A bottle of orange oil pressed from the peel, 86 to 96 percent of which is a single molecule. The smell that makes an orange lives in the few percent that remain. Three words on the label, rectified, folded, terpeneless, record the times an oil was sent back to the still to deal with that large part.
One Bottle of Orange, Three Words on the Label
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 three words rectified, folded and terpeneless describe three levels of intervention. The ISO 9235:2021 standard defines each. A rectified essential oil is one that has undergone fractional distillation to change the content of certain compounds, its colour, or both. A concentrated or folded oil is one processed by a physical process to concentrate one or more components of interest. A terpeneless oil is a rectified oil from which fractions consisting mainly of monoterpene hydrocarbons have been partly removed.
These words appear most often on citrus oils, and on a small group of dry distilled wood oils such as cade. The two groups of materials are very different, but the logic is the same: remove the unwanted part to keep the wanted one.
To see why the flavour and fragrance industry needs these forms, it helps to start with the molecule that fills almost the whole bottle of orange.
Limonene: The Large, Faint Part
Limonene is a monoterpene hydrocarbon with a molecular weight of about 136 and no oxygen atom. Its smell is described as fresh orange, citrus peel, lightly sweet; a defining top note of the citrus family. Yet relative to its quantity in the oil, its contribution to the smell is small.
In cold pressed sweet orange oil, (+) limonene makes up 86 to 93 percent in Brazilian oil, 94 to 96 percent in Italian oil and about 96 percent in Spanish oil. Brazil is the largest producer, with more than half of world output; sweet orange is the most produced essential oil by volume, largely a by product of the orange juice industry. In cold pressed lemon oil limonene is lower, about 57 to 76 percent, with beta pinene and gamma terpinene.
The characteristic smell lies in the small remainder: aldehydes such as decanal, octanal and citral, alcohols such as linalool, and esters. These are oxygenated molecules, often grouped as oxyterpenes. Removing part of the limonene raises their share.
Limonene has another weakness. Exposed to air it oxidises into hydroperoxides that can sensitise skin, and its smell turns sharp and turpentine like. Pooled data suggest oxidation may raise its allergenic potential roughly tenfold. Limonene is also on the EU list of fragrance allergens that must be labelled.
Distilled Again: Rectified
Rectified means redistilling an oil, usually under vacuum, keeping the middle cut and discarding the head or the tail. The aim may be to remove colour, to remove burnt or grassy notes from the first run, or to lower the content of an unwanted compound. ISO gives rectified mint oil as its example.
The vacuum matters. Lower pressure lowers boiling points, so the oil is separated at lower temperature and heat sensitive aldehydes are not damaged. Monoterpene hydrocarbons boil lower than most oxygenated compounds, so they come over first; the heart keeps the odour bearing molecules.
On the same principle, some oils are processed to remove one specific compound. Bergamot FCF has had most of its furocoumarins, such as bergapten, removed. Bitter almond oil FFPA has had its hydrocyanic acid removed, a highly toxic compound released with benzaldehyde when the kernels are macerated and distilled. ISO places decolourised, washed or iron free oils among post processed essential oils, named by an adjective for the process.
Rectified does not mean better or worse. It means one more step, and that step should be recorded. The certificate of analysis of a rectified oil should show what was removed and what remains.
Concentrated: 5 Fold, 10 Fold
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. What is removed is mostly limonene. The higher the fold, the fewer terpenes remain; at very high folds the product approaches a terpeneless oil.
There are three main ways to remove terpenes. The most common in industry is vacuum fractional distillation, as described. The second is liquid to liquid extraction based on polarity: the oil is shaken with aqueous ethanol; the oxygenated compounds move into the alcohol phase while the hydrocarbons stay behind. Recent studies have tested ionic liquids in place of ethanol. The third is chromatographic adsorption through a silica column: the hydrocarbons elute first, while 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.
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 than the fold number.
The separated terpenes are not discarded either. They are sold on their own; ISO calls them terpenes, as in orange terpenes. One bottle of orange can thus become two products: the concentrated scent and an almost pure limonene.
Terpeneless and Solubility in Alcohol
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. That is why the flavour industry uses terpeneless lemon oil for clear drinks, and why perfumery uses concentrated forms when it needs a clean citrus note in a water and alcohol solution.
Stability is the second reason. Terpene hydrocarbons oxidise easily; removing most of the limonene helps the oil keep better in storage. Strength is the third: because the odour bearing part has been concentrated, a small amount of folded or terpeneless oil matches several times its weight of the parent oil.
But there is a cost. A terpeneless oil loses the light, fresh lift the terpenes gave. Its smell sits closer to the heart of the fruit than to the moment of peeling. On a blotter the parent orange oil opens bright and leaves quickly; a 10 fold oil opens more slowly, deeper, leaning toward the aldehydes.
A processed 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. A stronger oil needs a new dilution calculation, not the old one for ordinary orange oil.
Cade Rectified: Removing the Unwanted
Cade is the name of the oil; the source plant is Juniperus oxycedrus, the prickly juniper. The oil is made by destructive distillation, without water or steam, so the wood is pyrolysed. As the essential oil in the wood comes over, it is followed by a tar like material that dissolves in it. Commercial cade oil is a mixture of both. Its principal constituent is recorded as delta cadinene.
Pyrolysing wood produces polycyclic aromatic hydrocarbons such as benzo[a]pyrene, the same class found in cigarette smoke. Redistillation removes most of them. IFRA accepts only rectified cade and rectified birch tar, with the combined level of benzo[a]pyrene and 1,2 benzanthracene in the finished product not exceeding 1 ppb. IFRA recommends that crude cade oil not be used as a fragrance ingredient in any product.
Here the word rectified means something quite different from the orange bottle. For orange, rectified or folded is a matter of smell, solubility and stability. For cade, rectified is the condition of use. Even after this step, benzo[a]pyrene may remain in rectified cade oil at up to 20 ppb, so the oil must be used very low for the finished product to meet the limit.
Two bottles, two reasons for the same word. One goes round again to become stronger; the other goes round again to lose what is not allowed. How citrus oil is taken from the peel in the first place is told in orange peel under the press.
Nine tenths of the orange bottle is limonene.
The smell of orange lives in the remaining tenth.
Each extra distillation is a choice of what to keep and what to let go.