CO2 extract

The raw material is dried and milled, then loaded into a pressure vessel. CO2 is pumped in, compressed and warmed until it passes the critical point. In this state its density is close to that of a liquid, so it dissolves aromatic oils; its low viscosity and high diffusivity let it penetrate the material.

The loaded CO2 flows through one or more separators. There the pressure drops, the CO2 loses its solvent power and the extract settles out. The CO2 is recompressed and returned to the start of the circuit. With several separators held at different pressures, the operator can drop plant waxes in the first vessel and collect the aromatic fraction in the next.

The solvent power of CO2 rises with its density. The composition of the extract is therefore chosen by setting pressure and temperature.

Flavex, a German producer, states that it uses CO2 of volcanic origin and works at pressures up to 500 bar.

CO2 extract labels usually carry one of two words: select (or selective) and total. The words describe the pressure used and the range of compounds taken out. They are not quality grades.

A select extract is made at lower pressure. According to the review by Capuzzo and colleagues (2013), the volatile fraction is usually extracted at about 90 to 200 bar and 35 to 55 °C. The product is a mobile liquid, close in composition to the essential oil of the same species, often with extra heavier molecules that steam carries poorly.

A total extract is made at higher pressure, around 250 to 350 bar. At that density CO2 also dissolves waxes, fats and pigments such as carotenoids. The result is often thick or sets on cooling, is deeply coloured, and smells closer to the raw material than to the essential oil. Flavex compares select extracts with steam distillates and total extracts with hexane extracts.

When CO2 is kept liquid below its critical point, usually colder and at lower pressure, the process is called subcritical extraction. Liquid CO2 is a weaker solvent, so it takes mainly light compounds and carries less wax.

Hops are a well studied case. One comparative study found that subcritical CO2 favoured monoterpenes and alpha acids, while supercritical conditions took more of the less volatile compounds. On a label the word subcritical is the supplier's own; there is no shared convention on the pressure it implies.

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 compounds poorly. A CO2 extract therefore holds none of the plant's water-soluble fraction.

Compared with the essential oil from the same source, a CO2 extract carries fewer by-products of heat and hydrolysis. In ginger, the CO2 extract keeps the gingerols, pungent compounds that do not travel with steam. In black pepper, a total extract carries piperine, while the distilled oil is mostly terpenes.

When the pressure is released, all the CO2 leaves at room temperature, so the extract has no organic solvent residue in the sense that a hexane extract might. That does not make the extract harmless. IFRA limits and cosmetic rules apply by composition, as for the essential oil of the same species, and a total extract may be richer in sensitising compounds than the oil.

Rosemary extract rich in carnosic acid, made with supercritical CO2 or with acetone or ethanol, is authorised in the EU as a food antioxidant under the code E 392 (Regulation (EU) 231/2012).

A complete label gives the Latin name, the plant part, the word select or total, and whether a carrier is present. Many total extracts are thick enough that the producer pre-dilutes them in vegetable oil. Flavex, for example, sells a total hop extract standardised with sunflower oil. With a carrier present, the share of aromatic compounds is lower than in the neat extract.

A total extract may solidify in the cold and need gentle warming before weighing. Some leave a waxy deposit in alcohol. To obtain a product fully soluble in alcohol, the producer washes the extract with ethanol, chills and filters it, and removes the ethanol; ISO 9235 calls the result an absolute and states that absolutes can be made from supercritical fluid extracts.

An essential oil holds only what travels with steam, or what is pressed from citrus peel. A select CO2 extract is close to the essential oil but a little broader. A total CO2 extract is close to a solvent-made spice oleoresin, since both carry taste compounds and pigments. Concretes and resinoids are solvent extracts too, but their solvents are hexane, ethanol or similar liquids that must be distilled off afterwards.