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An overview of extraction terminology, specific types of extraction methodologies, and clarification on some common misconceptions
Extraction—this term can mean different things to different people. It can refer to huge holes dug in the earth to extract minerals (mining) or pulling a tooth at the dentist. Solvent extractions are familiar to scientists and nonscientists alike; the process of making coffee or tea is a solvent extract and you drink the “raw extract” without further refinement or cleanup. Solvent extractions are the type used in the cannabis industry and the term extract is used as a noun for a wide variety of products in the cannabis industry. This column series on extraction begins with an overview of the terminology and attempts to clarify some misconceptions. We also cover more specific types of extraction methodologies and the physical parameters that make them similar or different from one another. Finally, we discuss the trend towards ultra-refinement after the extraction process to focus on isolation of specific compounds and the impact that has on medical and recreational use formulations.
In the cannabis industry, the term extraction usually refers to solvent-based extractions unless the product was made using a physical process such as “kiefing” (removing the trichomes) or rosin-pressing. Examples of this latter type of physical extraction of oils can be seen in other industries, for example, when making olive oil or other seed oils-extreme pressure bursts plant cells and releases the raw oil. Rosin can be pressed from high-quality flower material in a similar way partly because the cannabinoids and terpenes are highly concentrated within the trichomes and are not more evenly distributed throughout the plant itself. When making cannabis rosin, heat is also utilized to help liquify and aid the cannabinoids and terpene mixtures to release from the plant cells. Rosin is the only true solventless cannabis extract. Kief and hash are scientifically not considered extracts because they are only physical separations of the trichomes, which are kept mostly intact. Kief and hash are, however, considered concentrates because the active components are more concentrated than they are in the original plant material.
It is also a common misunderstanding that bubble hash or ice water hash uses water as the solvent. While water is certainly a good solvent in many situations, in this case water is not the solvent because it’s not dissolving the compounds of interest. It’s simply used to help keep the temperature cold and provide a way to agitate the trichomes off the flowers more gently so they can then be more easily filtered out and dried.
Many different solvents can be utilized to extract cannabinoids and terpenes from cannabis plant material. However, for human consumption and high-throughput processing, only a small handful are currently being used at large scale in the cannabis industry: carbon dioxide (CO2), butane or propane, and ethanol.
For cannabinoid extractions, the gas-phase carbon dioxide that we’re often familiar with is not used, nor is it the solid “dry ice” form-it’s the supercritical phase, which means the CO2 looks like a clear liquid and can be pumped and moved in similar ways. The difference is that the supercritical phase occurs at higher temperature and pressure combinations, which changes the polarity of the molecules to be more specific for dissolving cannabinoids. Subcritical CO2 (liquid phase CO2) can also be used for terpene extraction, but that requires somewhat more technical skills for the operator and an extraction vessel with tunable parameters. Separate fractions can also be collected at different pressures that are enriched in certain cannabinoids. For instance, a high tetrahydrocannabinolic acid (THCA) raw extract can be collected that will have different viscosity, boiling, and melting points than if you adjust the collection vessel pressures to different settings that can collect lower-molecular weight mixtures. Adjusting the pressures all the way down to the subcritical or liquid phase will preferentially extract the smaller terpene molecules. While creating a super-critical phase changes the polarity to prefer cannabinoid solubility, extended extraction times and relatively warmer temperatures dissolve many other compounds in the plant that processors often need to remove in subsequent steps. However, the molecular tunability and safety-it’s both nonflammable and generally nontoxic at low levels-and the ease of removing the solvent (reduce the pressure, it drops all the raw extract in your vessel as it transitions towards the liquid and gas phase) is very desirable for safe end products. It can also be recycled, which reduces costs.
Hydrocarbons-Mainly Butane and Propane
Because of their nonpolar nature, almost all hydrocarbon solvents-and there are many that chemists use-are useful for extracting cannabinoids and terpenes. However, the most common chemistry-laboratory solvents aren’t very safe for human consumption or worker safety and they can be expensive and difficult to source. These are the main reasons butane was historically used in clandestine operations; it’s cheap and easily obtainable.
There are two main reasons butane (and also propane) have somewhat bad reputations; one of which is already taken care of with laboratory design approval processes, and the other can be addressed in the regulated market if companies are trustworthy and use high-quality, clean sources for their solvents. First, the outdated “open blasting” method of extraction is very dangerous and while it isn’t performed in licensed, regulated facilities, there are still many people who don’t realize the difference between this method and the much safer methods using qualified technicians in closed-loop systems (CLS). The high flammability of these hydrocarbons do require a “C1D1” facility-that is, a class one, division one safety-rated room to house the equipment-and some jurisdictions have bans on using this method altogether. Secondly, consumers are often afraid of seeing residual amounts of these solvents in concentrates, but as long as the source was food or pharmaceutical grade with no impurities (it’s low-level contaminants such as benzene or xylene that can be concentrated during purging that are worrisome), low parts-per-million (ppm) levels of residual butane or propane are not considered an inhalation hazard to humans. After all, butane lighters are used all the time for combusting cannabis and folks are generally not concerned about inhaling residual butane from lighters. If there is 1000 ppm in a 0.5 g cartridge, that is a very small amount (0.5 mg) and that’s in the entire cartridge.
Butane and propane’s solvent properties can be tuned somewhat by mixing them in various ratios and utilizing different temperatures during extraction. The low cost and relatively short extraction times are desirable for high-throughput processing. Terpenes are quite soluble as well, but there are always challenges in balancing complete solvent removal and terpene retention because of the volatility of terpenes, which makes them easy to lose in post-processing.
More recently, ethanol-based cannabis extractions have increased dramatically. One huge advantage is the rapid extraction times and high throughput for large capacities as well as the relative safety of residual levels in finished products. Additionally, the molecular properties are such that even at very cold temperatures, it is still a good solvent for cannabinoids, while it is not a good solvent for fats and lipids, so a winterization step is not necessary. Scientists outside the cannabis realm would call “winterization” a phase separation or precipitation in which the mixture of raw extract dissolved in ethanol is chilled and the fats and lipids solidify and can be physically separated from the liquid cannabinoid–ethanol solution. This is commonly used on raw extract that may come from various solvent extractions. Ethanol in large amounts is also flammable, but like the other hydrocarbons, it can be performed quite safely in closed systems that aren’t complicated to operate.
While the “big 3” solvents already discussed are the main ones currently being used, there are a few other solvents out there. Let’s take a closer look at a few of them.
RSO is an acronym for “Rick Simpson Oil,” named after the man (Rick Simpson) who taught medical patients how to make concentrated extracts at home. The term RSO doesn’t refer to a specific solvent, but describes the full spectrum extracts that don’t utilize any further cleanup steps after removing the solvent, and they’re usually taken orally. Originally, naphtha was used as the solvent, but the toxic properties of naphtha and difficulty in effectively purging it quickly made it obsolete. Isopropyl alcohol (rubbing alcohol [IPA]) can be used as well as high-proof ethanol because these are all liquids at room temperature and one doesn’t need specialized equipment to utilize them. IPA is particularly difficult to remove completely from finished products given its higher boiling point. It also dissolves many of the water-soluble compounds that make the finished product taste harsh.
One lesser-known solvent is 1,1,1,2-tetrafluoroethane, known as “R134a” as well as many other trade names. It is used mainly as a refrigerant and to a lesser extent as a botanical extraction solvent. THCA does not have high solubility in this solvent, so decarboxylation of the plant material before extraction is best to increase cannabinoid recovery. R134a is currently being used by a small number of extraction companies where its nonflammability and relatively low cost are used as selling points. The extraction equipment is similar to other closed-loop pressurized systems because R134a is a gas at atmospheric conditions. It’s unclear if this will gain traction as a solvent moving forward or continue to be used only by a small number of manufacturers.
We all understand at this point that the cannabis plant synthesizes the acidic version of all the cannabinoids, which can then further degrade naturally or be sped up with something like heating with a lighter. While studies are beginning to focus more on these acidic versions for use in medical formulations, decarboxylated or activated cannabinoids are usually targeted for large-scale production for a couple of reasons. First, if the concentrate will be used for edibles or oral ingestion formulations, THCA is not psychoactive. Additionally, if distillation is used as a final step to post-processing, the heat involved converts all the cannabinoids to their decarboxylated form. A smaller consideration is the viscosity difference between the acidic and decarboxylated forms-but vape cartridge processors who have a highly standardized process are aware of this and may desire one form or the other depending on their desired viscosity range. The acid forms may or may not be decarboxylated during the extraction process-decarboxylation is highly dependent on the method of extraction. Heat is the fastest and easiest way to remove the CO2 group, so colder extraction methods such as ethanol or hydrocarbons won’t cause much decarboxylation during the extraction itself. Often, the majority of the decarboxylation occurs during the post-processing steps.
Relatedly, sometimes the fact that CO2 is used as a solvent and the fact that a CO2 group is lost during activation of THCA is confusing to nonscientists. While there will be some complicated chemical equilibrium between the two during the actual extraction, it’s really the elevated pressures combined with moderate heat that are the driving force behind any decarboxylation during CO2-based extractions. Most often, the decarboxylation rate after CO2 extraction is ~25–60% in the raw extract and some sort of additional processing needs to occur if 100% conversion is desired.
This might be obvious to chemists or folks who work in cannabis processing, but there are a lot of people who don’t know the difference between distillation and extraction. Scientifically, they are different in the following ways: extraction is a process that uses some sort of solvent to dissolve desired compounds. The “raw extract” that results directly from an extraction will vary widely depending on the solvent and method. Distillation uses boiling point differences (and relatedly molecular weight or size of molecules) as a way to separate a mixture. The end product is a high-percentage cannabinoid compound that is a glassy solid at room temperature and almost clear to yellowish amber in color. Distillates are related to extracts because it’s necessary to create an extract first before the distillation step. The method and solvent can vary, but the feedstock or starting point of the distillation process is going to be a partially refined oil with already relatively high concentrations of cannabinoids that was made via some extraction.
Distillation alone is not a way to separate the cannabinoids into individual isolated compounds. The boiling points of the cannabinoids are too similar to one another to make distillation a viable option because of molecular degradation as well as time, energy, and efficiency issues. Pure, unformulated distillate does not have any residual solvents or terpenes because these compounds have lower boiling points than the cannabinoids and are separated out before or during the distillation process.
The unscientific language of classifying different strains or chemovars as “indica” or “sativa” will not be covered in this column, but these terms are particularly incorrect to use when referring to distillate product. The molecular profile of distillates is not very complex and the vast majority (~80–95%) are cannabinoids. While there can certainly be differences in the cannabinoid ratio profiles, any other differences in the biomolecular profiles have essentially been removed. There are processors that have the scientific wherewithal to isolate the cannabis terpenes from each batch in-house before they’re lost in any post-processing. There are a variety of ways to accomplish terpene isolation that won’t be covered here. Terpenes can be formulated back in with finished distillate, either the native cannabis-derived versions or mixtures formulated to mimic certain strains from other botanical sources. However, given the complicated molecular profile of the cannabis plant, and the fact that different strains don’t have scientifically defined chemical profiles, it’s arguable at what point the strain name should be used to label a highly refined mixture of cannabis-derived concentrate.
Related to highly-refined extracts and distillate, color or clarity are often used within retail and in marketing to describe products. Often, the point being made is that “clear” or lighter in color is “better.” Subsequent editions to this column series will address the “full-spectrum” or “whole-plant” formulations versus high refinement in more detail, but there is overwhelming scientific evidence that the more pure a cannabis formulation is, and the less there are of other compounds naturally found in cannabis, the intensity and experience of the therapeutic effect or high is different and shorter lasting. So, most patients and educated consumers recognize that while stripping away all the other compounds and focusing only on the cannabinoids will clarify the color of the final product, the physical or mental effect may not be what they’re seeking.
Pure, white, powdery cannabidiol (CBD) is a common sight if one searches the internet to buy bulk CBD. THCA can also form a white or pale yellow powder when fully purified and isolated to 98–100% purity. These types of end products are only made with even more refining past the extract or distillate stage using chromatographic separations. Large-scale chromatographic separations are commonly performed in chemistry, pharmaceutical, and biology-related industries when highly purified compounds are desired. This is the only method available to separate and isolate individual cannabinoids or any of the other various biomolecules in an extract or mixture. Many CBD products available online or in nonregulated cannabis retail outlets are made using CBD isolate because it helps ensure the concentration of THC is below the federally legal limit of 0.3% by weight and makes formulations more reliably reproducible. This also means no other biomolecule was part of the CBD addition and the entourage effect is not relevant unless very careful and complicated processing and formulation is done.
While some people may not think it matters much if a budtender or salesperson interchanges “extract” with “distillate” in conversation, scientifically they mean different things. As the public’s understanding of these products grows, it’s important that industry representatives use correct language and represent products correctly to prevent perpetuating misconceptions or stereotypes. There is still the issue of not necessarily having clear definitions for end products (for example, what exactly is the difference between wax, budder, and crumble and where do you draw the distinction?), but that is a conversation for another day. Stay tuned for more in-depth conversations on extraction-related topics in future installments of “Extraction Science.”
Dr. Amber Wise is currently the Science Director at Medicine Creek Analytics, a certified cannabis testing laboratory in Washington state. She was previously the Science Director at Avitas, a licensed cannabis grower and processor in Washington and Oregon. Direct correspondence to: firstname.lastname@example.org
A. Wise, Cannabis Science and Technology 2(1), 20-26 (2019).