Essential oil is any concentrated, hydrophobic (immiscible with water), typically lipophilic (oil or fat soluble) liquid of plants that contains highly volatile aroma compounds and carries a distinctive scent, flavor, or essence of the plant. This large and diverse class of oils also are referred to as volatile oils or ethereal oils. They usually are named for the plants from which they are extracted, such as oil of clove or peppermint oil. Essential oils do not as a group need to have any specific chemical properties in common, beyond conveying characteristic fragrances.
Essential oils are found in diverse parts of plants, including leaves, seeds, flowers, roots, and bark. They are extracted by a variety of techniques, including distillation, expression, and solvent extraction. Essential oils can be very complex chemically, with some essential oils with more than 200 identified chemical substances (Dupler and Odle 2005).
For the plant, essential oils are thought to be vital for the life of the plant, containing compounds that help to fight parasites and infections; many essential oils have anti-bacterial, anti-fungal, and anti-parasitic properties (Dupler and Odle 2005).
For people, essential oils are used in perfumes, cosmetics, and bath products, for flavoring food and drink, for scenting incense and household cleaning products, and for medicinal purposes. They have a long history, being used by the ancient civilizations of Egypt, Greece, India, and Rome; more than 5,000 years ago, the ancient civilizations of Mesopotamia utilized machines for obtaining essential oils from plants (Dupler and Odle 2005).
Interest in essential oils has revived in recent decades, with the popularity of aromatherapy, a branch of alternative medicine which claims that the specific aromas carried by essential oils have curative effects. Oils are volatilized or diluted in a carrier oil and used in massage, or burned as incense, for example.
Today, most common essential oils, such as lavender, peppermint, and eucalyptus, are distilled, using water and steam to remove the oils from the fresh or dried plants. Raw plant material, consisting of the flowers, leaves, wood, bark, roots, seeds, or peel, is put into an alembic (distillation apparatus) over water. As the water is heated, the steam passes through the plant material, vaporizing the volatile compounds. The vapors flow through a coil where they condense back to liquid, which is then collected in the receiving vessel.
Most oils are distilled in a single process. One exception is Ylang-ylang (Cananga odorata), which takes 22 hours to complete through a fractional distillation.
The recondensed water is referred to as a hydrosol, hydrolat, herbal distillate, or plant water essence, which may be sold as another fragrant product. Popular hydrosols are rose water, lavender water, lemon balm, clary sage, and orange blossom water. The use of herbal distillates in cosmetics is increasing. Some plant hydrosols have unpleasant smells and are therefore not sold.
Most citrus peel oils are expressed mechanically, or cold-pressed, using machines to squeeze the oil from the plant material. Due to the large quantities of oil in citrus peel and the relatively low cost to grow and harvest the raw materials, citrus-fruit oils are cheaper than most other essential oils. Lemon or sweet orange oils that are obtained as by-products of the citrus industry are even cheaper.
Prior to the discovery of distillation, all essential oils were extracted by pressing.
Most flowers contain too little volatile oil to undergo expression and their chemical components are too delicate and easily denatured by the high heat used in steam distillation. Instead, a solvent such as hexane or supercritical carbon dioxide is used to extract the oils. Extracts from hexane and other hydrophobic solvent are called concretes, which is a mixture of essential oil, waxes, resins, and other lipophilic (miscible with other fats, oils, and so on) plant material.
Although highly fragrant, concretes contain large quantities of non-fragrant waxes and resins. As such another solvent, often ethyl alcohol, which only dissolves the fragrant low-molecular weight compounds, is used to extract the fragrant oil from the concrete. The alcohol is removed by a second distillation, leaving behind the absolute.
Supercritical carbon dioxide is used as a solvent in supercritical fluid extraction. This method has many benefits, including avoiding petrochemical residues in the product. It does not yield an absolute directly. The supercritical carbon dioxide will extract both the waxes and the essential oils that make up the concrete. Subsequent processing with liquid carbon dioxide, achieved in the same extractor by merely lowering the extraction temperature, will separate the waxes from the essential oils. This lower temperature process prevents the decomposition and denaturing of compounds and provides for a superior product. When the extraction is complete, the pressure is reduced to ambient and the carbon dioxide reverts back to a gas, leaving no residue. Although supercritical carbon dioxide is also used for making decaffeinated coffee, the actual process is different.
Essential oils are very concentrated. One drop of essential oil is equivalent in concentration of plant essence to roughly thirty cups of herbal tea (Dupler and Odle 2005). Some essential oils produced from rose plants require 4,000 pounds of rose petals to make a pound of essential oil and thus are very expensive; however, only one 1000 pounds of lavender plant material is required to produce a pound of lavender oil (Dupler and Odle 2005).
Estimates of total production of essential oils are difficult to obtain. One estimate, compiled from data in 1989, 1990, and 1994 from various sources, gives the following total production, in metric ton, of essential oils for which more than 1,000 metric ton were produced (ISO Technical Committees (ISO/TCs 2004).
Oil Metric tons Sweet orange 12,000 Mentha arvensis 4,800 Peppermint 3,200 Cedarwood 2,600 Lemon 2,300 Eucalyptus globulus 2,070 Litsea cubeba 2,000 Clove (leaf) 2,000 Spearmint 1,300
Types of essential oils and plant part
Essential oils are derived from various sections of plants. Some plants, like the bitter orange, are sources of several types of essential oil.
- Bay leaf
- Common sage
- Lemon grass
- Tea tree
- Clary sage
The most well-known essential oil is probably rose oil, produced from the petals of Rosa damascena and Rosa centifolia. Steam-distilled rose oil is known as "rose otto" while the solvent extracted product is known as "rose absolute."
Essential oils are usually lipophilic (literally: "Oil-loving") compounds that usually are not miscible with water. Instead, they can be diluted in solvents like pure ethanol (alcohol), polyethylene glycol, or oils.
Essential oils have been used for thousands of years. In addition to ancient civilizations in Mesopotamia having machines for obtaining essential oils, they have been found in 3,000-year-old tombs in the pyramids of Egypt, and early Greek physicians, such as Hippocrates mentioned the use of plant essences and oil massages for healing and enhancing mood (Dupler and Odle 2005).
Essential oils are used as perfumes, medicines, food flavoring, and as additives for antiseptic and fragrant purposes. They have been used in such healing systems as aromatherapy, massage therapy, and Ayurvedic medicine. They are used for skin conditions (acne, burns, cuts, athlete's foot, sunburn, and so on), muscle and circulation problems (arthritis, high blood pressure, varicose veins), respiratory problems, infections, allergies, asthma, colds, flu, as a digestive aid, and for anxiety, depression, exhaustion, and so forth (Dupler and Olde 2005). Treatment claims are now subject to regulation in most countries, and have grown correspondingly more vague, to stay within these regulations.
Aromatherapy is a form of alternative medicine, in which healing effects are ascribed to the aromatic compounds in essential oils and other plant extracts. Many common essential oils have medicinal properties that have been applied in folk medicine since ancient times and are still widely used today. For example, many essential oils have antiseptic properties (Prabuseenivasan et al. 2006). Many essential oils are also claimed to have an uplifting effect on the mind. The claims are supported in some studies (Komiya et al. 2006; Kuriyama et al. 2005), and unconfirmed in others (Lehrner et al. 2005).
Dangers associated with use
Because of their concentrated nature, essential oils generally should not be applied directly to the skin in their undiluted or "neat" form. Some can cause severe irritation, or provoke an allergic reaction. Instead, essential oils should be blended with a vegetable-based "carrier" oil (also known as, a base, or "fixed" oil) before being applied. Common carrier oils include olive, almond, hazelnut, and grapeseed. Common ratio of essential oil disbursed in a carrier oil is 0.5–3 percent (most under 10 percent), and depends on its intended purpose. Some essential oils, including many of the citrus peel oils, are photosensitizers (that is, increasing the skin's vulnerability to sunlight, making it more likely to burn). Lavender oil, though generally considered the mildest essential oil, is cytotoxic to human skin cells (Prashar et al. 2004).
Estrogenic and antiandrogenic activity have been reported by in vitro study of tea tree oil and lavender essential oils. Case reports suggest that the oils may be implicated in some cases of gynecomastia, an abnormal breast tissue growth, in prepubescent boys (Henley et al. 2007.; BBC 2007).
While some advocate the ingestion of essential oils for therapeutic purposes, this should never be done except under the supervision of a professional who is licensed to prescribe such treatment. Some very common essential oils, such as eucalyptus, are extremely toxic internally. Pharmacopoeia standards for medicinal oils should be heeded. Essential oils should always be kept out of the reach of children. Some oils can be toxic to some domestic animals, cats in particular. Owners must ensure that their pets do not come into contact with potentially harmful essential oils (Bischoff and Buale 1998). The internal use of essential oils should be fully avoided during pregnancy without consulting with a licensed professional, as some can be abortifacients in dose 0.5–10 ml.
The smoke from burning essential oils may contain potential carcinogens, such as polycyclic aromatic hydrocarbons (PAHs). Essential oils are naturally high in volatile organic compounds (VOCs).
- BBC. 2007. Oils "make male breasts develop." BBC News February 1, 2007.
- Bischoff, K., and F. Guale. 1998. Australian tea tree (Melaleuca alternifolia): Oil poisoning in three purebred cats. Journal of Veterinary Diagnostic Investigation 10 (108): 208-210. Retrieved September 27, 2008.
- Dupler, D., and T. G. Odle. 2005. Essential oils. In J. L. Longe, The Gale Encyclopedia of Alternative Medicine, Farmington Hills, Mich: Thomson/Gale, ISBN 0787693960.
- Henley, D. V., N. Lipson, K. S. Korach, and C. A. Bloch 2007. Prepubertal gynecomastia linked to lavender and tea tree oils. New England Journal of Medicine 356(5): 479–85. PMID 17267908. Retrieved September 27, 2008.
- ISO Technical Committees (ISO/TCs). 2004. ISO TC 54 Business Plan: Essential oils. ISO Technical Committees. Retrieved September 27, 2008.
- Komiya, M., T. Takeuchi, and E. Harada. 2006. Lemon oil vapor causes an anti-stress effect via modulating the 5-HT and DA activities in mice. Behav Brain Res 172(2): 240–9. PMID 16780969. Retrieved September 27, 2008.
- Kuriyama, H., S. Watanabe, T. Nakaya, I. Shigemori, M. Kita, N. Yoshida, D. Masaki, T. Tadai, K. Ozasa, K. Fukui, and J. Imanishi. 2005. Immunological and psychological benefits of aromatherapy massage. Evid Based Complement Alternat Med 2(2): 179–184. PMID 15937558. Retrieved September 27, 2008.
- Lehrner, J., G. Marwinski, S. Lehr, P. Johren, and L. Deecke. 2005. Ambient odors of orange and lavender reduce anxiety and improve mood in a dental office. Physiol Behav 86(1-2): 92–5. PMID 16095639. Retrieved September 27, 2008.
- Prabuseenivasan, S., M. Jayakumar, and S. Ignacimuthu. 2006. In vitro antibacterial activity of some plant essential oils. BMC Complement Altern Med. 6(39): 39. PMID 17134518.
- Prashar, A., I. C. Locke, and C. S. Evans. 2004. Cytotoxicity of lavender oil and its major components to human skin cells. Cell Proliferation 37(3): 221-229. Retrieved September 27, 2008.
- Schnaubelt, K. 1999. Advanced Aromatherapy: The Science of Essential Oil Therapy. Healing Arts Press. ISBN 0892817437.
- Sellar, W. 2001. The Directory of Essential Oils. Essex: The C.W. Daniel Company. ISBN 0852073461.
- Tisserand, R. 1995. Essential Oil Safety: A Guide for Health Care Professionals. Churchill Livingstone. ISBN 0443052603.
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