An essential oil is a mixture of tens to hundreds of molecules, each with its own boiling point. Fractional distillation splits that mixture into parts by boiling range. Light parts come first, heavy parts later.
There are two ways to do it. The first is to divide the primary distillation itself: the distillate is collected in separate receivers over time, as with ylang ylang. The second is to take a finished oil and redistil it through a fractionating column, usually under vacuum, as with patchouli heart cuts.
ISO 9235:2021 has no separate entry for fractional distillation. The nearest term is rectified essential oil: an essential oil that has undergone fractional distillation to change the content of certain compounds, its colour, or both.
| Method | Distillate collected over time; or redistilled through a column, often under vacuum |
| Solvent | None |
| Temperature | Lower under reduced pressure; molecular distillation uses a very deep vacuum |
| Yield range | The fractions add up to the original amount of oil |
| Physical form | Liquid; the head is light, the tail thick and dark |
| Typical materials | Ylang-ylang, patchouli, mint, citrus oils, birch, cade |
| ISO 9235 term | rectified essential oil (3.25); concentrated essential oil (3.6) |
When an oil is redistilled through a column, the operator divides the distillate into at least three parts. Heads are the lightest molecules, usually monoterpene hydrocarbons. The heart (coeur in French) is the middle range. Tails and residue are the heavy molecules, colour bodies and non-volatiles.
Which part is kept depends on the goal. For a pale oil free of smoky or earthy edges, the heart is kept. For a more concentrated oil, the terpene-rich heads are removed, giving a terpeneless or folded oil; see rectified and terpeneless. For a single molecule, the cut is made very narrow around its boiling point, and the product is an isolate.
Ylang ylang flowers (Cananga odorata forma genuina) are distilled in one continuous run lasting many hours; sources give from about 10 to more than 20 hours. The distillate is not pooled but collected in successive receivers, by time and by density. The earliest portion is denser and rich in esters and aromatic ethers such as benzyl acetate, p-cresyl methyl ether and linalool. Later portions become lighter and richer in sesquiterpenes such as beta caryophyllene.
The trade calls these portions Extra, I (premières), II (secondes) and III (troisièmes). Extra is usually the portion collected in roughly the first one to two hours. ISO 3063:2004, for ylang ylang oil from Madagascar, Mayotte and the Comoros, sets characteristics such as relative density, refractive index, optical rotation and ester value for each grade.
Complete is a confusing name. Some sources use it for oil distilled in one run without fractions. Others use it for a blend reassembled from the fractions. The word on a label does not say which, so the supplier's technical sheet is where to check.
Cananga comes from the same species in another form, Cananga odorata forma macrophylla, and is distilled mainly in Java, Indonesia. It has its own standard, ISO 3523:2002. Cananga oil is usually sold as a single oil, without the Extra, I, II, III grading used for ylang ylang.
Lowering the pressure lowers boiling points, so the oil can be separated at lower temperature with less heat damage. A packed column with reflux separates compounds whose boiling points are close. Molecular distillation, also called short path distillation, uses very high vacuum and very short heat contact.
Patchouli is the familiar example. Crude patchouli oil is dark, with earthy and musty edges. Redistilled under vacuum, the light heads and the dark residue are removed; the heart is richer in patchoulol and paler. One study of molecular distillation of patchouli oil reached about 62% patchoulol in the product. Suppliers sell patchouli heart, or coeur de patchouli, with the patchoulol content stated on the technical sheet.
Extra, I, II, III and complete belong to ylang ylang. Heart, coeur or fraction indicate a heart cut. MD usually indicates molecular distillation. Rectified indicates an oil redistilled to change its composition or colour; ISO uses mint as the example of a rectified essential oil (see peppermint). For birch tar and cade, rectified is also an IFRA condition for fragrance use, because the step lowers PAH content.
| Ylang-ylang | |
| Cananga | |
| Patchouli | |
| Peppermint | |
| Birch tar | |
| Cade (rectified) | |
| Bergamot (FCF) |
| essential oil |
| rectified terpeneless |
| isolate |
| expression |
Updated 09/2026. Figures follow the year stated in each source.
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