Improvement of Essential Oil Steam Distillation by Microwave

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Improvement of Essential Oil Steam Distillation by Microwave Pretreatment A. Navarrete, S. Wallraf, R. B. Mato,* and M. J. Cocero Department of Chemical Engineering and Environmental Technology, School of Industrial Engineering (Dr. Mergelina), University of Valladolid, 47011 Valladolid, Spain ABSTRACT: Microwave irradiation is proposed as a pretreatment method to accelerate the conventional steam distillation of rosemary essential oil. Microwave extraction methods are based on the capacity of radiation to break the oil-containing glands, allowing faster extraction rates. An analytical method is developed to quantify the fraction of essential oil inside and outside trichomes. This method is used to evaluate the effect of microwaves on the fraction of oil released outside of trichome glands. Steam distillation rates are measured after using different microwave pretreatment energies, and the influence of liquid moisture released from plant cells is also analyzed.

1. INTRODUCTION Essential oils are the ethereal fraction obtained by physical means from a plant. Essential oils have been extracted from over 3000 plants, 200
300 of which are commonly traded on world markets.1 Main uses are in the perfume and flavor industries, although they are also used in the food and pharmaceutical industries as preservatives (mainly antioxidants) and flavors. Also, compounds in essential oils with a phenolic structure have been identified to be active against micro-organisms.2 The industrial extraction of essential oils is usually carried out via steam distillation. Other methods of extraction include hydrodistillation, solvent extraction, or supercritical solvent extraction. Recently, several extraction techniques have also been published using microwave energy.3 The latter are mainly intended for analytical purposes; however, the reported reduction in energy requirements and operating time4
6 makes them suitable for industrial processing. Some preliminary studies6,7 indicate that the cost of microwave extraction could be competitive with traditional steam distillation. However, two characteristics diminish this advantage of reduced energy and shorter operating time: (1) high investment cost of the continuous microwave oven, which must operate separating vapor and exhausted plant, and (2) high operating cost of electricity, in comparison with the much less-expensive cost of fuels used to operate steam distillation. Most of the operating cost in both steam distillation and microwave extraction corresponds to the energy required to generate the mass of steam necessary to extract all the essential oil present in the aromatic plant. Essential oil is mostly present in the plant in special glands named trichomes. During steam distillation, as steam flows over the oil-containing glands, the trichomes slowly release their content, because of high temperature, and, finally, become deflated when they release all their essential oil content.8,9 However, when microwave energy instead of steam is used, the interaction of the electromagnetic field with the polysaccharide molecules and moisture present in trichome walls causes wall breakage. So, essential oil is quickly released outward from trichomes and is ready to be evaporated by the surrounding r 2011 American Chemical Society

steam.10,11 This mechanism reveals the advantage of using microwaves for essential oil extraction since, from the initial period of the process, oil may be evaporated by steam in an oil: steam ratio close to vapor
liquid equilibrium. In contrast, in pure steam distillation, oil is only released slowly from the inside of the trichomes. Based on these facts, a new coupled process may be suggested, which takes advantage of both methods of extraction. First, a short microwave pretreatment is applied to the plant in order to break all trichome walls and release the entire oil content on the surface of the plant. Second, inexpensive conventional steam distillation is performed in the pretreated plant. This process depends on the performance of the first stage. If trichome wall breakage is fast, and no steam is generated from plant moisture at this stage, then energy consumption will be low, and, moreover, no vapor recovery device will be required in the continuous microwave oven. Next, conventional steam distillation will extract oil at a faster rate, reducing energy demand and operating time, and, consequently, equipment size. To the knowledge of the authors, no method has been proposed to date to measure and evaluate the extent of trichomes breakage. The aim of this paper is to determine how trichome wall breakage evolves under microwave irradiation, and how their main operating variables affect the process. Microscopic methods (optical and electronic) have been previously used to verify the status of trichomes before and after oil extraction, both in steam distillation and in microwave extraction.6 However, microscopy was discarded as a convenient measuring technique, because of the difficulty to quantify the ratio of exploited to intact glands, from a representative sample. Instead of the number of trichomes, the concepts of “oil trapped inside trichomes” and “oil released out of trichomes” were chosen to quantify the effect of microwave pretreatment. Received: November 2, 2010 Accepted: March 1, 2011 Revised: January 16, 2011 Published: March 15, 2011 4667

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Industrial & Engineering Chemistry Research The released oil fraction is defined as mr Y ¼ mr þ mt

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ð1Þ

where mr is the mass of oil released out of trichomes, accessible to be evaporated by steam, and mt is the mass of oil that remains trapped inside of trichomes. The essential oil content in aromatic plants is usually determined by the standard analytical method, using a Clevenger distiller.12 However, this method extracts oil by distillation and does not allow one to distinguish between released and trapped oil fractions. Since both oil fractions have the same chemical composition, an alternative method has been developed to quantify their values. This method uses solvent extraction and is based on the differences in extraction kinetics for both fractions. Once trichome breakage kinetics are known, the influence of the released oil fraction of the plant material on steam distillation evolution is determined, performing conventional steam distillation with a plant pretreated with microwave radiation.

Figure 1. Experimental setup. (Legend: [1] microwave oven, [2] glass flask, [3] rotary fitting, and [4] condenser.)

2. EXPERIMENTAL PROCEDURES 2.1. Plant Material. For all experiments, rosemary (Rosmarinus officinalis L.) was selected, because it is a local representative of aromatic plants grown in this region. Rosemary was collected in September 2009 in Pe~ nafiel (Valladolid, Spain). Plants were stored at 4 °C until needed for the extractions. The maximum storage time before use was one month. At the moment of being used, entire leaves were carefully separated from the stems and used in the experiments without crushing. 2.2. Analytical Techniques. 2.2.1. Moisture Measurement. Moisture content was determined using the gravimetric method as the ratio of water to total mass. A sample of ∼0.10 kg was weighed before and after 24 h of oven drying at 105 °C. Moisture content was in the range of 49.2%
62.5%. 2.2.2. GC-MS Analysis. Oil composition was obtained using gas chromatography coupled to mass spectrometry (GC-MS). GCMS analyses were carried out with an Agilent 6890/5973 (Agilent Technologies, Palo Alto, CA, USA). The column used was manufactured by Hewlett
Packard, Model HP-5MS; carrier gas, He; flow, 0.7 mL/min; mode, split; split gas, He; split ratio, 200:1; injection temperature, 250 °C; and injection volume, 1 μL. Column temperature started at 70 °C, hold for 1 min, and then increased at 3 °C/min until 160 °C. To measure the evolution of oil components over time, an internal standard (linalyl acetate 0.5 wt %) was added to the solvent (n-hexane in steam distillation and ethanol in solvent extraction) to improve analytical accuracy. The relative areas between a given component and the internal standard were correlated versus their relative masses in the sample. The exact amount of the extracted compound of interest was determined from the peaks ratio and the known composition of internal standard in the solvent. The measured components were chosen to represent rosemary essential oil, in terms of composition and volatility: R-pinene (CAS No. 80-56-8), eucalyptol (CAS No. 470-82-6), and camphor (CAS No. 76-22-2).8 2.3. Trichome Breakage. 2.3.1. Microwave Pretreatment. The plant material was exposed to microwave radiation before steam distillation experiments, in order to promote breakage of the trichome wall. The experimental setup is shown in Figure 1. Exposition of rosemary leaves (100 g approx.) to microwave

Figure 2. Experimental conditions used in microwave pretreatment.

energy was carried out in a 500-mL glass flask introduced in a modified microwave oven (Panasonic Model NN-GD 566 M). During microwave heating, the sample rotates inside of the cavity. This setup allowed the samples to move through the uneven electromagnetic field pattern formed inside the oven, allowing for a somewhat more uniform energy absorption in the material plant. This setup configuration significantly improved the reproducibility of experimental results. In order to prevent the loss of essential oil, the oven was adapted to condense the possible evaporation of volatile material leaving the plant container. However, condensation out of the flask was not observed in any of the experiments, because of the low energy values used in the pretreatment. The experimental conditions used in microwave pretreatment are shown in Figure 2. Different combinations of oven power and operation time were selected to irradiate energies up to 1 kJ/g, on a dry-plant basis. After microwave pretreatment, the plant container was immediately taken out of the oven and subjected to one of the two following procedures: (1) steam distillation, to determine extraction kinetics, or (2) solvent extraction, to evaluate the trapped and released oil fractions, as explained in the following section. 2.3.2. Solvent Extraction. The aim of solvent extraction experiments was to determine the fractions of “oil trapped inside trichomes” and “oil released out of trichomes”, after microwave pretreatment. The quantification method exploits differences in the solvent extraction kinetics for both fractions. 4668

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Figure 3. Time evolution of solvent extraction of R-pinene at temperatures of 2.6, 13.9, and 24.8 °C.

Solvent selection must be carefully done, since some solvents (e.g., n-hexane) cause trichome wall damage, releasing part of the oil from the glands.13 Ethanol, however, has proven to be a more friendly solvent to trichome walls, allowing different extraction kinetics to released and trapped oil.13,14 For this reason, ethanol was chosen to perform this part of the work. The use of an impeller to improve oil extraction was discarded, to avoid damage to the trichomes. Instead, the use of a rotavapor (Buchi RE 111) was chosen, to provide gentle stirring to the plant material, along with the solvent, without altering the integrity of the trichomes. Finally, an experimental screening of extraction time and temperature conditions was performed to determine the optimal conditions needed to clearly separate the fractions of released and trapped oil. A flask with 50 g of fresh rosemary leaves was connected to the rotavapor and deposited in a bath of glycol at a preset temperature, allowing rotation at 5.5 rpm. Simultaneously, a flask with a solution of linalyl acetate (0.5% w/w) in ethanol was also thermostatted in the bath. Once thermal equilibrium was reached, solvent extraction was started by adding 200 mL of ethanol solution to the leaves. To measure solvent extraction evolution, several samples of 1.5 mL of solvent were taken at different extraction times and then filtered (0.2 μL nylon filter, Millex-GN) into amber sampling flasks. Figure 3 shows time evolution of the extracted mass of Rpinene, at different temperatures. In all cases, there was a fast extraction stage in the initial 30 min, where it is assumed that the entire fraction of released oil was extracted by the solvent. At low temperature (2.6 °C), no oil trapped inside trichomes was extracted by the solvent within the next 5 h. At higher temperature, the trapped oil extraction rate from inside trichomes was increased, overlapping with the released oil extraction stage at 24.8 °C. From these results, the measurement procedure was established as follows: first, the released oil fraction was extracted at 3 °C for 60 min, and, after taking a sample of solvent, the bath temperature was increased to 40 °C for 180 min. A new sample was taken to determine the remaining oil fraction that was initially trapped inside trichomes. 2.3.3. Steam Distillation. Steam distillation experiments were performed with fresh and microwave-pretreated plant material, in order to evaluate the influence of the released oil fraction on distillation kinetics.3,8

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Figure 4. Released oil fraction as a function of microwave pretreatment energy, calculated from solvent extraction with ethanol at 3 °C for 60 min.

Fresh plant (l00 g approx.) was placed in a flask for microwave pretreatment. Next, the flask was connected to the steam distillation unit. The steam was produced in a separate boiler by electrical heating (380 w) and passed through the fresh plant. The vapor leaving the plant flask was condensed, and a solution of linalyl acetate (0.5% w/w) in hexane was used to collect the distilled oil. This method reduces oil loss over condenser walls. During the distillation process, seven fractions, corresponding to consecutive distillation periods, were taken to examine the evolution of oil extraction. The samples were deposited in amber flasks and kept at 4 °C until GC/MS analysis.

3. RESULTS AND DISCUSSION 3.1.1. Solvent Extraction. Solvent extraction experiments to determine the fraction of oil released out of trichomes were performed with fresh and microwave-pretreated plant material, according to procedures described in section 2.3.2, using ethanol at 3 °C for 60 min. Figure 4 shows time evolution of the extracted mass of main oil components in solvent extraction, after using different microwave energies in plant material pretreatment. Figure 4 illustrates how an increasing amount of irradiated energy significantly increased the fraction of released oil caused by the breakage of trichomes. The three components chosen to characterize the oil show a very similar behavior, represented by the average line. Differences in release of three components at intermediate power input may be explained by the steep slope of the curve at these energy values. Therefore, the oil composition was not modified during the extraction process, indicating that there was no effect of the fractionation of oil at the microwave pretreatment stage. The use of irradiation energies below 0.30 kJ/gdry-plant had little effect on the integrity of trichomes. However, in the range of 0.30
0.60 kJ/gdry-plant, almost all trichomes on the plant became broken, and their trapped oil was released to be easily accessible. 3.1.2. Steam Distillation. Steam distillation experiments were carried out to quantify the influence of the microwave pretreatment on distillation kinetics, since the rate of extraction of essential oil during steam distillation was expected to be related to the released oil fraction. Experimental results, for the case of Rpinene, are shown in Figure 5. The condensed mass of R-pinene is represented as a function of the condensed water mass, in order to compare the obtained saturation level of the vapor phase using 4669

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Figure 5. Time evolution of the extracted mass of R-pinene obtained by steam distillation, after microwave pretreatment with 0.00, 0.10, 0.45, 0.59, and 0.90 kJ/gdry-plant.

Figure 6. Yield of R-pinene, eucalyptol, and camphor (g/gdry-plant) obtained by steam distillation, as a function of pre-irradiated microwave energy.

different preirradiated energy levels. A similar behavior was observed with eucalyptol and camphor and, therefore, is assumed to be representative of the total oil content. As expected, experiments with 0.00 and 0.10 kJ/gdry-plant (both below the 0.30 kJ/gdry-plant threshold) showed similar behavior. When the irradiated energy is increased up to 0.45 kJ/gdry-plant, the rate of extraction increased greatly and a much larger quantity of essential oil was obtained with less steam. However, the use of further increases in the preirradiated energy (0.59 kJ/gdry-plant) progressively reduced the rate of extraction, returning to nonirradiated behavior when the maximum energy value (0.90 kJ/ gdry-plant) was used. This behavior is clearly seen in Figure 6, where the steam-distillation yield of the three analyzed components is presented as a function of the preirradiated microwave energy. The effect of the pretreatment is not clear below 0.30 kJ/ gdry-plant. A maximum value is reached at 0.45 kJ/gdry-plant, and the effect decreases again when larger energies are used. In the microwave pretreatment process, there was no condensation of water or oil in the condenser located out of the oven to prevent the possible evaporation of volatile material leaving the plant container. Therefore, this reason was discarded as a possible explanation for the decrease in yield at higher microwave power input (shown in Figure 6).

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Figure 7. Time evolution of the extracted mass of eucalyptol, obtained by steam distillation after microwave pretreatment with 0.54 kJ/gdry-plant. Comparison between experiments with and without intermediate plant drying.

This behavior was attributed to the liquid moisture released from plant cells when they are irradiated, as it happens with essential oil. In fact, in the Solvent Free Microwave Extraction (SFME) process,4 this plant moisture is used to distill the essential oil from the plant without a need for additional steam. The effect of moisture on plant surface in the steam distillation process has been previously described.15,16 Steam only condenses into water surfaces in immediate contact with oil, at the oil/water interface, where latent heat is released to vaporize the oil. Oil and water on the plant surface intermingle at the interface by capillary action, under ideal conditions. This situation allows a large contact perimeter and, therefore, fast extraction rates. However, when the liquid moisture released by radiation is too high, the excess of water may flood the herb surface and overwhelm its absorptive capacity. This fact leads to the formation of circular drops of oil on water, with a thin circular line of contact. The area into which steam could usefully condense would be insignificant and very little oil would vaporize. Based on the previous argument, one experiment (0.54 kJ/ gdry-plant) was repeated but, between the microwave pretreatment and the steam distillation operations, the leaves were dried in the ambient environment for one day, to reduce the water content on the plant surface. The rate extraction for this experiment, corresponding to eucalyptol, is shown in Figure 7, in comparison with the same experiment carried out without the intermediate drying. When the water content over the plant surface is reduced before steam distillation, the rate of extraction is clearly increased and the operation time is reduced. This behavior lays the groundwork for a possible application of microwaves as pretreatment previous to steam distillation extraction of essential oils.

4. CONCLUSIONS Traditional steam distillation is a slow and high-energydemanding process. These two negative characteristics are a consequence of the slow thermal release of essential oil from inside trichome glands. This article studies the use of microwave energy to promote the breakage of trichome walls and release the essential oil to make it available to steam distillation. A novel technique to measure the fraction of essential oil inside and 4670

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Industrial & Engineering Chemistry Research outside trichomes is described, based on differences on extraction rate kinetics with temperature, using ethanol as the solvent. When fresh rosemary leaves are irradiated with microwave energy, trichome breakage starts to be effective at 0.30 kJ/gdry-plant, and it is completed at an energy input of 0.60 kJ/gdry-plant. Steam distillation experiments confirm how low-energy irradiation pretreatment (up to 40 kJ/gdry-plant) improves the steam distillation kinetics. However, when a higher energy is used to promote complete breakage of trichomes, steam distillation efficiency is reduced. As an effect of microwaves, the liquid moisture released by plant cells covers the plant material, reducing its absorptive capacity. The oil and water interface diminishes and the distillation kinetics are decreased. When pretreated material plant was dried in an ambient environment after irradiation, the absorptive capacity of the plant surface was recovered, and the steam distillation rates were clearly improved again.

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(10) Raman, G.; Gaikar, V. G. Microwave-Assisted Extraction of Piperine from Piper nigrum. Ind. Eng. Chem. Res. 2002, 41 (10), 2521–2528. (11) Lucchesi, M. E.; Smadja, J.; Bradshaw, S.; Louw, W.; Chemat, F. Solvent free microwave extraction of Elletaria cardamomum L.: A multivariate study of a new technique for the extraction of essential oil. J. Food Eng. 2007, 79 (3), 1079–1086. (12) Clevenger, J. F. Content of essential oil in plants. Am. Perfum. Essent. Oil Rev. 1928, 23, 467–503. (13) Chen, S. S.; Spiro, M. Kinetics of microwave extraction of rosemary leaves in hexane, ethanol and a hexane þ ethanol mixture. Flavour Fragrance J. 1995, 10 (2), 101–112. (14) Spiro, M.; Chen, S. S. Kinetics of solvent extraction of essential oil from rosemary leaves. Flavour Fragrance J. 1994, 9 (4), 187–200. (15) Denny, E. F. K. Distillation of the lavender type oils: Theory and practice. In Lavender: The Genus Lavandula; Lis-Balchin, M., Ed.; Taylor and Francis: London, New York, 2002. (16) Guenther, E. The Essential Oils—Vol. 1: History, Origin in Plants, Production, Analysis; D. Van Nostrand: New York, 1948.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ34 983 423 177. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors wish to thank the financial support of the Agricultural Technology Institute of the Junta de Castilla y Leon (ITACyL, project reference VA-12-C2-1), and the Junta de Castilla y Leon (Project GR11-2008). ’ REFERENCES (1) Bakkali, F.; Averbeck, S.; Averbeck, D.; Idaomar, M. Biological effects of essential oils—A review. Food Chem. Toxicol. 2008, 46 (2), 446–475. (2) Dorman, H. J.; Deans, S. G. Antimicrobial agents from plants: antibacterial activity of plant volatile oils. J. Appl. Microbiol. 2000, 88 (2), 308–316. (3) Farhat, A.; Ginies, C.; Romdhane, M.; Chemat, F. Eco-friendly and cleaner process for isolation of essential oil using microwave energy. Experimental and theoretical study. J. Chromatogr. A 2009, 1216 (26), 5077–5085. (4) Lucchesi, M. E.; Chemat, F.; Smadja, J. Solvent-free microwave extraction of essential oil from aromatic herbs: Comparison with conventional hydro-distillation. J. Chromatogr. A 2004, 1043 (2), 323–327. (5) Ferhat, M. A.; Meklati, B. Y.; Smadja, J.; Chemat, F. An improved microwave Clevenger apparatus for distillation of essential oils from orange peel. J. Chromatogr. A 2006, 1112 (1
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