By Ezra Pryor
As Chairperson of the Cannabis Chemistry Subdivision at the American Chemical Society and as President of EZ Chem Consultancy, Inc., the question I am asked most often is “what is the perfect solvent for extraction?” People are hoping to hear that the solvent they are using is perfect. However, there is no perfect solvent.
Central to this issue is the fallacy of a one-step extraction method. In no case is extraction with the use of a chemical solvents a single step (except for a tincture of something into ethanol or glycerin). From Dr. Otto Stitcher’s paper “Natural Product Isolation,” botanical extraction can be broken into five distinct steps:
(i) desorption of the compound from the active sites of the matrix;
(ii) diffusion into the matrix itself;
(iii) solubilisation of the analyte in the extractant;
(iv) diffusion of the compound in the extractant;
(v) collection of the extracted solutes.
Additionally, Dr. Stitcher states, “Ideally, an extraction process should be exhaustive with respect to the constituents to be analysed or isolated, rapid, simple, inexpensive, and—at least for routine analysis—amenable for automation.”
Because not all of us are scientists, what do Dr. Stitcher’s guidelines mean regarding cannabis extraction? As applied to cannabis extraction, it means:
(i) Cannabinoids diffuse from within the trichome;
(ii) the solvent freely travels into the plant material;
(iii) cannabinoids dissolve in the solvent;
(iv) cannabinoids spread out in the solvent;
(v) cannabinoids can be collected from the solvent.
Furthermore, he goes on to state that in the best case scenario, the process should be complete, fast, easy, cheap and something that can be done by a machine.
Dr. Stitcher was speaking specifically about the isolation of individual components. He would take a plant product beyond getting an acceptable extract; his goal was to purify to the Nth degree a component of interest. (Dr. Stitcher’s recommendations for complete purification can be found on page 538 in his paper here.
In any case, though, we are asking a lot of the solvent. Additional considerations for the modern age include the safety of the process and making the end product fit for human consumption in small amounts. Starting with a “safer” solvent is ideal because some extract manufacturers forgo residual solvent testing when allowed by law (though not recommended).
So, what is a perfect solvent or extraction process? It should be fast, easy, cheap, exhaustive (complete extraction), automatable, safe for the processor and safe for consumption in small amounts. This is a lot to ask of a solvent or even a solvent mixture, and herein lies the problem. Selectivity might be another factor to consider.
CO2 extraction is cheap, automatable, exhaustive, safe to consume in even more than small quantities; but it is not easy and it is not fast. A CO2 extraction can take 8–24 hours and require precise parameters to obtain acceptable selectivity. It also needs extremely high pressures to be useful, leading to exceptionally high equipment costs. I rate CO2 extraction 4 out of 6 stars.
People seem most attached to using butane and other small hydrocarbons as solvents. This article will consider closed-loop systems only. Blasting is not safe. Butane can make quick work of an extraction and is exhaustive and automatable, but pure butane is not cheap, and the explosive nature of gaseous butane means it is not safe outside of a class one, division one building and the whole process takes place in a fume hood (in which case you have exchanged easy for safe). Butane and its shorter-chain cousin, propane, get 3 out of 6 stars.
Alcohol is safe for human consumption in small amounts (as long as it is ethanol). It is flammable but not explosive, so it is generally safe for the processor. It is automatable, exhaustive, easy, but not fast (especially running chilled ethanol), nor is it in the least bit cheap thanks to the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). Now, for those who think “isopropanol is good and cheap,” yes it is, but you have just exchanged safe for cheap, so there is no net gain. Ethanol and its more poisonous cousins—methanol and isopropanol—get 4 of 6 stars.
Water can be used to extract cannabis, although the practice is not common in the US. In Europe, cannabis tea is widely used, but it is more of an infusion because the end product is not a concentrate.
Olive and vegetable oils are effective at making infusions, but again, the oil may not easily be removed to make a concentrate. Instead the end product is infused oil. The same goes for milk and butter.
What about naptha? Naptha is a petroleum distillate, a brain toxin and not a single substance, but a mixture of chemicals. It is a flammable liquid that contains various components with various boiling points, and its composition may vary greatly—it may be heavy, light or anywhere inbetween. Naptha is dangerous to humans at very low levels and is difficult to remove from an extract due to the various boiling points of its assorted toxic components. Extracts made with naphtha should be thrown away. Naphtha will extract cannabinoids and get you stoned but so will chloroform—as it destroys brain cells. Consuming products made with naptha is not worth the risk, because neurotoxins can damage health.
For those who are still not convinced, Romano and Hazekamp’s paper reports on various extraction methods, which you can read here. It establishes that even with Rick Simpson’s instructions followed to the letter, naphtha persists in the extract at unsafe levels for human consumption.
Cannabis extraction is a complex issue with no easy answers, except to make safety the highest priority. Lawmakers, unfortunately, seem to always find an easy answer, such as banning extracts—no matter how misguided or ill-informed their rationale. So what can we expect from new laws as they emerge?
Lawmakers will likely put heavy restrictions or taxes on volatile, organic solvents, including butane and propane. Some states may make the use of butane illegal or legal only with a hefty licensing fee. CO2 may be less restricted because the process and product are safer. Tinctures and alcohol extractions may eventually be overseen by the ATF, which doesn’t seem likely in the near future because is a federal entity.
Many ill-informed laws against extraction may be unenforceable with regard to personal use, especially in the production of tinctures and infused oils, because the end result is not a concentrate. We may see the hot press or “rosintek” method grow in popularity because it would not be affected by growing restrictions on the use of solvents to make extracts.
In summary, there is no such thing as a perfect solvent. Each solvent has advantages and disadvantages. Some are downright bad ideas, and at least one of those ideas is, unfortunately, popular. Hopefully this will help to weigh the risks against the benefits.
Lastly, help us continue this discussion by becoming a member of the American Chemical Society’s Cannabis Chemistry Subdivision. This allows participating in a monthly discussion of cannabis science at the International Cannabis Journal Club and at the ACS national meeting in San Francisco, where there will be symposiums and workshops on cannabis extraction. Learn more at dchas.org/CANN