Every essential oil starts as a large quantity of raw plant material and ends up as a small bottle of concentrated liquid. How it gets from one state to the other isn't arbitrary, and it isn't a marketing choice — it's dictated almost entirely by which part of the plant the oil comes from, and how tightly that part holds on to it. Three methods account for nearly every essential oil on the market: steam distillation, cold pressing, and, for a smaller number of resins and delicate materials, CO2 extraction. Each works on a different physical principle, and — this is the part that's easy to miss — each one pulls a slightly different chemical result out of the same starting plant. This is a plain look at how each method actually works, why the method is matched to the material rather than picked at random, and what that means for what genuinely ends up in the bottle.
Steam distillation: the method behind most oils
Steam distillation accounts for the majority of essential oils on the market. Most flowers, leaves, needles, wood and root material are processed this way.
The plant material is loaded into a still, either suspended above boiling water so that only steam reaches it, or — for dense resins — submerged directly into the boiling water itself, a variant usually called hydrodistillation. Heat and steam soften and rupture the tiny structures where a plant stores its aromatic compounds: oil glands on the surface of a leaf, small sacs within a flower's petals, resin ducts running through a piece of wood. Once ruptured, the freed aromatic compounds are volatile enough to evaporate and travel along with the steam. That vapour then passes into a condenser, cools back into liquid, and separates into two layers: the essential oil itself, which doesn't mix with water, and the hydrosol — the fragrant water left behind — with the oil floating above or, less often, sinking below it, depending on its density relative to water.
Why temperature, pressure and duration vary by plant
Commercial stills typically run somewhere between 60°C and 100°C, at a modest pressure of roughly 15–20 psi — but the exact figures, and especially the duration, vary enormously depending on what's being distilled. Lavender, for instance, is a comparatively quick job: a full run typically takes one to three hours, and as much as three-quarters of the total oil comes off in the first thirty to sixty minutes. Dense woods sit at the opposite end of the scale — sandalwood commonly takes anywhere from fourteen to well over thirty hours to fully distil, since its aromatic compounds are held far more tightly within the wood's structure and need much longer exposure to steam and heat to work free.
This is also why a resin such as frankincense is usually hydrodistilled rather than steam-distilled in the strictest sense: the resin is dense and hard enough that steam passing over it doesn't penetrate effectively, so producers submerge it directly in gently boiling water instead, stirring continuously so it doesn't scorch or foam over. In every case, the operator is balancing the same trade-off — enough heat and time to free the aromatic compounds, without so much that the more delicate ones degrade before they ever reach the condenser.
Why some oils need many times their own weight in plant material
Yield varies just as much as duration does, because it reflects how much of a plant's weight is actually made up of the aromatic compounds being extracted — and that fraction differs enormously between species. Rose is the standout example: producing a single kilogram of rose otto oil typically takes several tonnes of rose petals, which is why genuine rose oil is among the most expensive essential oils to produce. Lavender is far more efficient by comparison — roughly 100kg of flowering tops for a litre of oil — but that's still nowhere near a one-to-one ratio. Neither figure is a flaw in the plant or the process; it's simply a reflection of how concentrated (or dilute) a given species' aromatic compounds are within its raw material.
Cold pressing: the citrus exception
Cold pressing — also called expression — is used specifically, and almost exclusively, for citrus peel oils: Wild Orange, Lemon, lime, Bergamot, grapefruit and mandarin among them.
No heat and no steam are involved at any stage. The peel is mechanically abraded or punctured to rupture the large oil-bearing sacs sitting just beneath its surface, releasing a mixture of essential oil and juice. That mixture is then spun through a high-speed centrifuge, which separates the lighter oil from the heavier juice and pulp by density. Historically, this was done by hand using sponges — a technique long associated with Sicilian bergamot production specifically — though today it's done mechanically, at much greater scale and speed.
Citrus peel works this way because of where its oil actually sits: large, close to the surface, and structurally fragile enough to rupture under mechanical pressure alone. Most other plant material holds its aromatic compounds far more deeply — in glandular structures, resin ducts or dense fibre — which is exactly why it needs steam's heat to release them, rather than pressure by itself. There's a practical, waste-reducing side to this too: cold-pressed citrus oil is often effectively a co-product of the fruit juice industry, since the peel would otherwise be a discarded by-product of juicing.
CO2 extraction: a smaller-scale alternative for resins
CO2 extraction is far less common than the two methods above, but worth understanding for the handful of materials it suits particularly well. Supercritical CO2 extraction pressurises carbon dioxide into a state that behaves partway between a gas and a liquid, in which it acts as a solvent — drawing aromatic compounds out of a raw material at comparatively low temperatures. Once extraction finishes, releasing the pressure returns the CO2 to an ordinary gas, and it disperses, leaving no solvent residue behind in the finished extract.
It's used on a much smaller commercial scale than steam distillation, mostly for resins such as frankincense and myrrh, and for some delicate floral material that heat handles poorly. For a dense resin in particular, CO2 extraction can lift out a broader range of compounds than distillation alone — including some heavier, less volatile ones that steam or hydrodistillation simply leaves behind in the spent material.
Why extraction method changes what's actually in the bottle
This is the point where extraction method stops being a production detail and starts being a genuine chemistry question. Steam distillation and hydrodistillation only ever carry over compounds volatile enough to travel with steam vapour — anything too heavy, or not volatile enough, to vaporise simply gets left behind in the plant material, however long the distillation runs. Cold pressing has no such filter. Because it's a purely mechanical process, with no heat and no vapour stage at any point, everything present within the peel's surface oil sacs ends up in the finished oil — lighter and heavier molecules alike, carried straight across without a vapour-and-condense step to sort between them.
A concrete example makes this tangible: furanocoumarins (bergapten is one) are present in cold-pressed citrus oils, because mechanical pressing carries over everything sitting in the peel's oil sacs. Those same compounds are largely absent from a steam-distilled oil made from the same fruit, because they're heavy and non-volatile enough that they never travel with the steam in the first place. That's a genuine, measurable compositional difference between two oils that can otherwise share a common name and a common plant of origin — a lemon oil, strictly speaking, isn't automatically the same chemical mixture as another oil also labelled lemon oil, once the extraction method behind each one is taken into account. It's a fact about chemistry and processing, not a claim about which version is better.
Which method, for which oil family
| Plant material | Typical extraction method | Examples |
|---|---|---|
| Citrus peel | Cold pressing (expression) | Wild Orange, Lemon, Bergamot, Grapefruit, Lime |
| Flowers | Steam distillation | Lavender, Rose, Ylang Ylang |
| Leaves and herbs | Steam distillation | Peppermint, Eucalyptus, Rosemary |
| Wood | Steam distillation (often long-duration) | Sandalwood, Cedarwood |
| Resin | Hydrodistillation, or CO2 extraction | Frankincense, Myrrh |
| Roots | Steam distillation (often long-duration) | Vetiver, Ginger |
FAQ
Is steam-distilled or cold-pressed oil "better"?
Neither — they're matched to the plant material rather than ranked against one another. Citrus peel is cold-pressed because its oil sacs are large, close to the surface and mechanically fragile; almost everything else is steam-distilled because its aromatic compounds sit more deeply and need heat and steam to release. Comparing the two head-to-head is a bit like asking whether a kettle is "better" than a juicer — they're built to do different jobs.
Why don't all essential oils use the same extraction method?
Because plant structures differ enormously in where they hold their aromatic compounds, and how tightly. A citrus peel's oil sacs rupture under gentle mechanical pressure; a piece of sandalwood or a lump of frankincense resin doesn't, and instead needs steam, sustained heat, or a pressurised solvent in the case of CO2 extraction, to release what it's holding.
Does extraction method actually affect an oil's chemical make-up?
Yes — it's one of the more measurable differences in the whole category. Steam distillation only carries over compounds that vaporise with steam; cold pressing carries over everything mechanically present in the peel's surface oil, heavy and light alike. Two oils made from the same plant can have a genuinely different constituent profile purely because of which method produced them.
What is CO2 extraction used for?
Mostly resins — frankincense and myrrh in particular — plus some delicate floral material, on a far smaller commercial scale than steam distillation. It uses pressurised carbon dioxide as a solvent at comparatively low temperatures, then releases it as an ordinary gas afterwards, leaving no solvent residue in the finished extract.
Why does distillation take so much longer for some oils than others?
Because the aromatic compounds are held differently depending on the plant structure involved. A flower's petals release their oil quickly once heat and steam reach it — lavender is usually done within a few hours. Dense wood holds on far more stubbornly, which is why something like sandalwood can take well over a day of continuous distillation to fully process.
The short version
Extraction method isn't a marketing choice — it's a physical one, dictated by where in the plant the oil actually sits and how firmly it's held there. Citrus peel is cold-pressed because its oil sacs are large, shallow and mechanically fragile; almost everything else — flowers, leaves, wood, roots and most resins — is steam or hydrodistilled, at a temperature, pressure and duration matched to how tightly that particular plant holds on to its oil, with a small number of dense resins also suited to CO2 extraction. None of that is incidental detail. It's the reason two oils that share a plant's name on the label can still differ, measurably, in what they actually contain.