How Fragrance Is Extracted: Distillation, Enfleurage and Absolutes

An essential oil, a concrete and an absolute aren't three grades of the same thing: they come out of three different processes. Steam distillation and the pressing of citrus peel give essential oils; solvents give concretes, which are then washed with alcohol to give absolutes. Wikipedia's overview of fragrance extraction sums up why the choice matters: heat, chemical solvents or exposure to oxygen can denature some aromatic compounds, changing their smell or leaving them odourless.

So the plant decides the method. Peppermint and rose petals can take steam; lemon peel is simply pressed; jasmine and tuberose, too delicate for heat, went into fat and later into solvents; supercritical carbon dioxide offers a gentler route at around 30 °C.

What's the difference between an essential oil, a concrete and an absolute?

An essential oil is the fragrant oil that steam or pressure separates from a plant, with no solvent added. A concrete is what remains when a hydrocarbon solvent such as hexane has pulled fragrant oils, waxes, fats and pigments out of the plant and has then been removed for re-use: a waxy mass, too thick or even solid at room temperature to be useful as it is. An absolute is that concrete washed with ethyl alcohol, which dissolves only the small fragrant molecules and leaves the waxes behind; once the alcohol is removed, the oil left is the absolute.

The same flower can exist in more than one form. Wikipedia's entry on rose oil separates rose otto, extracted by steam distillation, from rose absolute, obtained by solvent extraction, and notes that the absolute is the one used more commonly in perfumery. The material itself has its own page: rose as a perfume note.

The methods side by side:

How does steam distillation work?

Steam distillation carries the volatile compounds of a plant away in steam, then cools everything back into liquid in a condenser. Because most essential oils don't mix with water, they form their own layer and can simply be separated off. Wikipedia's article on steam distillation finds the method in perfume recipes attributed to the Arab philosopher al-Kindi, who died in 873, and records that Avicenna (980 to 1037) distilled rose petals with water to make essential oils.

Distillation gives two products: the essential oil and the water it came out with, called a hydrosol. Rose water, lavender water and orange flower water are all hydrosols. Some distillations are slow: ylang-ylang takes 22 hours and is collected in fractions sold as grades, "extra", I, II, III and "complete".

Steam has one clear limit, and it's heat. The conditions are harsh, around 100 °C for hours, and the compounds you're after can break down. Delicate flowers such as jasmine and tuberose would be destroyed or denatured at those temperatures, which is why jasmine and its absolute took a different path.

Why is citrus oil pressed rather than distilled?

Citrus oil is pressed because the peel gives up its oil to mechanical pressure, with no heat at all, and expression is the dominant method for the whole citrus family: the peel is pressed, squeezed or compressed by machine and the oil collected, much of it alongside the orange juice industry. The fragrance extraction overview lists bergamot, grapefruit and lemon oils among the expressed oils, with one exception worth remembering: lime oil is usually steam distilled.

If you drink Earl Grey, you already know bergamot, the citrus note pressed from the peel. The bergamot orange grows mostly on a short stretch of the Ionian coast around Reggio di Calabria, in southern Italy; an essence from its peel flavours Earl Grey and Lady Grey teas, and bergamot oil is one of the most commonly used ingredients in perfumery. It belongs, with lemon and grapefruit, to the citrus family of fragrances.

How did enfleurage capture jasmine and tuberose?

Enfleurage captured scent in fat. In cold enfleurage, as Wikipedia's enfleurage article describes it, a large framed glass plate called a chassis was smeared with animal fat, usually lard or tallow, and left to set. Flowers were laid on the fat for one to three days, then replaced with fresh ones, again and again, until the fat was saturated. In hot enfleurage, solid fat was heated and the flowers stirred into it, strained out and replaced.

The scented fat is called an enfleurage pomade. It could be sold as it was, or washed in ethyl alcohol to draw out the fragrant molecules; once the alcohol had evaporated, what remained was the absolute. Enfleurage was costly and inefficient, but it was the only way to extract the scent of delicate flowers such as jasmine and tuberose, the overpowering white flower. The article dates the method to the seventeenth and eighteenth centuries, alongside the perfume craft of Grasse, the French perfume capital, and calls it largely obsolete today, replaced by solvent and supercritical CO2 extraction.

Capturing scent in fat is far older than Grasse. The history of perfumery published by the Grasse museums notes that the ancient Egyptians knew how to capture fragrances in fat, by cold maceration or hot decoction, but knew nothing of distillation. Distillation spread much later with the still, after Arabic treatises were translated by physicians of the School of Salerno in the twelfth century.

What happens in solvent extraction?

Solvent extraction soaks plant material in a solvent that dissolves its aromatic compounds. A hydrocarbon such as hexane or toluene pulls out the fragrant oils along with waxes, fats and pigments; the solvent is then removed under vacuum and reused, leaving the concrete. Ethanol washes the concrete and leaves the waxes behind, and the result, from flowers such as jasmine and rose, is an absolute. Because the process runs at low temperatures, the fragrance extraction overview notes, an absolute may be more faithful to the original scent of the raw material than a distilled oil.

Wikipedia's article on absolutes adds a detail you can picture: the ethanol extract is chilled, to around -15 °C for example, so that the waxes solidify and can be filtered out cold. The Grasse museums credit François Coty, whom they call the father of modern perfumery, with making regular use of natural absolutes from volatile solvent extraction alongside the new synthetic materials, the story told in the class on natural versus synthetic fragrance materials.

What does supercritical CO2 extraction change?

Supercritical CO2 extraction uses carbon dioxide pushed above its critical temperature of 31 °C and critical pressure of 74 bar, where it works as a solvent. Wikipedia's article on supercritical fluid extraction says carbon dioxide is the most used supercritical fluid, that the same technique decaffeinates coffee and tea and extracts hops for brewing, and that a plant's volatile oils can be drawn out at relatively low pressures.

Its main advantage comes at the end: release the pressure and the CO2 turns back into a gas, leaving no trace of itself in the extract, which is why such materials are sometimes sold as "CO2 extracts". The work happens at around 30 °C, gentle compared with steam. The catch is cost: the high pressures make the equipment expensive.

How can you tell the method from a material's name?

The name of a natural material tells you how it was made:

• Essential oil, or otto for rose: steam distillation, or pressing for citrus peel.
• Hydrosol: the water that comes out of the still with the oil.
• Concrete: a first solvent extract, waxy and thick.
• Absolute: a concrete, or an enfleurage pomade, washed in alcohol.
• CO2 extract: drawn out by supercritical carbon dioxide.

Beauty magazines, Mademoiselle Bulle among them in its guide to choosing a perfume, talk about fragrance families and notes. The club goes one step further back, to the material itself: when you open a note page, look for that one word first, because it tells you whether the flower met steam, fat or solvent.

Exercise: three kitchen extractions with an orange

You'll need an unwaxed orange, a small jar with a lid, a spoonful of unsalted butter, a saucepan, paper smelling strips or strips of plain white paper, and a notebook. Nothing goes on your skin, and keep the peel away from your eyes.

1. Pressing. Cut a strip of zest, fold it in half over a paper strip, coloured side down, and squeeze. Watch what sprays out and the mark it leaves. Smell straight away, then again ten minutes later.
2. Steam. Drop another piece of zest into a pan of water, bring it to a simmer and waft the steam towards you from a distance with your hand. Compare it with the paper strip: what did the heat add, and what did it take away?
3. Fat. Lay a few strips of zest on the butter, close the jar and leave it in the fridge overnight. Next day, lift out the zest and smell the butter: that's kitchen enfleurage. Cook with the butter; don't put it on your skin.

Write three words for each result side by side in your notebook, and share them in the Learning to Smell circle.