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An essential oil is a concentrated, hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile, ethereal oils or aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove. An oil is "essential" in the sense that it carries a distinctive scent, or essence, of the plant. Essential oils do not as a group need to have any specific chemical properties in common, beyond conveying characteristic fragrances.
Essential oils are generally extracted by distillation. Other processes include expression, or solvent extraction. They are used in perfumes, cosmetics, soap and other products, for flavoring food and drink, and for scenting incense and household cleaning products.
Various essential oils have been used medicinally at different periods in history. Medical application proposed by those who sell medicinal oils range from skin treatments to remedies for cancer, and are often based on historical use of these oils for these purposes. Such claims are now subject to regulation in most countries, and have grown more vague to stay within these regulations.
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, diffused in the air by a nebulizer or by heating over a candle flame, or burned as incense, for example.
The techniques and methods first used to produce Ethereal oil (Essential oil) was first mentioned by Ibn al-Baitar (1188-1248), an Andalusian physician, pharmacist and chemist.[1]
Today, most common essential oils, such as lavender, peppermint, and eucalyptus, are distilled. 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 include 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. Due to the relatively large quantities of oil in citrus peel and 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 are a mixture of essential oil, waxes, resins, and other lipophilic (oil soluble) 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 and the loss of some "top notes" when steam distillation is used. 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. When the extraction is complete, the pressure is reduced to ambient and the carbon dioxide reverts back to a gas, leaving no residue. An animated presentation describing the process is available for viewing.
Supercritical carbon dioxide is also used for making decaffeinated coffee. However, although it uses the same basic principles it is a different process because of the difference in scale.
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 tonnes, of essential oils for which more than 1,000 tonnes were produced.[2]
| Oil | Tonnes |
|---|---|
| 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 |
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.[3] Many are also claimed to have an uplifting effect on the mind.
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 are derived from various sections of plants. Some plants, like the bitter orange, are sources of several types of essential oil.
Rhizome
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Peel
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The most well-known essential oil world-wide is eucalyptus oil, produced from the leaves of "Eucalyptus globulus". Steam-distilled eucalyptus oil is utilized throughout Asia, Africa, Latin America and South America as a primary cleaning/disinfecting agent added to soaped mop and countertop cleaning solutions, possessing insect and limited vermin control properties.
The 2nd 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".
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, provoke an allergic reaction and over time prove hepatotoxic. Essential oils should not be used with animals as they possess extreme hepatotoxicity and dermal toxicity for animals, especially for cats. Instead, essential oils should be blended with a vegetable-based "carrier" oil (a.k.a., a base, or "fixed" oil) before being applied. Common carrier oils include olive, almond, hazelnut and grapeseed. Only neutral oils should be used. A common ratio of essential oil disbursed in a carrier oil is 0.5–3% (most under 10%), depending on its purpose. Some essential oils, including many of the citrus peel oils, are photosensitizers, increasing the skin's vulnerability to sunlight. Industrial users of essential oils should consult the material safety data sheets (MSDS) to determine the hazards and handling requirements of particular oils.
Essential oils can be aggressive toward rubbers and plastics so care must be taken in choosing the correct handling equipment. Glass syringes are often used but have coarse volumetric graduations. Chemistry syringes are ideal as they resist essential oils, are long enough to enter deep vessels, and have fine graduations, facilitating quality control. Unlike traditional pipettes which have difficulty handling viscous fluids, the chemistry syringe has a seal and piston arrangement which slides inside the pipette, wiping the essential oil off the pipette wall. This improves accuracy, and the inside of the pipette is easy to clean and reuse immediately. Chemistry pipetting syringes are equal in accuracy to the best laboratory equipment and are available in sizes from 1 mL through 25 mL.
The use of essential oils in pregnancy is not recommended due to inadequate published evidence to demonstrate evidence of safety.
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.[4][5]
There is some concern about pesticide residues in essential oils, particularly those used therapeutically. For this reason, many practitioners of aromatherapy buy organically produced oils.
Essential oils are used extensively as GRAS flavoring agents in foods, beverages and confectionaries according to strict GMP (Good Manufacturing Practice) and flavorist standards, and while some advocate the ingestion of essential oils for therapeutic purposes in extremely small amounts, this should never be done except under the supervision of someone licensed to prescribe such treatment and/or who is well versed and experienced in "clinical" aromatherapy. Some common essential oils such as Eucalyptus are toxic internally. Pharmacopoeia standards for medicinal oils should be heeded. Some oils can be toxic to some domestic animals, cats in particular.[6] The internal use of essential oils can pose hazards to pregnant women, as some can be abortifacients in dose 0.5–10 ml, and thus should not be used during pregnancy.
The flash point of each essential oil is different. Many of the common essential oils such as tea tree, lavender, and citrus oils are classed as a Class 3 Flammable Liquid as they have a flash point of 50–60 °C.
LD50 of most essential oils or their main components are 0.5–10 mg/kg (orally or skin test). This compares with, for example, the LD50 of arsenic acid, one of the most common arsenic based poisons which is 6 mg/kg (as tested on rabbits).[7]
In 2002, ISO published ISO 4720 in which the botanical names of the relevant plants are standardized [8]. The rest of the standards with regards to this topic can be found in the section of ICS 71.100.60 [9]