Supercritical CO₂

CO2 is compressed to above 74 bar and held at around 31 to 60 degrees. In this state it is no longer a gas, nor quite a liquid, and it dissolves like a solvent.

The stream of supercritical CO2 passes through the raw material, carrying off the aromatic part. The pressure is then lowered; the CO2 returns to a gas and leaves completely, without leaving any trace.

Adjusting pressure and temperature makes it possible to select the group of compounds to extract — an ability no other method has.

Closer to the fresh material than distillation, but closer is not always better. There are smells people have grown used to in their heat-processed form, and the more faithful version can sound stranger.

The equipment is expensive, so costs are high. The broad extraction range is also a drawback: it pulls in waxes and resins that nobody sometimes wants in the bottle.

CO₂ products are usually labelled clearly because it is a selling point, so the word CO₂ on a label is not suspicious. The question worth asking is whether it was extracted below or above the critical point, because the two regimes give quite different products.

On the nose and in the hand, the easiest sign is texture: the supercritical version is usually thicker and darker than a distilled oil of the same kind, because it carries resins and waxes. Ginger CO₂, for example — thick, dark and noticeably more pungent than distilled ginger, because it captures the gingerols, which are too heavy to travel with steam.

If a bottle labelled CO₂ is thin and clear like an ordinary essential oil, ask again.

Supercritical CO2 extraction was developed from the 1970s, originally for the food industry — decaffeinating coffee is the best-known application. The aromatics industry adopted it later because of equipment costs.

It is the youngest method here, and the only one whose advantages are still being debated among formulators.