An oil leaving the still or the press is not always the finished product. Some oils are distilled a second time to remove impurities, dark colour or off-notes. Some are concentrated to raise the share of odour-bearing compounds. Some lose part of their terpene hydrocarbons, the group that fills most of the volume but adds little to the smell.
The words rectified, folded and terpeneless describe three levels of intervention. This page explains the methods, what the fold number means, the technical reasons, and examples in the library: from sweet orange and lemon to cade and bitter almond FFPA.
ISO 9235:2021 defines a rectified essential oil (3.25) as one that has undergone fractional distillation to change the content of certain compounds, its colour, or both. A concentrated or folded oil (3.6) is an essential oil treated by a physical process to concentrate one or more components of interest. A terpeneless oil (3.30) is a rectified oil from which fractions consisting mainly of monoterpene hydrocarbons have been partly removed; a terpeneless and sesquiterpeneless oil (3.29) also loses part of its sesquiterpene hydrocarbons.
| Method | Vacuum fractional distillation; liquid–liquid extraction; chromatographic adsorption |
| Solvent | None (distillation) or aqueous ethanol; ionic liquids in research |
| Temperature | Lowered by vacuum |
| What is removed | Terpenes, waxes, colour, PAH, HCN depending on oil |
| Physical form | Liquid; often paler, stronger in odour than the parent oil |
| Typical materials | Orange, lemon, mint, eucalyptus, cade, birch tar, bitter almond |
| ISO 9235 terms | Rectified (3.25) · Concentrated/folded (3.6) · Terpeneless (3.30) · Terpeneless and sesquiterpeneless (3.29) · Terpenes (3.31) |
Rectification means redistilling an oil, usually under vacuum, keeping the middle cut and discarding the head or tail. The aim may be to remove colour, burnt or grassy first-run notes, or to lower the content of an unwanted compound. ISO gives rectified mint oil as its example.
The clearest case is dry-distilled wood oil. Crude cade and crude birch tar contain PAHs such as benzo[a]pyrene, formed when wood is pyrolysed. Rectification removes most of them. IFRA accepts only the rectified grades of these two oils, with limits on PAH content.
Bitter almond FFPA is another kind of treatment. FFPA stands for free from prussic acid: the oil has had its hydrocyanic acid removed, a highly toxic compound released alongside benzaldehyde when the kernels are macerated and distilled. Bergamot FCF is also a treated oil, with most furocoumarins such as bergapten removed.
ISO 9235 places decolourised, washed or iron-free oils under post-treated essential oils, named by an adjective for the treatment.
Cold-pressed sweet orange oil holds about 86 to 96 percent limonene, depending on origin. The characteristic smell comes from the small remainder: aldehydes such as decanal and citral, alcohols such as linalool, and esters. Removing part of the limonene raises their share.
The fold number is the ratio of starting oil to finished product. A 5-fold orange oil is five parts of the parent oil reduced to one. The higher the fold, the fewer terpenes remain; at very high folds the product approaches a terpeneless oil. The separated terpenes are sold too; ISO calls them terpenes, as in orange terpenes.
Vacuum fractional distillation is the most common industrial route. Monoterpene hydrocarbons boil lower than most oxygenated compounds and come over first. The vacuum keeps the temperature low so aldehydes are not damaged.
Liquid to liquid extraction works on polarity. The oil is shaken with aqueous ethanol; oxygenated compounds move into the alcohol phase and the hydrocarbons stay behind. Recent studies have tested ionic liquids in place of ethanol.
Chromatographic adsorption passes the oil through a silica column: the hydrocarbons elute first, the oxygenated compounds are held and then washed out with a more polar solvent. It separates cleanly but uses a lot of solvent, so it suits small batches. Supercritical CO2 and molecular distillation are also used.
Solubility: limonene and other terpene hydrocarbons dissolve poorly in dilute alcohol. Clear drinks or low-alcohol colognes made with ordinary oils tend to turn cloudy. Terpeneless oils dissolve clear.
Stability: terpene hydrocarbons oxidise easily. Limonene left exposed to air forms hydroperoxides, which can sensitise skin, and its smell turns sharp and turpentine-like. Removing most of the limonene makes the oil keep better in storage.
Strength: because the odour-bearing part is concentrated, a small amount of folded or terpeneless oil matches several times its weight of the parent oil.
A terpeneless oil also loses the light, fresh lift the terpenes gave. Its smell sits closer to the heart of the fruit than to the moment of peeling.
Labels read rectified, redistilled, 5-fold, 10-fold, terpeneless or sesquiterpeneless. The fold number says nothing about the method; two 10-fold oils may differ if one was distilled and the other extracted. Refractive index, optical rotation and aldehyde content on the certificate of analysis say more.
A treated oil still falls under IFRA limits by composition. As limonene falls, the share of citral or linalool rises, so the use limit can be tighter than for the parent oil.
| Sweet orange | |
| Lemon (expressed) | |
| Cade (rectified) | |
| Birch tar | |
| Bitter almond FFPA | |
| Bergamot FCF | |
| Peppermint |
| essential oil |
| expression |
| fractional distillation |
| isolate |
Updated 09/2026. Figures follow the year stated in each source.
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