Subcritical CO2 Extraction of Terpenes

Carbon dioxide (CO₂) extraction is considered a superior method of extracting compounds from the cannabis plant. It’s environmentally friendly and non-toxic, as it allows cannabinoids, terpenes, and other compounds to be extracted from the cannabis plant while leaving no traces of solvent behind.

CO₂ extraction involves the use of pressurized carbon dioxide to draw out specific phytochemicals from the cannabis plant. At certain temperatures and pressure levels, CO₂ can act like a solvent without the dangers that can come with solvents. While it is initially the most expensive method of extraction, it’s also widely considered the safest and greenest commercial plant extraction method available.

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Supercritical CO₂ extractions are more often discussed, but this is just one method of CO₂ extraction available. There are other extraction methods available including subcritical CO₂ extractions.

With the subcritical method, CO₂ is kept below its critical temperature and pressure but above conditions where it reverts to gas (see image). This liquid CO₂ is sent through cannabis plant material where it dissolves specific phytochemicals. The liquid is then depressurized to a gaseous form, removing the CO₂ gas from the terpenes and other desired compounds. The gas liquefies again after being passed through a condenser and is recycled to be used again.

With the CO₂ pressure and temperature lowered to a subcritical state, the more volatile compounds are less likely to be lost. Since temperature and pressure are tunable, extractors can fractionate the process with subcritical and supercritical extractions that capture most molecules. The result is a product that more closely resembles the original plant material.

This method of extraction takes longer to complete and produces smaller yields compared to supercritical CO₂ extraction. This is because it has lower solvency power and, therefore, its gentler conditions leave behind waxes and most of the cannabinoids. That said, subcritical CO₂ extraction is optimal for terpenes, essential oils, and other volatile chemicals in the cannabis plant. While supercritical extraction may be able to produce higher yields, the higher pressure and temperatures used can damage thermally labile chemicals including terpenes.

For this reason, subcritical CO₂ extraction can be a more attractive way to separate terpenes and other volatiles from the cannabis plant, as it’s more delicate on these sensitive compounds. One study illustrated that supercritical CO₂ effectively eliminated monoterpenes while concentrating monoterpene alcohols and sesquiterpenes. [1]

Although supercritical has been the more commonly-known method of CO₂ extraction, subcritical CO₂ extraction has become increasingly popular for preserving terpenes, particularly in terms of developing full-spectrum cannabis products.

Image source: Eugenio Cuppone from Pixabay

Reference:

1. Sexton M, et al. Evaluation of cannabinoid and terpenoid content: cannabis flower compared to supercritical CO₂concentrate. Planta Med. 2018; [Impact Factor: 3.352; Times Cited: 24 (Semantic Scholar)]