An essential oil is a mixture of volatile organic compounds biosynthesised by a plant, carrying its scent and biological signature. A single oil contains anywhere from a few dozen to several hundred molecules, most belonging to the terpene and phenylpropanoid families. Their identities and ratios determine odour, stability and how the oil interacts with skin, mucous membranes and the nervous system.
Plants do not produce these compounds for human use. They serve ecological functions: attracting pollinators, deterring herbivores, signalling to other plants. Once removed from the tissue, the mixture retains most of its original character but becomes more vulnerable to oxygen, heat and light.
This page covers the nature of essential oils, the principal isolation methods, the main chemical classes, the chemotype phenomenon and the commercial risks of adulteration.
The same plant can yield very different oils depending on the method. Cold-pressed citrus retains furocoumarins, distilled does not. Jasmine yields a dense absolute via solvent extraction; steam distillation is inefficient.
Plants synthesise two distinct kinds of oil. Fixed oils are esters of glycerol and fatty acids, the heavy non-volatile oils pressed from seeds. Essential oils are mixtures of small, volatile, scent-bearing molecules. The two behave very differently: fixed oils are stable and non-aromatic, while essential oils evaporate at room temperature, leaving a fading trail of odour on paper.
Of the roughly 350,000 plant species on earth, about 17,500 are aromatic. Only around 400 are processed commercially, drawn from ten principal families: Apiaceae, Asteraceae, Cupressaceae, Lamiaceae, Lauraceae, Myrtaceae, Pinaceae, Poaceae, Rutaceae, Zingiberaceae.
Essential oils can be drawn from any part of a plant: flowers (Rose), leaves (Peppermint), fruit peel (Bergamot), seeds (Fennel), roots (Vetiver), wood (Sandalwood), bark (Cinnamon bark), resin (Frankincense), dried buds (Clove bud).
Steam distillation is the dominant method. Plant material sits in a sealed vessel, hot steam passes through, and volatile molecules ride the vapour out. The condensed mixture separates into a light oil layer floating on a water layer known as the hydrosol. The method suits most leaves, stems, flowers and woods. Lavender, Rosemary and Tea tree are produced this way.
Cold expression applies to citrus peels. The fresh peel is mechanically crushed, rupturing the oil sacs inside the rind. The oil and juice are then centrifugally separated. Expressed citrus oils carry the heavier, less volatile compounds, including the furanocoumarins responsible for photosensitisation. Bergamot, Lemon (expressed) and Lime (expressed) are typical examples. Citrus oils can also be distilled, producing variants such as Lemon (distilled) and Lime (distilled), which are free of furanocoumarins and not phototoxic.
Solvent extraction handles delicate flowers that cannot survive distillation heat. Hexane or cyclohexane washes the petals to yield a waxy substance called a concrete. The concrete is then washed with ethanol to separate the scent molecules from the plant waxes. After the ethanol is evaporated, what remains is an absolute. Jasmine and certain tuberose products use this route. Absolutes commonly include larger, less volatile molecules such as benzyl benzoate, indole and phytol.
Supercritical CO₂ extraction uses high-pressure carbon dioxide as solvent. The output sits closer to the smell of the fresh plant than a distilled oil, because no high heat is applied. Several resins and spices are produced this way.
A small number of oils are made by dry distillation, in which the raw material is burned directly. Cade oil and birch tar are examples. The result is smoky in character, chemically distinct from a steam-distilled oil.
| Method | Suitable material | Example |
|---|---|---|
| Steam distillation | Leaves, flowers, stems, woods | Lavender |
| Cold expression | Citrus peel | Bergamot |
| Solvent extraction | Delicate flowers | Jasmine absolute |
| Supercritical CO₂ | Resins, spices | Frankincense |
| Dry distillation | Wood, bark | Cade, birch tar |
A typical essential oil contains between 20 and 200 compounds. One or two of them usually dominate and define the oil's character. Peppermint is roughly 40% Menthol, Eucalyptus globulus roughly 75% 1,8-Cineole. The remainder consists of many compounds at low or trace concentrations, some below 0.1 %, yet capable of shaping odour, stability or safety profile.
Compounds are classified by carbon skeleton and by functional group. The dominant skeleton is the terpene framework, built from 5-carbon units: monoterpenes (10 carbons), sesquiterpenes (15 carbons), diterpenes (20 carbons). The second framework is phenylpropanoid, a benzene ring with a 3-carbon side chain, present in Eugenol, Anethole, Safrole.
Functional groups attached to these skeletons define the chemical families: hydrocarbons (plain terpenes such as Limonene and α-Pinene), alcohols (Linalool, Geraniol, Terpinen-4-ol), phenols (Carvacrol, Thymol, Eugenol), aldehydes (Citral, Citronellal), ketones (Camphor, Thujone, Pulegone), esters (Linalyl acetate, Methyl salicylate), ethers and oxides (1,8-Cineole, Methyleugenol).
Functional group governs biological behaviour. Phenols tend to irritate skin and mucous membranes. Aldehydes and sesquiterpene lactones are common sensitisers. Monoterpene ketones can affect the central nervous system. Certain ethers including Methyleugenol, Safrole and Estragole are classed as rodent carcinogens.
| Class | Example constituent | Oil rich in this class |
|---|---|---|
| Monoterpene hydrocarbon | Limonene | Bergamot, Lemon |
| Alcohol | Linalool | Lavender, Basil (linalool CT) |
| Phenol | Eugenol | Clove bud, Cinnamon leaf |
| Aldehyde | Citral | Lemongrass, May chang |
| Ketone | Camphor | Camphor (white), Sage (Dalmatian) |
| Ester | Linalyl acetate | Bergamot, Lavender |
| Oxide | 1,8-Cineole | Eucalyptus globulus, Rosemary |
Two batches of the same oil can differ substantially in composition. The variation comes from cultivar, geography, elevation, soil, harvest timing within the year and within the day, distillation technique and even equipment. 1,8-cineole in Moroccan Eucalyptus globulus ranges from 62.4 % in May to 82.2 % in July. Menthone in French peppermint moves from around 6 % in October to nearly 55 % in June.
When the variation is large and genetically fixed, a species can produce distinct chemotypes. Chemotypes are plants identical in appearance but yielding oils with different dominant constituents. Within Thymus vulgaris alone there are chemotypes for thymol (phenol-rich, strong, more irritant), linalool (gentle, suitable for sensitive skin), thujanol and geraniol. Within Ocimum basilicum there is a linalool chemotype (safe) and an estragole chemotype, which contains methyleugenol and estragole and carries restrictions.
Chemotype labelling is not decorative. It determines safety, indications and dose. Two bottles marked simply "thyme" can carry very different risk profiles depending on whether the content is thyme thymol/carvacrol CT or thyme linalool CT.
Essential oils are valuable, particularly the floral oils that are difficult to extract. Adulteration is common and takes several forms. The simplest is dilution with odourless materials: ethanol, isopropyl myristate, glycols, phthalates, cheap vegetable oils. More sophisticated is the addition of synthetic versions of compounds already present in the target oil, for example synthetic linalool into lavender, synthetic citral into may chang, synthetic terpinen-4-ol into tea tree. The most extreme is passing off, where a fully synthetic blend is sold under the name of a natural oil.
The most expensive oils suffer the most adulteration: jasmine absolute, rose absolute, rose otto, neroli, sandalwood. The cheapest oils are rarely faked: sweet orange, eucalyptus.
Authentication relies on three layers. Gas chromatography (GC), usually paired with mass spectrometry (GC-MS), reveals the full constituent profile. The appearance of an unexpected compound, or a concentration outside the natural range, is a flag. Physical parameters such as specific gravity, refractive index and optical rotation expose odourless extenders. Olfactory evaluation by a trained nose catches anomalies that instruments sometimes miss, particularly in absolutes.
No single test guarantees purity. Trust is built over time through transparent sourcing, chemotype documentation, batch records, distillation dates and routine analysis.
Some indirect indicators: a pure oil evaporates cleanly from paper, leaving no greasy ring. A drop in water remains insoluble and floats. The scent develops in layers, with top notes lifting first and middle and base notes appearing after a few minutes. Adulterated oils tend to read flat, missing the later layers.
A complete label should carry the trade name, the botanical name (genus and species), the plant part, the extraction method, the geographic origin, the chemotype where relevant, batch number and distillation date, expiration date, dilution percentage if applicable, and the relevant safety warnings.
Botanical name is the only reliable anchor. "Lavender" could mean Lavandula angustifolia, Lavandula x intermedia (lavandin) or Lavandula stoechas (Spanish lavender), three distinct products in composition and safety. "Eucalyptus" could be Eucalyptus globulus or Eucalyptus radiata, with different guidance for use around children.
The plant part matters as well. Cinnamon bark is rich in cinnamaldehyde, a strong skin irritant. Cinnamon leaf is rich in eugenol, a different safety profile altogether. Two parts, two products, two sets of instructions.
An essential oil is a mixture of volatile molecules synthesised by a plant and isolated by distillation, cold expression, solvent extraction or CO₂. Each oil contains 20 to 200 compounds, mostly terpenes and phenylpropanoids, with functional group governing biological behaviour. Composition varies with cultivar, season, location and technique. Chemotype distinguishes products of the same species but different profile.
Adulteration exists at several levels and is detected through a combination of laboratory analysis, physical measurement and olfactory evaluation. A clear label with botanical name, plant part, method and chemotype is the minimum requirement for any judgement of safety and fit.