Microwave-assisted Extraction of Essential Oils

Herbs and spices are invaluable resources, useful in daily life as food additives, flavors, fragrances, pharmaceuticals, colors, or directly in medicine. This use of plants has a long history all over the world, and over the centuries humanity has developed better methods for extraction of essential oils from such materials. 

Microwaves in Organic Synthesis, Second edition. Edited by A. Loupy Copyright 2006 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-31452-0 

This chapter presents a complete picture of current knowledge on microwave extraction of essential oils. It provides the necessary theoretical background and some details about essential oils and their extraction, the technique, and safety precautions. We will also discuss some of the factors which make the combination of extraction and microwaves one of the most promising topics of research in modem chemistry. 

Essential oils are also known as volatile oils in contrast with fatty vegetable, animal, and mineral oils. Thus a drop of essential oil on a piece of doth or paper disappears within a few minutes or few days at most, depending on the temperature this is not so for fatty oils. 

Essential oils are highly concentrated aromatic oily liquids obtained from a variety of spices and aromatic plant materials. Numerous publications have presented 

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One of the first methods using microwave-assisted extraction of essential oil was presented in 1989 by Craveiro [44]. The essential oil of Lippia sidoides was extracted using microwave energy and compressed air only. Inspired by classical steam distillation, the CAMD technique used compressed air instead of vapor to extract the volatile oil. Typically, plant material is placed in a reactor inside the microwave cavity and heated. At the same time, a compressor, located outside the cavity, forces compressed air into the reactor. Volatile oil and vapor are then driven to the recovery flask outside the cavity. In 5 min CAMD provides an essential oil which is qualitatively and quantitatively identical with that produced by the conventional hydrodistillation method. 

Extraction of essential oils is one of the most time- and effort-consuming processes in the analysis of the constituents of plants. Various extraction methods were traditionally employed, depending on the material or the available devices. The most commonly used methods are steam distillation and distillation-solvent extraction. The introduction of innovative extraction methods, such as microwave-assisted extraction (MAE) and supercritical fluid extraction (SEE), has led to significant improvement, not only in the analytical performance, but also in the accuracy and reproducibility of methods. 

Supercritical fluid extraction and microwave-assisted extraction have been recently applied to the extraction of essential oils. Both techniques are based on the application of high pressures and temperatures for the total extraction of analytes. Supercritical fluid extraction has... 

Microwave-assisted extractions (MAE) can be performed in open (focused MAE) or closed (pressurized MAE) flasks. This technique is commonly used for extractions from complex and difficult sample matrices, replacing time- and solvent-intensive Soxhlet extractions or hydrodistillations.46 MAE is also widely applied to environmental samples, for example, for extracting polycyclic aromatic hydrocarbons (PAH) from soil, methylmercury from sediments, and trace metals and pesticide residues from plant material47 48 The use of microwave treatment instead of hydrodistillation offers a solvent-free separation technique essential oils are heated and dry-distilled 46... 

An essential oil (EO) is internationally defined as the product obtained by hydro-, steam-, or dry-distillation of a plant or of some of its parts, or by a suitable mechanical process without heating, as in the case of Citrus fruits (AFNOR, 1998 Council of Europe, 2010). Vacuum distUladon solvent extraction combined offline with distillation simultaneous distillation extraction supercritical fluid extraction microwave-assisted extraction and hydro-distiUation and static, dynamic, and high concentration capacity headspace sampling are other techniques used for extracting the volatile fraction from aromatic plants, although the products of these processes cannot be termed EOs (Faleiro and Miguel, 2013). 

The use of essential oils is increasing because of the increase in the number of their apphcations and in the framework of natural and environmentally friendly materials. Many times the analysis of their components is quite complex due to the high number and the diversity of compounds in their composition. In this entry a general overview of the extraction methods is given by comparing conventional hquid-liquid and sohd-hquid methods with new alternative ones, such as supercritical fluid extraction and microwave-assisted extraction. Gas chromatography methods and examples are treated and important issues such as detection systems, modem hbraries for compounds identification, as well as multidimensional or hyphenated techniques are discussed. The use of these modem techniques and methods has improved resolution and sensitivity in essential oils determination and could open the possibihty of future work in this area of chromatography. 

Another possibility for essential oils extraction is the use of microwave-assisted extraction (MAE). This methodology appeared to be quite attractive for the isolation of essential oils. The main advantage of using MAE for essential oils extraction is the effective heat transfer that allows quicker times of extraction as compared to classical methods. In the last few years, different applications using microwave energy have been developed. A recent modification of this technique is solvent-free microwave extraction (SEME), where the sample is placed in the reactor inside the microwave oven without any solvent.A cooling system outside the micro-wave oven cool the extract continuously. Einally, the mixrnre of water and essential oil is collected and separated in a vessel. 

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