering⁷⁶ 6.dehydration⁷⁷ 7.blanching⁷⁸

8. Aroma extraction⁷⁹

             TPH: Total Petroleum Hydrocarbons, PUF: Polyurethane Foam). PAHs polycyclic aromatic Hydrocarbons, PCBs polychlorobiphenyls,

 

APPLICATIONS

The first publications which dealt with the efficiency of microwave heating for organic extraction appeared in 1986. Ganzler et al.^22,23 have developed extraction protocols for lipids, antinutritives and pesticides from soils, seeds, foods and feeds in a few millilitres of solvent, irradiated for 30 s up to 7 times in a domestic oven (1140 W). Also in 1986, Lane and Jenkins²⁴ proposed the desorption of hydrocarbons from particulate matter by microwave irradiation with gas flow to replace extraction by solvent or desorption by infrared radiation which allows only superficial heating. The recovery of essential oils is also one of the first applications in the organic extraction field. In 1989, Craverio et al.²⁵ have compared extraction by steam flow and microwave assisted extraction of fresh leaves of Lippia sidoides. Pare et al.²⁶ have also developed an extraction process of natural products in water, soils, plants and animals (MAPTM Microwave Assisted Process) in liquid and gas phases. This method enhances analytical capabilities such as selectivity and sensitivity, with similar or better linearity and reproducibility factors than traditional methods. Since than several types of samples have been extracted, some of them have been listed in table 4.

CONCLUSION:

Thermal technology dictates the quality, economics and environmental impact of any processing plant. The rising number of Green engineering regulations calls for more efficient energy usage and more environment friendly raw materials as well as effluents.⁸⁰ Electric heating technologies such as radio frequency, microwave, ohmic and infrared are fast emerging; among them microwave shows a highly promising future.⁸¹ Microwave energy for heating has been in commercial use since 1950.⁸² But it is only recently that its benefits as a environmentally- friendly source of thermal energy has been widely appreciated. Efficiency demands a bare minimal processing of natural materials. The hallmark of microwave extraction (MAE) is accelerated dissolution kinetics as a consequence of the rapid heating processes that occur when a microwave field is applied to a sample. It has gained acceptance as a mild and controllable processing tool. The main advantages of MAE are shorter extraction times (typically 15 minutes), shorter cooling times (2 minutes) and less use of solvent (10 mL for MAE versus 250 mL for Soxhlet). The drastic reduction in extraction time results in a higher sample throughput without significant losses in analyte recovery. MAE is a viable candidate for performing extractions due to its applicability over a wide range of sample types because the selectivity can be easily manipulated by altering solvent polarities.

 

 

REFERENCES:

1.        H. M. Kingston and L.B. Jassie (Editors), Introduction to Microwave Sample Preparation, American Chemical Society, Washington, DC, 1998.

2.        Lew A, Krutzik PO, Hart ME, Chamberlin AR. Journal of Combinatorial Chemistry. 2002; 4(2): 95-105.

3.        Huie CW. A review of modern sample preparation techniques for the extraction and analysis of medicinal plants. Anal. Bioanal. Chem 2002; 373: 23-30.

4.        Zygmunt B, Namiesnik J. (2003): Preparation of samples of plant material for

5.        Cannell RJP. How to approach the isolation of a natural product. Pp. 1-51. In Cannell, RJP. (ed.): Methods in biotechnology 4. Natural products isolation. Humana Press, Totowa, New Jersey, USA. 1998.

6.        Neas ED, Collins MJ. Introduction to microwave sample preparation. American Chemical Society: Washington D.C., 1988, Chap. 2, 7-32.

7.        Garcia-Ayuso LE, Luque De, Castro MD. A multivariate study of the performance of a microwave-assisted Soxhlet extractor for olive seeds. Anal. Chim. Acta. 1999; 382: 309-316.

8.        Garcia-Ayuso LE, Luque De, Castro MD. Employing focused microwaves to counteract conventional soxhlet extraction drawbacks. Trends Anal. Chem. 2001; 20: 28- 34.

9.        Starmans DAJ, Nijhuis HH. Extraction of secondary metabolites from plant material: a review. Trends Food Sci. Technol. 1996; 7: 191-197.

10.     Hudaib M, Gotti R, Pomponio R, Cavrini V. Recovery evaluation of lipophilic markers from Echinacea purpurea roots applying microwave-assisted solvent Extraction versus conventional methods. J. Sep. Sci. 2003; 26: 97-104.

11.     Zlotorzynski. The application of microwave radiation to analytical and environmental chemistry, Critical reviews in analytical chemistry.1995; 25, 43-75.

12.     Haswell SJ, Kingston HM. Microwaved-enhanced chemistry. American chemical society, Washington, DC, 1997.

13.     Poole C, Poole S. Trends in extraction of semivolatile compounds for environmental analysis. Analytical communications. 1996; 33: 11H-14H.

14.     CEM. Microwave-accelerated reaction system, model mars - x, for the extraction of organic pollutants from solid matrices. Evaluation report. NC: CEM Corporation. 2000.

15.     Raman G, Gaikar VC. Microwave-assisted extraction of piperine from piper nigrum. Ind. Eng. Chem. Res. 2002; 41: 2521-2528.

16.     Li HB, Chen L, Nie, Yao S. Solvent effects on focused microwave assisted extraction of polyphenolic acids from Eucommia ulmodies Phytochemical analysis 2004; 15: 306-312.

17.     Kaufmann B, Christen P. Recent extraction techniques for natural products: microwave assisted extraction and pressurized solvent extraction. Phytochemical analysis.2002; 13:105-113.

18.     Milestone. 2005. http://www.milestone-general.com

19.     Lucchesi ME, Fchemat, Smadja J. Solvent-free microwave extraction of essential oil from aromatic herbs: comparison with conventional hydro-distillation. Journal of chromatography A. 2004; 1043: 323–327.

20.     Gao S, Han W, Deng X. Study of the mechanism of microwave-assisted extraction of Mahonia Bealei (Fort.) leaves and Chrysanthemum morifolium (Ramat.) petals. Flavor and Fragrance Journal. 2004; 19: 244-250.

21.     Nelida EG, Witte L. International society of ecology 99 – Marseille,      Poster   presentations abstracts http://www.chemecol.org. 1999.

22.     Ganzler K, Salgo A, Valko KJ. Chromatogr. 1986; 371: 299-306.

23.     Ganzler K, Salgo AZ. Unters. Forsch. 1987; 184: 274-276.

24.     Lane D, Jekins SWD. Polynucl. Aromat. hydrocarbons: Chem., Charact. Carcinog., Int. Symp., 9th. 1986; 437-449.

25.     Craverio AA, Matos FJA, Alenca JW, Plumel MM. Flavour and flagrance J. 1989; 4: 43-44.

26.     Pare JRJ, B langer JMR, Stafford SS. Trends Anal. Chem. 1994; 13: 176-184.

27.     Csiktusnadi Kiss GA, Forgacs E, Cserhati T, Mota T, Morais H, Ramos A. Optimisation of the microwave- assisted extraction of pigments from paprika (Capsicum annuum L.) powders. J. Chromatogr. A 2000; 889: 41-49.

28.     Pan X, Liu H, Jia G, Shu YY. Microwave-assisted extraction of glycyrrhizic acid from licorice root. Biochem. Eng. J. 2000; 5: 173-177.

29.     Joong-Ho Kwon,Young-Hui Choi, Hyung-Wook Chung, Gee-Dong Lee. The characteristics of a microwave extraction process used for saikosaponins from Bupleurum falcatum root. International Journal of Food Science & Technology. 2005; 41(1): 67-75.

30.     Brachet A, Christen P, Veuthey JL. Focused microwave- assisted extraction of cocaine and benzoylecgonine from coca leaves. Phytochem. Anal. 2002; 13: 162-169.

31.     Pare JRJ, Sigouin M, Lapointe J. Microwave assisted natural products extraction U S patent no. 5 002 784, March 26, 1991.

32.     Wang Z, Ding L, Li T, Zhou X, Wang L, Zhang H, Liu L, Li Y, Liu Z, Wang H, Zeng H, He H. Improved solvent free microwave extraction of essential oil from dried Cuminum cyminum L. and Zanthoxylum bungeanum Maxim. J. Chromatogr. A. 2006; 1102: 11-17.

33.     Fulzele DP, Satdive RK. Comparison of techniques for the extraction of the anticancer drug camptothecin from Nothapodytes foetida. J. Chromatogr. A. 2005; 1063: 9-13.

34.     Stashenko EE, Jaramillo BE, Martinez JR. Comparison of different extraction methods for the analysis of volatile secondary metabolites of Lippia alba (Mill.) N.E. Brown, grown in Columbia and evaluation of its in vitro antioxidant activity. J. Chromatogr. A. 2004; 1025: 93-103.

35.     Zhang F, Chen B, Xiao S, Yao S. Optimization and comparison of different extraction techniques for sanguinarine and chelerythrine in fruits of Macleaya cordata (Wild) R. Br. Sep. Purif. Technol. 2005; 42: 283-90.

36.     Pan X, Niu G, Liu H. Microwave assisted extraction of tanshinones from Salvia miltiorrhiza bunge with analysis by high performance liquid chromatography. J. Cromatogr. A. 2001; 922: 371-75.

37.     Li H, Chen B, Nie L, Yao S. Solvent effects on focused microwave assisted extraction of polyphenolic acids from Eucommia ulmodies Phytochemical analysis 2004;15: 306- 312.

38.     Zhou H, Liu C. Microwave assisted extraction of solanesol from tobacco leaves. J. Chromatogr. A. 2006; 1129: 135-39.

39.     Latha C. Microwave assisted extraction of embelin from Embelia ribes. Biotechnol. Lett. 2006; 9243: 105-29.

40.     Hao J, Han W, Huang S, Xue D, Deng X. Microwave assisted extraction of artemisnin from Artemisia annua L. Sep. Purif. Technol. 2002; 28: 191-196.

41.     Waksmundzka-Hajnas M, Petruczynik A, Dragan A, Wianowska D, Dawidowicz AL, Sowa I. Influence of the extraction mode on the yield of some furanocoumarins from Pastinaca sativa fruits. J. Chromatogr. B. 2004; 800: 181-87.

42.     Martino E, Ramaiola I, Urbano M, Bracco F, Collina S. Microwave assisted extraction of coumarin and related compounds from Melilotus officinalis L. as an alternative to Soxhlet and ultrasound assisted extraction. J. Chromatogr. A. 2006; 1125: 147-51.

43.     Sterbova D, Matejicek D, Vlcek J, Kuban V. Combined microwave assisted isolation and solid- phase purification procedures prior to the chromatographic determination of phenolic compounds in plant materials. Anal. Chim. Acta. 2004; 513: 435- 44.

44.     Kiss GAC, Forgacs E, Serati TC, Mota T, Morais H, Ramos A. Optimisation of the microwave assisted extraction of pigments from paprika (Capsicum annum L.) powders. J. Chromatogr. A. 2000; 889: 41-49.

45.     Kwon J, Lee G, Belanger JMR, Pare JRJ. Effect of ethanol concentration on the efficiency of extraction of ginseng saponins when using a microwave assisted process. Int. J. Food Sci. Technol. 2003; 38: 615-22.

46.     Talebi M, Ghassempour A, Talebpour Z, Rassouli A, Dolatyari L. Optimization of the extraction of paclitaxel from Taxus baccata L. by the use of microwave energy. J. Sep. Sci. 2004; 27: 1130-36.

47.     Japon-Lujan R, Luque-Rodrigue JM, Luque de Castro MD. Multivariate optimization of the microwave assisted extraction of oleuropein and related biophenols from olive leaves. Anal Bioanal Chem. 2006; 385: 753-59.

48.     Hemwimon S, Pavasant P, Shotiprule A. Microwave assisted extraction of antioxidative anthraquinones from roots of Morinda citrifolia. Sep. Purif. Technol. Article in press (2007).

49.     McMillin R, Miner LC, Hurst L. Abbreviated microwave extraction of pesticides and PCBs in soil. Spectros. Int. J. 1997; 13(1): 41-50.

50.     Vioricalopez-Avila , Janetbenedicto, Chatancharan, Andrichardyoung, Wernerf, Beckert. Determination of PCBs in Soild Sediments bv Microwave-Assisted Extraction and GC/ECD or ELISA. Environ. Sci. Technol. 1995; 29: 2709-12.

51.     Llompart MP, Lorenzo RA, Cela R, Li K, B langer JMR, Pare JRJ. Evaluation of supercritical fluid extraction, microwave-assisted extraction and sonication in the determination of some phenolic compounds from various soil matrices J. Chromatogr. A 1997; 774: 243-251.

52.     Llompart MP, Lorenzo RA, Cela R, Li K, B langer JMR, Pare JRJ. J. Chromatogr. A 1997; 757 : 153-164.

53.     Li K, Blanger JMR, Llompart MP, Turpin RD, Singhvi R, Pare JRJ. Spectroscopy. 1997 ; 13: 1-13.

54.     Molins C, Hogendoorn EA, Heusinkveld HAG, van Harten DC, van Zoonen P, Baumann RA. Microwave assisted solvent extraction (MASE) for the efficient determination of triazines in soil samples with aged residues. Chromatographia 1996 ; 43: 527-532.

55.     Hoogerbrugge R, Molins C, Baumann RA. Anal. Chimi. Effects of Parameters on Microwave Assisted Extraction of Triazines from Soil: Evaluation of an Optimisation Trajectory.Acta. 1997 ; 348: 247-253.

56.     Frost SP, Dean JR, Evans KP, Harradine K, Cary C, Comber MHI. Extraction of hexaconazole from weathered soils: A comparison between Soxhlet extraction, microwave-assisted extraction, supercritical fluid extraction and accelerated solvent extraction. Analyst. 1997 ; 122: 895-898.

57.     Stout SJ, Da Cunha AR, Allardice DG. Microwave Assisted Extraction Coupled with GC/ECD/MS for the Simplified Determination of Imiazolinone Herbicides in Soil at the ppb Level Anal. Chem. 1996; 68: 653-658.

58.     Lopez-Avila V, Young R, Teplitsky NJ. AOAC Int. 1996; 79(1): 142-156.

59.     Chiu C, Poole G, Tardif M, Miles W, Turle R. Determination of PCDDs/PCDFs Using Microwave- Assisted Solvent Extraction. Organohalogen Compounds.1997; 31: 175-180.

60.     Barnabas IJ, Dean JR, Fowlis IA, Owen SP. Extraction of polycyclic aromatic hydrocarbons from highly contaminated soils using microwave energy. Analyst. 1995; 120: 1897-1904.

61.     Punt MM, Raghavan GSV, Belanger JMR, Pare JRJ. Microwave-assisted process (maptm) for the extraction of contaminants from soil. Soil and sediment contamination (formerly Journal of Soil Contamination). 1999; 8(5): 577 –592.

62.     Onuska F, Terry KA. Chromatographia 1993; 36: 191- 194.

63.     Lopez-Avila V, Young R, Kim R. J. Chromatogr. Sci. 1995; 33: 481-484.

64.     Hasty E, Revesz R. American Lab. 1995: 66-74.

65.     Lane D, Jekins SWD. Polynucl. Aromat. hydrocarbons: Chem., Charact. Carcinog., Int. Symp., 9th. 1986: 437-449.

66.     Chiu C, Poole G, Shu Y, Thomas R, Turle R. Microwave vs. Soxhlet for the extraction of dioxins and furans from solid samples. Organohalogen Compounds. 1996; 27: 333- 338.

67.     Andreasen M. New technologies to improve susceptor efficiencies in microwave packages. In Proceedings of the of 31st Annual Technical Conference of the Society of Vacuum Coaters, San Francisco, CA: SVC. 1988; 133-138.

68.     Turpin CH, Hoese TC. 1978. Microwave heating package and method US Patent No. 4190757.

69.     Benzanson A, Learson R, Teich W.. Defrosting shrimp with microwaves. Microwave energy applied news. 1973; 6(4): 3- 8.

70.     Cathcart WH, Parker JJ. Defrosting foods with high- frequency heat. Food Res. 1946; 11: 341-344.

71.     Martin EF. Electronic device defrosts frozen fruits. Ice Cream Field 1945; 45(5): 18- 20.

72.     Brastad WA. Method and Material for Preparing Food to Achieve Microwave Browning. US Patent No. 4230924. 1980.

73.     Brastad WA. Packaged food item and a method for achieving browning thereof. US Patent No. 4267420. 1981.

74.     Turpin, C. 1989. Browning and crisping: the functions, design and operation of susceptors. Microwave World 10 (6): 8-13.

75.     Mallikarjunan P, CHung Y, Brackett RE, Hao YY. Modeling Heat and Mass Transfer and Microbial Inactivation Kinetics During Microwave Cooking of Chicken Breast. Winter meeting sponsored by ASAE. 13-16 December. Atlanta, GA: ASAE, 1994.

76.     Meisel N. Tempering of meat by microwaves. Microwave energy applied news. 1972; 5(3): 3-7.

77.     Kiranoudis T, Tsami E, Maroulis ZB. Microwave vacuum Drying kinetics of some fruits. Drying Technology 1997; 15(10): 2421–2440.

78.     Wrolstad RE, Lee DD, Poei MS. Effect of microwave blanching on the color and composition of strawberry concentrate Journal of food science. 1980; 45:1573- 1577.

79.     Ramanadhan B, Meda V. Utilization of microwaves in applied natural product chemistry. Paper Number: RRV03- 0033, ASAE Intersectional meeting ASAE. 2003.

80.     Allen DT, Shonnard DR. Green engineering. Environmentally conscious design of chemical processes, Upper Saddle River, NJ: Prentice Hall. 2002.

81.     Chang VCT. Understanding microwave heating cavities 1st edition, London: Artech House Publishers. 2000.

82.     Edgar RH. Handbook of microwave technology for food applications, Datta AK and Anantheswaran. R.C. eds New York: Marcel Dekker, Inc. 2001; 215.