What is an essential oil? The science behind the scent of thyme, lavender and sage

In early July, on the road to the village of Kuyucak in the Keçiborlu district of Isparta, in south-western Türkiye, the scent reaches you before you reach the village. The fields stretch away in purple rows and bees move from flower to flower. Months later, the scent of lavender from those same fields lives on in small glass bottles, in soaps and in dried bunches. So what exactly is this scent? Why does the plant make it, how do we separate it from the plant, and why should we be careful with these powerful liquids?

Where the scent hides: glandular hairs

Thyme, lavender, sage, mint, basil and rosemary all belong to the same family: the mint family (Lamiaceae). Look at the leaves of these plants, or at the sepals of their flowers, through a magnifying glass and you will see tiny, shiny, round dots on the surface. These are glandular hairs (trichomes): small groups of cells, each topped with a tiny sac full of essential oil. When you touch or rub a leaf, some of the sacs burst and the scent spreads into the air.

The plant does not make these oils for our benefit. Many of the compounds put off plant-eating insects and other animals and make it harder for fungi and bacteria to grow on the leaf, while the scents of flowers attract pollinators. It is no accident that the hot, dry hillsides of the Mediterranean are so full of fragrant plants.

Is an essential oil really an “oil”?

An essential oil is a mixture of strongly scented molecules made by plants; they evaporate easily at room temperature and hardly dissolve in water at all. (The “essential” in the name has nothing to do with being necessary: it comes from “essence”, the concentrated scent of the plant.) Essential oils have no chemical kinship with vegetable oils such as olive or sunflower oil; the only thing they share is that they do not mix with water. A simple experiment shows the difference. A drop of olive oil on paper leaves a lasting greasy mark. But if you bend and squeeze a piece of orange peel over the paper, the essential oil that sprays out evaporates, scent and all, within a short time and usually leaves no obvious mark.

Hundreds of molecules in one bottle

A single essential oil is made up not of one substance but of dozens, even hundreds, of compounds. Usually, though, a few of them dominate and give the oil its character:

• Thyme: thymol and carvacrol, two “twin” molecules built from the same atoms arranged in slightly different ways (chemists call such pairs isomers). Thyme’s sharp, warm smell comes from them.
• Lavender: linalool and linalyl acetate, which give a floral, slightly sweet smell.
• Sage: 1,8-cineole, camphor and thujone. In Anatolian sage (Salvia fruticosa, also known as Greek sage), which is common along the Aegean and Mediterranean coasts, 1,8-cineole usually dominates; common sage (Salvia officinalis) can contain more thujone and camphor.

These proportions are not fixed. Plants of the same species growing in different places can produce oils dominated by different compounds; these chemical varieties are called chemotypes. Harvest time, climate, soil and drying method change the mix too. In Türkiye the picture is even more colourful, because the name kekik, usually translated as “thyme”, is given not to a single plant but to several genera, including Thymus (true thyme), Origanum (oregano), Thymbra and Satureja (savory). So it is no surprise if two packets of “kekik” from a Turkish herb-and-spice shop smell different.

Steam distillation: separating the scent from the plant

Most essential oils are obtained by steam distillation. The method has a few steps:

1. The harvested plants are packed into a large vessel.
2. Steam is passed up through the plant material from below. The steam bursts open the oil sacs and sweeps the volatile molecules upwards.
3. The mixture of steam and oil passes through a condenser, cools and turns back into a liquid.
4. In the separating vessel the oil, which does not mix with water and is usually lighter than it, forms a separate layer on top. The faintly scented water left underneath is called hydrosol, or floral water; rose water is the best-known example.

Rose oil is made slightly differently: the rose petals are usually boiled together with water, a method called water distillation. But the basic idea is the same: steam carries the scent away from the plant.

There is some elegant physics hidden here, too. Thymol boils at about 233 °C, so how can steam at 100 °C carry it away? When two liquids that do not mix are heated together, their vapour pressures add up, and the mixture boils at a lower temperature than either of them would on its own: just below 100 °C. This means delicate molecules can be carried off with the steam without being overheated and broken down.

One bottle of oil, many kilograms of plants

Distillation yields are low. For most aromatic plants, the oil makes up roughly a few tenths of a per cent to a few per cent of the plant’s mass. At a 1% yield, 100 kilograms of lavender give only about 1 kilogram of oil. Roses yield far less: depending on the source, 1 kilogram of rose oil takes 3–5 tonnes of rose petals.

These numbers matter for safety as well. A bottle of essential oil is many kilograms of plant in concentrated form. Compounds found in small amounts in the plant itself become tens or even hundreds of times more concentrated in the oil. A cup of thyme tea and a drop of thyme oil are not the same thing.

“Natural” does not mean harmless

Hemlock (Conium maculatum) and foxglove are natural too, and both can kill. Whether a substance is harmful depends not on where it comes from but on what it is and how much of it there is. That is why some rules about essential oils apply to everyone:

• Never swallow them. Essential oils are not for drinking, and they are not to be added to water, tea or food. Even small amounts can cause serious poisoning, especially in children. If oil is swallowed by accident or gets into the eyes, get help straight away: in Türkiye call 114 (the National Poison Information Centre); elsewhere, call your local poison control centre.
• Never put them on the skin undiluted. Neat oil can irritate or even burn the skin, and over time it can cause an allergy. Thyme oil, rich in thymol and carvacrol, is especially strong in this respect.
• Keep them away from the eyes and mucous membranes. If oil gets into the eyes, rinse them at once with plenty of water and seek medical advice.
• Keep them away from children and pets. Cats are very sensitive to essential oils, because their livers struggle to break down some of the compounds; dogs and birds can be affected too. Running a diffuser non-stop in a closed room with animals in it is not recommended.
• Pregnancy and breastfeeding, epilepsy, allergies and regular medication: if any of these apply to you, ask a doctor or pharmacist before using essential oils. Oils containing thujone and camphor can trigger seizures in large amounts, and herbs and oils can also interact with medicines.
• Treat medical claims with scepticism. A compound killing microbes in a laboratory dish does not show that the same oil, or a tea, does the same job safely inside the body. This post is not a treatment or a recommendation for use; for health problems, see a doctor.
• Do not try distilling at home. Boiling water, pressure and steam can cause serious burns.

Picking plants in the wild

Depending on the source, Türkiye’s flora includes about 12,000 plant taxa (species, subspecies and varieties), and nearly a third of them are endemic: they grow nowhere else in the world. Such richness brings responsibility. Never pick or eat a plant you cannot identify with certainty: there are poisonous look-alikes such as hemlock, which resembles parsley and wild carrot. Do not pick endemic or rare species, and never dig plants up by the roots. In Türkiye, collecting some native plant species from the wild and trading in them requires a permit and is subject to quotas. Even when you pick common plants such as thyme, cutting only part of each plant lets it grow back.

Summary

• Essential oils are mixtures of molecules stored in glandular hairs; they evaporate easily and do not dissolve in water, and they help protect the plant from plant-eaters and microbes.
• Thymol and carvacrol stand out in thyme, linalool and linalyl acetate in lavender, and 1,8-cineole, camphor and thujone in sage; the mix varies with chemotype and growing conditions.
• In steam distillation, steam carries the oil off, condenses in a cooler and the oil separates out on top of the water. Because yields are low, a bottle of oil is many kilograms of plant in concentrated form.
• Natural does not mean harmless: essential oils are never swallowed or put on the skin undiluted, they are kept away from children and pets, and in pregnancy or with a health condition you should ask a doctor or pharmacist first.

To try out in a simulation the conditions in which thyme grows best, have a look at the aromatic plants lesson in the Nature and farming topic.