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www.wjpps.com Vol 8, Issue 7, 2019.
Gayoor et al. World Journal of Pharmacy and Pharmaceutical Sciences
TOPIC-THE SCENARIO OF PHARMACEUTICALS AND
DEVELOPMENT OF MICROWAVE ASSISTED EXTRACTION
TECHNIQUES
Yezdani Umama1, Md. Khan Gayoor*2, Venkatajah G.3, Rav Shourabh2, Roshan
Kumar4, Arvind Verma2, Ayush Kumar2
1MRM College of Pharmacy, Telangana, Hyderabad.
2Truba Institute of Pharmacy, Bhopal, M.P.
3SSJ College of Pharmacy, Gandhipet, Hyderabad TN.
4SBS College of Pharmacy, Patti Punjab.
ABSTRACT
Microwave-assisted extraction (MAE) or simply microwave extraction
is a relatively new extraction technique that combines microwave and
traditional solvent extraction. Application of microwaves for heating
the solvents and plant tissues in extraction process, which increases the
kinetic of extraction, is called microwave-assisted extraction. The use
of MAE in natural products extraction started in the late 1980, though
the technological developments, it has now become one of the popular
and cost-effective extraction methods available today, and several
advanced MAE instrumentations and methodologies have become
available, e.g., pressurized microwave-assisted extraction (PMAE) and
solvent-free microwave-assisted extraction (SFMAE). This chapter provides an overview of
the MAE and presents a number of specific proto collateral products extraction.
KEYWORDS: Microwaves, Induction, Plant Matrix, etc.
INTRODUCTION
Microwave-assisted extraction (MAE) is a relatively new extraction technique that combines
microwave and traditional solvent extraction. Solvent extraction has been intensively used for
isolation of important compounds and for the qualitative and quantitative analysis in various
fields such as environmental analysis, food agricultural analysis, pharmacological drugs and
WORLD JOURNAL OF PHARMACY AND PHARMACEUTICAL SCIENCES
SJIF Impact Factor 7.421
Volume 8, Issue 7, XXX-XXX Research Article ISSN 2278 – 4357
*Corresponding Author
Md. Khan Gayoor
Truba Institute of Pharmacy,
Bhopal, M.P.
rk981487@gmail.com
officerliason0@gmail.com
Article Received on
10 May 2019,
Revised on 01 June 2019,
Accepted on 22 June 2019
DOI: 10.20959/wjpps20197-14204
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Gayoor et al. World Journal of Pharmacy and Pharmaceutical Sciences
herbal medicine. In MAE, the extraction occurs as a result of changes in the cell structure
caused by electromagnetic waves. It has been proposed that the extraction acceleration
observed in MAE may be due to the heat and mass transfer gradients working in the same
direction. Using microwaves for heating the solvents and plant tissues increases the kinetics
of extraction, and various advantages are thus obtained over traditional solvent extraction,
including shorter extraction times, higher extraction rates, lower costs, and less solvent use.
MAE has been employed in the extraction of various compounds from natural sources, in
leaves, terpene from must, and gossypol from cottonseeds. The potential natural anticancer
drugs like vincristine, vinblastine and Taxol can be the best example. Recent years have
shown a growing popularity and faith in the use of herbal medicine worldwide. This may be
because of the realization that modern synthetic drugs have failed to provide a “cure all”
guarantee to most of the human diseases with often producing undesirable side effects, which
at the end turnout to be more problematic than the actual disease itself. The herbal medicine
provides a ray of hope through its cocktail of phyto-compounds, which are believed to act in
a synergistic manner, providing excellent healing touch with practically no undesirable side
effects, provided its quality is assured off.
Principle: Even though dried plant material is used for extraction in most cases, but still
plant cells contain minute microscopic traces of moisture that serves as the target for
microwave heating. The moisture when heated up inside the plant cell due to microwave
effect, evaporates and generates tremendous pressure on the cell wall due to swelling of the
plant cell. The pressure pushes the cell wall from inside, stretching and ultimately rupturing
it, which facilitates leaching out of the active constituents from the ruptures cells to the
surrounding solvent thus improving the yield of phyto-constituents. This phenomenon can
even be more intensified if the plant matrix is impregnated with solvents with higher heating
efficiency under microwave. Higher temperature attained by microwave radiation can
hydrolyse ether linkages of cellulose, which is the main constituent of plant cell wall, and can
convert into soluble fractions within 1 to 2 min. The higher temperature attained by the cell
wall, during MAE, enhances the dehydration of cellulose and reduces its mechanical strength
and this in turn helps solvent to access easily to compounds inside the cell. In order to study
cell damage during the MAE experiments, tobacco leaf samples were examined by scanning
electron microscopy. Scanning electron micrographs of the untreated sample, heat-reflux
extraction sample and MAE sample revealed that there were no structural difference between
heat-reflux extraction and those of untreated samples, except few slight ruptures on the
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Gayoor et al. World Journal of Pharmacy and Pharmaceutical Sciences
surface of the sample. However, the surface of the sample was found greatly destroyed after
MAE. This observation suggests that microwave treatment affects the structure of the cell due
to the sudden temperature rise and internal pressure increase. During the rupture process, a
rapid exudation of the chemical substance within the cell into the surrounding solvents takes
place.
Oven Design: The magnetron tube generates microwaves at a fixed frequency. It consists of
a vacuum tube with a central electron-emitting cathode of highly negative potential which is
surrounded by a structured anode that forms cavities. They are coupled by the fringing fields
and have the intended microwave resonant frequency. The power output of the magnetron
can be controlled by the tube current or the magnetic field strength. The wave guide transmits
the microwave from the source to the cavity. It can itself be used as the applicator for
microwave heating when the material is introduced by wall slots and the waveguide is
terminated by the matched load.
This configuration is called a travelling wave device since the locations of the field maxima
change with time.The sample introduced in a dedicated extraction vessel (reactor) is then
placed in the cavity and the circulator is used for reflection and homogenization of radiation.
Mechanism: The fundamentals of the microwave extraction (MAE) process are different
from those of conventional methods (solid–liquid or simply extraction) because the extraction
occurs as the result of changes in the cell structure caused by electromagnetic waves. In
MAE, the process acceleration and high extraction yield may be the result of a synergistic
combination of two transport phenomena: heat and mass gradients working in the same
direction. On the other hand, in conventional extractions the mass transfer occurs from inside
to the outside, although the heat transfer occurs from the outside to the inside of the substrate.
In addition, although in conventional extraction the heat is transferred from the heating
medium to the interior of the sample, in MAE the heat is dissipated volumetrically inside the
irradiated medium.
During the extraction process, the rate of recovery of the extract is not a linear function of
time: the concentration of solute inside the solid varies, leading to a non-stationary or
unsteady condition. A series of phenomenological steps must occur during the period of
interaction between the solid-containing particle and the solvent effectuating the separation,
including
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(1) Penetration of the solvent into the solid matrix.
(2) Solubilisation and breakdown of components.
(3) Transport of the solute out of the solid matrix.
(4) Migration of the extracted solute from the external surface of the solid into the bulk
solution.
(5) Movement of the extract with respect to the solid.
(6) Separation and discharge of the extract and solid.
The development of MAE techniques: In general, MAE systems are classified into multi-
mode system and focused-mode system (mono-mode). Multi-mode system allows random
dispersion of microwave radiation in cavity by a mode stirrer while focused system (mono-
mode) allows focused microwave radiation on a restricted zone in cavity. Usually, the multi-
mode system is associated with high pressure while the mono-mode system is employed
under atmospheric operating pressure. However, mono-mode system can also run at high
pressure. To avoid confusion in the classification of MAE, „closed system‟ and „open system‟
are used to refer to the system that operates above atmospheric pressure and under
atmospheric pressure, respectively. For further understanding of the closed system and open
system, schematic diagrams are illustrated in Fig. In a closed MAE system, the extractions
are carried out in a sealed-vessel with different mode of microwave radiations. Extraction is
normally carried out under uniform microwave heating. High working pressure and
temperature of the system allow fast and efficient extraction. The pressure inside the
extraction vessel is controlled in such a way that it would not exceed the working pressure of
the vessel while the temperature can be regulated above the normal boiling point of the
extraction solvent. Recent advancements in the closed system have led to the development of
high pressure microwave-assisted extraction (HPMAE). The increase temperature and
pressure accelerates microwave-assisted extraction due to the ability of extraction solvent to
absorb microwave energy. Despite the fact that the closed system offers fast and efficient
extraction with less solvent consumption, it is susceptible to losses of volatile compounds
with limited sample throughput. Open system is developed to counter the shortcomings of
closed system such as the safety issues and it is considered more suitable for extracting
thermo-labile compounds. This system has higher sample throughput and more solvent can
be added to the sys- tem at any time during the process. Basically, open system operates at
more mild conditions. Its advantages and disadvantages are highlighted by luque-Garcia and
luque de Castro. Open MAE system is widely used in the extraction of active compounds and
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is also used in analytical chemistry. This system operates at atmospheric conditions and only
part of the vessel is directly exposed to the propagation of microwave radiation (mono-
mode). The upper part of the vessel is connected to a reflux unit to condense any vaporized
solvent. Besides that, multi-mode radiation can also be employed in open MAE system with
the reflux unit.
Microwave Theory: Microwaves are non-ionizing electromagnetic waves of frequency
between 300 MHz to 300 GHz and positioned between the X- ray and infrared rays in the
electromagnetic spectrum. In modern day science microwaves serves two major purpose
communication and as energy vectors. The latter application is the direct action of waves on
materials that has the ability to convert a part of the absorbed electromagnetic energy to heat
energy. Microwaves are made up of two oscillating perpendicular field‟s i.e. electric field and
magnetic field and the former is responsible for heating. Unlike conventional heating which
depends on conduction – convection phenomenon with eventually much of the heat energy
being lost to the environment. Whereas in case of MAE, heating occurs in a targeted and
selective manner with practically no heat being lost to the environment as the heating occurs
in a closed system. This unique heating mechanism can significantly reduce the extraction
time (usually less than 30 min) as compared to Soxhlet. The principle of heating using
microwave is based upon its direct impact with polar materials/solvents and is governed by
two phenomenon‟s: ionic conduction and dipole rotation, which in most cases occurs
simultaneously. Ionic conduction refers to the electrophoretic migration of ions under the
influence of the changing electric field. The resistance offered by the solution to the
migration of ions generates friction, which eventually heats up the solution. Dipole rotation
means realignment of the dipoles of the molecule with the rapidly changing electric field.
Classification of MAE techniques
In General, MAE systems are classified into two type
1) Multi-mode system and
2) Focused-mode system (mono-mode).
Multi-mode system allows random dispersion of microwave radiation in cavity by a mode
stirrer while focused system (mono-mode) allows focused microwave radiation on a
restricted zone in cavity. Usually, the multi-mode system is associated with high pressure
while the mono-mode system is employed under atmospheric operating pressure. However,
mono-mode system can also run at high pressure. To avoid confusion in the classification of
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Gayoor et al. World Journal of Pharmacy and Pharmaceutical Sciences
MAE, „closed system‟ and „open system‟ are used to refer to the system that operates above
atmospheric pressure and under atmospheric pressure. For further understanding of the closed
system and open system, schematic diagrams are given in Fig.
Close MAE system: In a closed MAE system, the extractions are carried out in a sealed-
vessel with different mode of microwave radiations. Extraction is normally carried out under
uniform microwave heating. High working pressure and temperature of the system allow fast
and efficient extraction. The pressure inside the extraction vessel is controlled in such a way
that it would not exceed the working pressure of the vessel while the temperature can be
regulated above the normal boiling point of the extraction solvent. Recent advancements in
the closed system have led to the development of high pressure microwave-assisted
extraction (HPMAE). The increase in temperature and pressure accelerates microwave-
assisted extraction due to the ability of extraction solvent to absorb microwave energy.
Despite the fact that the closed system offers fast and efficient extraction with less solvent
consumption, it is susceptible to losses of volatile compounds with limited sample
throughout.
Open MAE System: Open system is developed to counter the shortcomings of closed
system such as the safety issues and it is considered more suitable for extracting thermo-
labile compounds. This system has higher sample throughput and more solvent can be added
to the system at any time during the process. Basically, open system operates at more mild
conditions. Open MAE system is widely used in the extraction of active compounds and is
also used in analytical chemistry. This system operates at atmospheric conditions and only
part of the vessel is directly exposed to the propagation of microwave radiation (mono-
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Gayoor et al. World Journal of Pharmacy and Pharmaceutical Sciences
mode). The upper part of the vessel is connected to a reflux unit to condense any vaporized
solvent. Besides that, multi-mode radiation can also be employed in open MAE system with
the reflux unit.
Factors affecting the capacity of MAE: The efficiency of MAE depend on the selection of
the operating conditions and the factors affecting the extraction mechanism. The factors that
may affect the yield of extraction are solvent nature, extraction time, microwave power,
temperature, sample characteristic, effect of stirring, etc. It is important understand the effects
and interactions of these factors on the MAE processes.
1. Solvent Nature: The selection of suitable solvent in MAE extraction process depends on
the solubility of the target analyse, solvent‟s penetration and interaction with sample matrix
and its dielectric constant. Aqueous solution of certain organic solvent is desired for some
extractions as the presence of water would improve the penetration of solvent into sample
matrix and thus enhance heating efficiency. Other organic solvents such as ethanol, methanol,
and acetone are also found to be effective in extraction. For instance, methanol was used to
extract phenolic compounds from grape skins and seeds and higher yield of polyphenols was
obtained as compared to extraction using ethanol but the latter extract had stronger
antioxidant properties. Solvent toxicity is also evaluated in selecting suitable solvent for
MAE. It is important to note that the selection of a solvent for MAE can not be deduced from
the conventional extraction methods as solvents that work well in conventional techniques
might not be a good solvent for MAE. For example, diethyl ether that has been used
extensively in solubilizing steroids from fragrance family is not suitable as MAE solvent.
However, a modifier can be added to the solvent to enhance its overall performance. Water
was added as modifier to diethyl ether to enhance microwave heating efficiency in the
extraction of steroids from Rodgersia aesculifolia Batal. In addition, ethanol or water can be
added into poor microwave absorber such as hexane to improve the extraction efficiency as in
the case of ginger extraction by MAE. Moreover, acetone can be enhanced by adding portion
of methanol in MAE of curcumin.
2. Microwave Power: High microwave power might cause poor extraction yield due to the
degradation of thermal sensible compounds. In general, the extraction yield increases
proportionally with increasing microwave power up to a limit before the increase becomes
insignificant or decline. Microwave power provides localized heating in the sample and it
acts as a driving force for MAE to destroy the plant matrix so that analyse can diffuse out and
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dissolve in the solvent. Increasing the power will generally improve the extraction yield and
result in shorter extraction time.
3. Extraction Time and Cycle: Apart from interactive effect on temperature, the influence
of the microwave power can be extended to the extraction time. Over exposure to microwave
radiation even at low temperature or low operating power was found to decrease the
extraction yield due to the loss of chemical structure of the active compounds. In order to
avoid the risk of thermal degradation and oxidation, the extraction time of MAE usually
varies from few minutes up to half an hour with the exception of solvent-free microwave-
assisted extraction (SFME) where longer extraction time of 1 h is necessary for complete
extraction of essential oil.
4. Plant Matrix Characteristic: The characteristics of the sample also effects on the
performance of MAE. The extraction sample is usually dried, powdered and sieved into fine
powder prior to the extraction for optimum extraction yield. Moreover, fine sample treated by
solvent for 90 min prior to extraction can enhance the heating efficiency of MAE, promote
diffusion and improve mass transfer of active compounds to the solvent.
5. Temperature: Microwave power and temperature are very interrelated to each other and
needs to be given special attention particularly when working with closed vessel system. In
closed vessel systems, temperature may reach well above the boiling point of the solvent.
This elevated temperature does indeed result in improved extraction efficiencies since
desorption of analyse from active sites in the matrix will increase. Additionally solvents have
higher capacity to solubilize analyse at higher temperature while surface tension and solvent
viscosity decreases with temperature, which will improve sample wetting and matrix
penetration respectively. Increase in temperature is also associated with increase in pressure
in closed systems, which can raise safety concerns. Temperature was found to be a significant
factor in the extraction of paclitaxel. Temperature can be effectively controlled in open vessel
system by proper combinations of extracting solvents which heat up differently. Thorough
study of different MAE investigations and from the personal experience of the authors, we
present a brief schematic MAE for open vessel extraction systems, performed under
atmospheric pressure.
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General MAE procedures: The extraction of active ingredients from plants involves several
steps starting from the initial sample preparation followed by the extraction of active
ingredients to the clean up procedure and quantification analysis.
1. Sample Preparation: As illustrated in Fig. 4, sample preparation involves that the desired
parts of the plants are oven dried or sun dried to remove the moisture. The drying temperature
is kept between 40 - 60 ◦C to avoid thermal degradation. The dried sample is then pulverized
and sieved to sizes of 40–60 mesh number to promote efficient extraction when expose to
solvent. In certain cases, sample pre-treatment prior to extraction is conducted to enhance the
extraction process. This is done by soaking the samples in an extraction solvent such as water
and methanol. As previously briefed in the case of solvent-free microwave-assisted extraction
(SFME), the moisture of the sample matrix resulted from water pre-treatment is responsible
for the extraction of the active ingredients.
In other cases, some undesired components of the samples were removed by pre-treatment
with an organic solvent i.e. petroleum ether. This can be done by soaking the sample
overnight to remove the lipids portion and chlorophyll.
2. Extraction Process: After sample preparation and pre-treatment, the sample is subjected
to extraction. For modified MAE techniques, additional instruments are required. In the case
of open microwave system, reflux unit is initiated and the condensing tube is connected to the
extraction vessel inside the microwave cavity. As for UMAE, ultrasonic transducer is needed
and is normally set to a power of 50W and frequency of 40 kHz. In the case of VMAE, the
condensing tube is kept under vacuum to 40 KPa. In NPMAE, inert gas can be pressurized
through the condensing tube in which the vessel is vacuumed. After proper setup, the sample
is then irradiated under specific operating conditions. As in HPMAE of ginseng, the pressure
of the vessel is allowed to reach up to 400 Kp.
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Guidelines for selecting MAE techniques: The brief guidelines for selecting suitable
techniques i.e. DMAE, NPMAE, CMAE, SFME, UMAE are listed in this section. The
summary of the development of these extraction techniques are tabulated in Table 4 and the
applications, advantages and drawbacks for each technique are also presented. Standard MAE
is commonly employed either in open or closed systems to extract thermo stable compounds.
For extraction of degradable active compounds, there are various modified MAE techniques
that are suitable for the application. DMAE is suitable to extract degradable compounds that
require multiple extraction cycles as the technique performs under mild conditions and in
continuous manner. This technique promotes a fast and efficient analytical step, as it can be
on-line coupled with HPLC analysis system. The drawbacks of this technique are that it has
low throughput, inconvenient feeding and removal of sample and residue as well as
requirement of additional equipment setup such as pumps and valves. Besides, for highly
fragile compounds which pose high risks of oxidation and thermal degradation, VMAE is
suitable as the extraction is carried out in vacuum condition and at low temperature. The
extraction usually requires longer extraction time due to the mild condition. Alternatively
extraction of thermal degradable compounds can also be achieved through NPMAE. It gives
faster extraction than VMAE but requires additional extraction step. On the other hand,
SFME is more preferable to be used in essential oil extraction and it is more efficient than the
traditional HD method. In some circumstances in which the associated active compounds
have low diffusion and difficult to be extracted, UMAE can be employed as it improves the
mass transfer mechanism and reduces the extraction time. This technique can provide high
activation energy or the impact energy required for the extraction to proceed. However, for
other extraction cases, additional ultrasonic wave might not give higher yields due to the fact
that microwave radiation alone is sufficient to overcome the activation energy of the
extraction.
CONCLUSION
This method is more effective and cheaper than conventional method. In this technique time
required for complete extraction is less as compared to the conventional techniques. Solvent
required for extraction is less than the other techniques. Amount of extracted phenolic
compounds is increase MAE for open and closed vessel systems have been given separately.
Several studies have been reported on the comparison of MAE with other conventional
techniques. In most of the cases Soxhlet has been used as the control experiment. The main
advantage of MAE reside in the performance of the heating source. MAE is now widely
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accepted in analytical laboratories. The method developed using MAE, and a traditional
method of extraction such as magnetic stirring, it was confirmed that MAE is a much faster
method.
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