Journal of Microwave Power and Electromagnetic Energy, 47 (1), 2013, pp. 63-72.
A Publication of the International Microwave Power Institute
Microwave-assisted Extraction of
Essential Oils from Herbs
Gabriel Abraham Cardoso-Ugarte, Gladys Paola Juárez-Becerra, María Elena SosaMorales, Aurelio López-Malo
Departamento de Ingeniería Química, Alimentos y Ambiental, Universidad de las Américas
Puebla. Sta. Catarina Mártir, Cholula, Puebla 72820, México
Received: December 12, 2012
Accepted: March 7, 2013
ABSTRACT
Microwave-assisted extraction (MAE) has been recognized as a technique with several
advantages over other extraction methods, such as reduction of costs, extraction time,
energy consumption, and CO2 emissions. In this study, MAE was performed to obtain essential
oils from two different herbs (basil and epazote). A factorial design was conducted in order
to determine the effect of solvent quantity, power, and heating time on essential oil yields.
Chemical composition, physical properties and yield percentage of essential oils from MAE
were compared with essential oils obtained by steam distillation (SD). Amount of solvent and
heating time significantly affected the yields (p<0.05). Chemical composition and physical
properties of the essential oils from basil and epazote were not affected by the extraction
method (MAE or SD), with similar yielding obtained by both methods (p<0.05).
KEYWORDS: Microwave-assisted extraction, steam distillation, factorial design, essential
oils, basil, epazote.
INTRODUCTION
Microwaves are a form of non-ionizing electromagnetic energy at frequencies ranging
from 300 MHz to 300 GHz. This energy is transmitted as waves, which can penetrate in
biomaterials and interact with polar molecules into materials, such as water to generate heat
[Takeuchi et al., 2009]. Fast heating is the main advantage of microwaves; the application
in foods is performed at frequencies of 915 MHz at industrial scale and 2450 MHz in domestic
ovens [Routray and Orsat, 2012].
Due to economics and environmental issues, food and chemical industries are facing
the challenge of using new technologies in order to reduce energy consumption and CO2
emissions [Bousbia et al., 2009]. Separation technologies, such as extraction, distillation
and crystallization are promising areas of innovation which can promote the growth of
sustainable processes in the chemical and food industries [Perino-Issartier et al., 2010].
Application of microwaves in separation and extraction processes has shown to
reduce both extraction time and volume of solvent required, minimizing environmental
impact by emitting less CO2 in atmosphere [Lucchesi et al., 2004; Ferhat et al., 2006] and
consuming only a fraction of the energy used in conventional extraction methods such as
steam distillation, SD [Farhat et al., 2009]. Advances in microwave-assisted extraction
(MAE) have led in the development of various techniques such as compressed air microwave
International Microwave Power Institute
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Gabriel Abraham Cardoso et al., Microwave-assisted Extraction of Essential Oils from Herbs
distillation (CAMD), vacuum microwave
hydro distillation (VMHD), microwave
hydro distillation (MWHD), solvent-free
microwave extraction (SFME), microwave
accelerated steam distillation (MASD),
microwave by hydro diffusion and gravity
(MHG) [Perino-Issartier et al., 2010; Farhat
et al., 2009].
MAE is a current technology to extract
biological materials and has been regarded
as an important alternative in extraction
techniques because of its advantages which
mainly are: reduction of extraction time
and solvents, selectivity, volumetric heating
and controllable heating process. Various
researches has shown the efficiency of MAE
in the extraction of different compounds
such as essential oils, fragrances, pigments,
antioxidants and other organic compounds as
animal tissues, food and plants. In addition
of the reduction of time, solvent usage and
energy consumption, this process shows
even more benefits like a more effective
heating, faster energy transfer, size reduced
equipment, rapid onset of warming and
increased yields [Farhat et al., 2009].
The use of MAE in the isolation
of herbal essential oils is an interesting
alternative that provides more effectiveness
than other processes [Bousbia et al.,
2009] as the conventional extraction of
essential oils of herbs and spices by steam
distillation. Several authors have used
microwave extraction techniques to obtain
essential oils from herbs. Early reports
of MAE application to extract essential
oils were recorded in the 80’s [Letellier
and Budzinski, 1999]. Craverio et al.
[1989] compared this process with steam
distillation, the oils obtained from Lippia
sidoides leaves did not show qualitative
difference, but quantitative differences
were observed. More recently, MohammadTaghi et al. [2008] also compared these two
methods for obtaining essential oil from
Thymus vulgaris. Dragovic-Uzelac et al.
[2012] employed MAE to get polyphenols
from wild sage, and Kahriman et al. [2012]
64
obtained essential oil from Vicia herb using
hydro and microwave distillations.
Epazote (Chenopodium ambrosioides),
member of the family Chenopodiacea, is an
aromatic plant whose leaves have been used
to cure influenza, pneumonia, typhoid and as
vermifuge, they have shown anthelmintic and
analgesic properties so as to treat dysentery,
stomachache and as a flavoring agent; its
essential oil has been used as antifungal,
nematicide and insecticide [Kumar et al.,
2007]. Meanwhile, basil (Ocimum basilicum
L.) belonging to Lamiaceae family is an
aromatic herb that has been widely used in
food as a flavoring agent and in perfumery
and pharmaceutical industries [Ijaz et al.,
2008]. Evenly, in traditional medicine its
leaves and flowers have been used as remedy
for digestive [Politeo et al., 2007; Ebrahim,
2006] and respiratory diseases [Simon et al.,
1999]. On the other hand, its essential oil
has insecticidal, nematicidal, fungistatic
and antimicrobial properties [Politeo et al.,
2007]. Additionally, its phenolic compounds
and flavonoids have shown to be powerful
antioxidants [Ijaz et al., 2008], free radical
scavengers and metal chelators [Jayasinghe
et al., 2003].
Therefore, the aim of this study was:
1) to determine the influence of three main
factors (amount of water, power and heating
time) in the MAE process of essential oils from
two herbs (basil and epazote) by applying a
two-level factorial design in order to define
its optimal extraction conditions, and
2) to characterize and analyze the physical
and chemical properties of the essential
oils obtained by MAE to compare them with
the essential oils obtained by SD in order to
establish MAE as an alternative method to
extract essential oil from herbs.
MATERIALS AND METHODS
Raw material
Two different herbs were purchased
in a local market of Puebla in Mexico:
basil (Ocimum basilicum L.), and epazote
Journal of Microwave Power and Electromagnetic Energy, 47 (1), 2013
International Microwave Power Institute
Gabriel Abraham Cardoso et al., Microwave-assisted Extraction of Essential Oils from Herbs
(Chenopodium ambrosioides L.). The
leaves were separated and dried at room
temperature from which the essential oils
were extracted. The obtained essential oils
were kept into closed amber vials for their
further analysis.
Microwave-assisted extraction
The extraction of the essential oils was
performed using a domestic microwave oven
(600 W, Daewoo, China). From preliminary
tests, it was found that low power levels
(from 20 to 60% of the maximum power)
resulted in long extraction times, while high
powers slightly burned the leaves. The oven
was adapted (Figure 1) as follows: 100 g
of sample (dried leaves) were placed in a
1000 mL flat-bottomed flask and the solvent
amount (distilled water) to analyze was
added (400 or 500 mL); the flask containing
the sample was introduced to the microwave
oven and adjusted to a condenser connected
to a cold water recirculation system; the
microwave oven was turned on and the
desired conditions of time (20 or 30 min) and
power (70 or 80%) were set to allow heating
of the herb-water blend and the consequent
generation of vapors. No stirring or rotation
were possible within the flask, however,
the amount of water allowed acceptable
convection and enough homogeneity was
obtained. Vapors began to rise into the
flask’s neck until reach the condenser where
they were cooled, the extracted liquid was
received into a trap; the essential oils were
recovered and its volume was determined
using a micropipette, the remaining water
in oils was removed by adding anhydrous
sodium sulfate. Yields were calculated as
percentage (volume of extracted oil per
weight of dried herb). The oil was placed
into screw cap amber test tubes and stored
under refrigeration until its analysis.
Steam distillation extraction
For comparison, steam distillation
extractions of the essential oils were carried
out in a steam distillation apparatus which
consists of the following accessories: electric
hot plate, Pyrex glass flask, a 500 mL beaker
for the reception of the distillate, distillation
head, condenser adapted to the coolant,
clamps, hoses and insulating material. For
the extraction process, 1500 mL of distilled
water were added to the container with some
boiling beads and placing the flask on the hot
plate; subsequently, 150 g of fractionated
dried herb were placed in the upper flask,
around the flask an insulating material was
placed to prevent heat loss. The cooling
system was turned on to cool the water
inside the condenser and the distillation
Figure 1. Schematic diagram of the microwave oven adaptation to perform MAE of essential oils from herbs.
Journal of Microwave Power and Electromagnetic Energy, 47 (1), 2013
International Microwave Power Institute
65
Gabriel Abraham Cardoso et al., Microwave-assisted Extraction of Essential Oils from Herbs
process started by setting the hot plate to
boil water. Generated vapors began to rise
toward the head of the distillation apparatus
until the condenser where it cools.
The extracted liquid was received into
a trap. Once the process was over, apparatus
was disconnected and allowed to cool; volume
of oil obtained was determined and stored as
explained in the previous section.
Factorial design
A two-level factorial design with a
confidence level of 95% was analyzed using
Minitab 16 software (Minitab Inc., State
College, PA, U.S.A.) in order to determine
the effects of the three variables tested
(amount of water, microwave power and
extraction time). Table I presents the
factors and levels tested.
Table I. Parameter values of the three factors tested.
Factor
Low level (-)
High level (-)
Microwave power (W)
70
80
Time (min)
20
30
Amount of water (mL)
400
500
Optimization of the yields of basil and
epazote essential oils by MAE
In order to determine the influence
of the factors and their levels, the yields
obtained were analyzed and the p value
of the factors and their interactions were
calculated. Through the two-level factorial
design statistical analysis, contour plots
were constructed in order to optimize the
extraction conditions, that maximize the
yield of the essential oils.
Essential oils chemical composition
Chemical composition of essential
oils was determined by gas chromatography
coupled to mass spectrometry detector (GCMS) in a chromatograph (Agilent Technologies
model 6850N) equipped with a triple axis
mass detector (5975C). A capillary column
(HP5-MS) was used for the separation of
the components, using chromatographic
grade helium as carrier gas at constant flow
66
(1.5 mL/min) and the injector at 240 °C;
temperature in the column after an initial
period is hold for 10 min at 60 °C, later is
increased every 5 minutes to reach 240 °C,
this temperature was maintained for 50 min
[Marangon et al., 2008].
Essential oils physical properties
Refractive index was determined
using a digital refractometer (Atago, Japan).
The refractometer was calibrated with
distilled water and then dried with paper to
place a drop of essential oil; determination
was carried duplicate. Due to the unique
compound composition of each essential
oil, a significant change in the refractive
index regarding the extraction method let
conclude that the extraction method affects
the composition of the essential oil. Density
was determined using the mass volume ratio
by weighing 0.1 mL of essential oil.
RESULTS
The complete experiments and the
yields of basil and epazote essential oils
obtained in each condition of the factorial
design for MAE are shown in Table II. The
whole experiments were done for basil, and
for epazote, selected conditions were carried
out (those in which the highest, the lowest
and medium yields were observed for basil).
Higher yields (0.47 and 0.39% for basil
and epazote, respectively) were obtained
with the combination of 70% of power, 30 min
of microwave heating and 400 mL of water.
Basil essential oil yield was found not to
be significantly different (p<0.05) with
respect to the oil obtained by SD extraction
(0.50±0.06%); however, epazote essential oil
yield was found to be significantly different
(p<0.05) in comprison with the oil obtained
by SD extraction (0.20±0.008%). On the other
hand, the lowest yields (0.15 and 0.21% for
basil and epazote, respectively) were obtained
with the combination of 70% of power, 20 min
and 500 mL of water.
Figure 2 shows a visual comparison
of the higher yields of essential oil of basil
Journal of Microwave Power and Electromagnetic Energy, 47 (1), 2013
International Microwave Power Institute
Gabriel Abraham Cardoso et al., Microwave-assisted Extraction of Essential Oils from Herbs
Table II. Experiments order of the factorial design and yields of basil and epazote essential oil obtained.
Microwave power
(W)
Time
(min)
Amount of water
(mL)
70
30
80
Yield (%)
Basil
Epazote
500
0.36
0.33
20
400
0.32
----
70
20
500
0.15
0.21
70
20
400
0.25
----
80
30
500
0.41
0.36
70
30
400
0.47
0.39
80
30
400
0.45
----
80
20
500
0.15
----
Table III. p values of the evaluated factors and their
interactions.
P
Power
0.32
Time
0.002a
Water
0.015a
Time*Water
0.249
Time*Power
0.312
Power*Water
0.353
a
Figure 2. Comparison of yields of essential oils from
basil and epazote obtained by MAE and SD.
and epazote obtained through MAE and the
yields obtained by SD. It can be observed
the similarity between the basil essential
oil yields and the significant difference
between the yields obtained for epazote
essential oil obtained from both methods.
The p values obtained from the three
factors evaluated and their interactions
are shown in Table III. The heating time
(p = 0.002) and the amount of water
(p = 0.015) showed a p value minor to 0.05;
therefore, both variables significantly
affected the yield of the essential oils.
Once known the factors that significantly
affected the essential oil yields, their
interactions were evaluated. Figure 3
shows the contour plot with the interaction
of the heating time and amount of water
significant factors
in order to optimize the essential oil yield;
a combination of the high level of time
(30 min) and the low level of water
(400 mL) resulted to be the highest
(> 0.45%) yield.
The main compounds of the essential
oils of basil and epazote extracted by MAE
and SD are shown in Table IV; listed in order
of abundance.
Most abundant compounds detected in
the essential oil of basil were the same from
both studied extraction methods, but the
abundance order was different for linalool,
eucalyptol and cadinol. In both cases, the
major component was methyl cinnamate. On
the other hand, in the essential oil of epazote
two common compounds were identified
from both extraction methods: limonene
oxide cis, and (+)-4-carene, although they
were found in different abundance. The last
one was the most abundant compound in the
Journal of Microwave Power and Electromagnetic Energy, 47 (1), 2013
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Gabriel Abraham Cardoso et al., Microwave-assisted Extraction of Essential Oils from Herbs
Figure 3. Contour plot of the interaction of the two significant affecting parameters on the yield of essential oil.
Table IV. Main components in essential oil of two herbs obtained by MAE and SD.
Compound
Herb
MAE
SD
methyl cinnamate
linalool
eucalyptol
β-cubebene
cadinol
methyl cinnamate
cadinol
linalool
β-cubebene
eucalyptol
a, 4 - trimethyl -, acetate
(+) - 4 - Carene
limonene oxide, cis
azabicycle [2. 2. 2] octane - 3 - one
1 - cyclohexane - 1 - carboxaldehyde, 4
-(1-methylethyl)
(+) -4 - Carene
limonene oxide, cis
androst - 1 - en - 3 -one, 4, 4 -dimethyl
a-terpinene
cyclohexane, 1 - methyl -3 - (1 methylethylene)
Basil
Epazote
essential oil obtained by SD, meanwhile, 4trimethyl-acetate was the main component
in the oil extracted by MAE. In the essential
oil of basil obtained by MAE, 28 compounds
were detected and 21 by SD, while in the
essential oil of epazote 32 compounds were
detected by SD and 26 by MAE; compounds
detected as traces were not considered for
both methods in the two herbs.
Table V shows the physical
properties of essential oils of basil and
epazote extracted by both methods. The
recorded refractive index of the two herbs
did not show a significant difference
(p < 0.05) regarding the method of
extraction. For density, essential oil of
basil did not show a significant difference
(p < 0.05) between the extraction methods.
Table V. Physical properties of essential oils of two herbs obtained by MAE and SD.
68
Refractive index
Density (kg/m3)
Method
Basil
Epazote
Basil
Epazote
MAE
1.4984±0.001a
1.478±0.00a
927±0.01a
906±0.004a
SD
1.5079±0.011a
1.478±0.00a
937±0.01a
882±0.002b
Journal of Microwave Power and Electromagnetic Energy, 47 (1), 2013
International Microwave Power Institute
Gabriel Abraham Cardoso et al., Microwave-assisted Extraction of Essential Oils from Herbs
For epazote essential oil, a higher density
in the essential oil extracted by MAE was
observed.
DISCUSSION
Even there was not control of
the internal temperature, acceptable
repeatability between experiments were
achieved. This was attributed to a small
cavity and relative low power employed in
the study. Martin (2008) suggested than in
small cavities the waves are dispersed in
better way than big cavities, improving the
uniformity during microwave heating. The results of yield percentage
of essential oil obtained from basil and
epazote let say that in addition to provide
the advantages already mentioned the
MAE technique does not affect the final
yield of the oil. In 2007, Lucchesi et al.,
reported that for the extraction of Eletaria
cardamomum L. essential oil, that the
microwave power, the amount of solvent
and time of extraction significantly affected
the yield of the essential oil.
Results of basil essential oil yield
are similar to those reported by Ijaz et al.
[2008] in a previous study of extraction
of essential oils from basil, in which
yields were ranged between 0.5 and 0.8%.
Moreover, the low yields obtained with both
methods of extraction for the essential oil
of epazote may be attributed to the innate
composition of the herb. However, a yield
of 0.3% was reported for Alitonou et al.
[2012] in the extraction of essential oil of
Chenopodium ambrosioides L. harvested
in different months, and by Jardim et al.
[2010] in the extraction of essential oil
of Brazilian Chenopodium ambrosioides
L. In literature, similar yields have been
reported in the extraction of essential oils
from herbs using both methods, Chemat
et al. [2006] extracted essential oil from
lavender (Lavandula angustifolia Mill)
with MAE and SD and obtained very similar
results, 8.86% for the former and 8.75% for
the second; moreover, Beste et al. [2008]
reported slightly higher yields of essential
oil of oregano with MAE in respect to SD. In
addition to the innate composition of the
herb, several authors [Omer, 1999; Boyle
et al., 1991; Baranauskiene et al., 2003]
have attributed the high or low yields of
essential oils to the level of fertilization
of the soil where the herb is planted, by
directly affecting the phenolic compounds
content [Musika, 1993].
High temperatures and different
heating conditions which herbs are subjected
in both methods are a determinant factor
in the chemical composition of essential oil
extracted; Alitonou et al. [2012] describe
some compounds as fragile molecules and
sensitive to the thermal shocks and the
chemical aggressions. Owing to the different
temperatures reached in each process,
the variation of method can result in the
disappearance of certain compounds or in
a different abundance from one method to
other as shown in the chemical composition
of essential oils of epazote. Under these
specific circumstances, although the use
of MAE may offer advantages over SD, the
use of this technique can be discarded if it
is desired to obtain a specific compound of
the essential oil for a particular purpose.
Despite this, in 2008, Gomez conducted
a study in which identified the major
components of essential oil of epazote,
reporting that they mainly are ascaridol,
transpinocarveol,
aritasone,
β-pinene,
myrcene, phellandrene, camphor, limonene
and α-terpinene, the latter two are also
within the major compound detected in this
study.
For extraction of basil essential oil,
the method did not affect its chemical
composition, greater number of compounds
were detected in oil obtained by MAE,
resulting in a further advantage of the
method proposed for this specific herb. The
presence of linalool, methyl-cinnamate and
β-cubebene in basil essential oil was also
reported by Simon et al. [1999], which
described the major components of 6 kinds
Journal of Microwave Power and Electromagnetic Energy, 47 (1), 2013
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Gabriel Abraham Cardoso et al., Microwave-assisted Extraction of Essential Oils from Herbs
of essential oils from different crops and
chemotypes.
The physical characterization of basil
essential oil showed that the extraction
method does not interfere in density and
refractive index, thus obtaining a further
contribution to the method of MAE applied
to basil. Similar results were reported by
Ijaz et al. [2008], in which the extraction
method does not affect the physical
properties of basil essential oil as well as
the obtained values of refractive index
and density are similar in both studies.
Besides, although the refractive index of
epazote essential oil was not altered, the
difference in the density values suggests
a need of more determinations and
extractions to assign some reasons of the
results obtained; however, the density of
epazote essential oil extracted by SD was
compared with the value reported by Leon
[2009], who employed the same extraction
technique and a significant difference
(p < 0.05) between the value reported
and the obtained in this study was not
detected.
CONCLUSION
The factors affecting the yields during
the extraction of essential oil from basil and
epazote are amount of solvent (water) and
heating time; the combination of extraction
conditions to optimize the essential oil
yield resulted to be 30 min and 400 mL
of water and, taking into account that the
reduction of power does not affect the
yield, the process can be performed at 70%
of power. Due to the significant reduction of
time, solvent and that no significant changes
in the yields of essential oil was shown with
respect to SD, the MAE process is a good
alternative in the extraction processes of
essential oil from basil and epazote. The
results obtained in this study encourage
applying the MAE method for the extraction
of the essential oils of some other different
herbs.
70
ACKNOWLEDGEMENTS
Authors thank to CONACyT (Consejo
Nacional de Ciencia y Tecnología, México) and
Universidad de las Américas Puebla for providing
scholarships to G. Cardoso-Ugarte and G.P.
Juárez-Becerra for their Master studies.
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