A pressure vessel of steel, a few kilograms of dried, ground ginger, and carbon dioxide, the same gas found in sparkling water. At 31 degrees Celsius and close to 74 bar, that gas starts to behave like a solvent. This piece explains how, and why ginger or pink pepper extracted with CO2 smells closer to the fresh spice than distilled oil does.
A steel vessel and a gas
In a CO2 extraction plant, most of the space is steel: thick walled pressure vessels, piping, compressors, heat exchangers. The plant material is dried, ground and loaded into the extraction vessel. There is no pot of boiling water and no drum of hexane. The only solvent pumped in is carbon dioxide, sometimes with a small amount of ethanol as a co-solvent.
The standard ISO 9235:2021 names this product a supercritical fluid extract, entry 3.28: an extract obtained by processing a natural raw material with a fluid in the supercritical state, then recovering the product by letting the fluid expand. The two examples the standard names are CO2 extracts of coffee and of pink pepper.
Compared with steam distillation, a technique several centuries old, CO2 extraction is a recent method in the history of aromatic materials. It does not replace distillation. It gives another form of material, with another composition and therefore another smell, even when it starts from the same ginger root.
What supercritical means
Every substance has a critical point, where the boundary between liquid and gas disappears. For CO2, this point lies at 31.0 degrees Celsius and 73.8 bar, about seventy three times atmospheric pressure. Beyond both thresholds, CO2 is no longer a gas or a liquid in the usual sense. It becomes a supercritical fluid.
In this state, CO2 has two properties at once. Its density is close to that of a liquid, so it can dissolve aromatic oils. Its low viscosity and high diffusivity resemble a gas, so it moves deep into the ground plant tissue. Solvent power rises with density, and density follows pressure and temperature. The operator therefore chooses the composition of the extract by turning those two dials.
The stream of CO2 carries what it has dissolved into one or more separators. There the pressure drops, the CO2 loses its solvent power and returns to a gas; the extract settles at the bottom of the vessel. The CO2 is compressed again and sent back to the start of the system. With several separators at different pressures, plant waxes can drop out in the first while the aromatic fraction is collected in the next.
When CO2 is kept liquid below the critical point, colder and at lower pressure, the process is called subcritical extraction. Liquid CO2 is a weaker solvent, so it takes mainly the light compounds and little wax. On labels, the word subcritical is the supplier's own; there is no shared convention on the pressure behind it.
Low temperature, intact molecules
Steam distillation works around the boiling point of water, for hours, with the material surrounded by moisture. Some molecules survive that. Others are hydrolysed or rearranged, or do not travel with the steam and stay behind in the residue. An essential oil is what gets through the condenser, not everything the plant contains.
The volatile fraction in CO2 extraction is usually taken at about 35 to 55 degrees Celsius. At that temperature, few by-products of heat and water form. Heavier molecules that steam struggles to carry do dissolve in compressed CO2. The result is an extract with a wider composition than the essential oil of the same plant, and closer to the smell of the intact material.
CO2 is a non-polar molecule. It dissolves terpenes, esters, aldehydes and light aromatic compounds well; it dissolves sugars, salts, proteins and most strongly polar substances poorly. A CO2 extract therefore does not carry the water soluble part of the plant. When the pressure is released, the CO2 evaporates completely at room temperature, and the extract holds no organic solvent residue in the sense that hexane leaves.
No solvent residue does not mean harmless. IFRA limits and cosmetic regulations apply by composition, just as for the essential oil of the same plant. A more concentrated extract can carry more of a restricted substance than the oil, and has to be dosed like any other material.
Ginger CO2 and distilled ginger
Ginger is the clearest example of the difference between the two methods. Ginger oil distilled from dried rhizome is mostly sesquiterpenes: zingiberene at about 38 to 40 percent, ar-curcumene at about 17 percent. The smell is warm, dry, a little woody. What makes ginger hot on the tongue, the gingerols and shogaols, is almost non-volatile. It stays in the residue after distillation.
Ginger CO2 keeps those compounds. The library describes its smell as freshly cut ginger, lemon, hot spice, a little earth and wood. The total extract is clearly pungent on the tongue and warmer and rounder in smell than the oil; because it carries gingerols, it is also used in food flavouring for heat. The select extract is brighter and closer to fresh ginger.
There is a caution attached. Gingerol is a pungent principle and at high concentration can feel hot on skin. Ginger CO2 has no dedicated standard in the IFRA 51 index, but citral, when present from fresh type ginger, is restricted by IFRA and must be labelled as an allergen in Europe above the threshold. Three rhizomes from the same family are read separately in Ginger, Turmeric, Galangal.
Pink pepper CO2 gives another example. Pink pepper is the fruit of Schinus terebinthifolia, a small South American tree in the cashew family, not a true pepper. Its chemistry is dominated by monoterpenes: alpha phellandrene, limonene, myrcene, pinene. The CO2 extract is rounder and sweeter than the oil and keeps the red berry note better. Since the mid 1990s, pink pepper has become a familiar top note in perfume.
Select and total: two kinds of CO2
CO2 extract labels usually carry one of two words: select, or selective, and total. The words describe the pressure range and the range of substances taken out, not a quality grade. According to a 2013 review by Capuzzo and colleagues, the volatile fraction is usually extracted at about 90 to 200 bar; total extracts are made at about 250 to 350 bar. These figures are only a guide; each producer has its own process.
A select extract is a mobile liquid, close in composition to the essential oil but a little wider. A total extract also dissolves waxes, fats and pigments such as carotenoids. It is often thick, sets when cool, is dark in colour, and smells closer to the raw material than to the oil. One German producer compares select extracts with steam distilled products, and total extracts with hexane extracts.
For perfume, the select type is easier to use because it contains little fat and dissolves better in alcohol. A total extract can leave a waxy deposit when added to alcohol. To obtain a fully soluble product, producers wash the extract with ethanol, chill filter it and then remove the ethanol. For body oils and food flavouring, the total type is often preferred, precisely because it carries what the select type leaves behind.
CO2 on the materials shelf
Some materials give their smell almost only through a solvent. Vanilla has no distilled oil; vanilla pods, after curing, yield their scent through solvent extraction, and CO2 is one of those routes. Rosemary offers another example, outside the world of scent: rosemary extract rich in carnosic acid, made with supercritical CO2 or another solvent, is authorised in the European Union as a food antioxidant under the code E 392.
CO2 extracts need their own storage habits. Total extracts can solidify in the cold and need gentle warming before weighing. Many are so thick that they are sold pre-diluted in a vegetable oil such as sunflower; the percentage of aromatic compounds is then lower than in the original extract, and the label should name the carrier. The terpenes in pink pepper and ginger oxidise easily, so bottles should be dark, sealed, cool, with little air space.
A complete label gives the Latin name, the plant part, the word select or total, and whether a carrier is present. Those four facts are enough to know what is in the bottle: an extended essential oil, or almost the whole oil soluble part of a root, a fruit, a seed. Other ways of taking scent from plant material are compared in Four Ways to Take Scent from a Flower.
Boiling water selects what can fly.
Compressed CO2 selects what can dissolve.
Two ways of choosing, one ginger root, two smells.