The future of cannabis is extraction. For the past five years, the popularity of cannabis concentrates has grown to rival flowers and pre-rolls. Indeed, a 2018 report from Arcview Market Research and BDS Analytics speculates that concentrates will pull in $8.4 billion by 2022, just shy of the $8.5 billion expected from flower sales.
But, cannabis concentrates are for more than just smoking. Extracts can be infused into foods, used to fill capsules, and isolated to create entirely new pharmaceutical and nutraceutical drugs. And yet, regardless of whether it’s for recreational, food, or medical purposes, all extraction starts the same way: with a solvent.
How to Find the Best Cannabis Extraction Solvent
Solvents are the foundation of extraction. A solvent is a chemical substance that can dissolve other compounds. In the world of cannabis extraction, technicians use solvents to wash oily cannabis resin away from unwanted plant material. Cannabis resin is stored in external structures called trichomes, which contain the majority of the plant’s active chemical constituents.
The cannabis industry uses three primary solvents:
- Supercritical CO2*
- Hydrocarbons (butane and propane extraction)
All of these solvents work particularly well for cannabis extraction. However, no solvent is a perfect match. Each solvent comes with its own advantages and disadvantages. The type of product created, the size of the production facility and the ability to meet safety requirements are all factors that may influence the type of solvent chosen by a lab.
Here’s how the three compare.
Ethanol is an unusual solvent. What makes it unusual is it’s chemical structure; ethanol molecules feature both polar and non-polar regions, which gives ethanol the unique ability to both dissolve in water, but also do dissolve fats and oils. These qualities make ethanol a particularly strong solvent, which can dissolve a wide variety of substances.
Why Choose Ethanol Extraction?
Ethanol extraction is very clean, safe, and uses cold temperatures that prevent the degradation of cannabinoids and terpenes. It is a preferred solvent for many large-scale extraction professionals, thanks to its safety profile and overall efficiency. Overall, ethanol is rapidly becoming an industry favorite, for the following reasons:
- Ethanol is liquid at room temperature, meaning that there is no need to use high-pressures during extraction.
- Ethanol processing happens at cold temperatures, which prevents cannabinoid and terpene degradation.
- Ethanol is a stronger solvent than supercritical CO2, so ethanol will extract cannabinoids and terpenes more efficiently and yields a product with better chemical diversity.
- Ethanol is generally recognized as safe by the FDA and is one of the safest consumable solvents.
All of these factors make ethanol one of the most popular solvents for large-scale production operations.
Cons of Ethanol Extraction
Cannabis compounds like cannabinoids and terpenes are mostly non-polar, which means that they are extracted best by non-polar solvents. Ethanol, however, is considered polar overall. So, although it can dissolve fats and oils, it will also interact with polar solutes. When you extract with ethanol, you not only capture cannabinoids, but you capture any compound that is also soluble in water, including chlorophyll. Bottom line? Ethanol is a non-specific solvent; it will pull out far more than is desired.
Before extraction, the ethanol needs to be chilled to prevent the polar molecule from dissolving water-soluble molecules, a process that requires energy. At cold temperatures, ethanol will successfully separate fat-soluble molecules like cannabinoids while leaving water-soluble molecules behind.
Unlike CO2 extractions, ethanol may also need to be “purged” from the end product via decarboxylation or further distillation. However, the amount of post-processing required depends dramatically on the type of technology you are using. But, thankfully, most excess ethanol is removed during the initial extraction. The rest is usually taken care of by the decarboxylation or distillation that happens afterward.
Traditionally, ethanol extraction required more post-processing than any other type of extraction. It required additional filtration and chilling to separate the desired cannabinoid-rich oil from the unwanted fats, waxes, and chlorophyll. This purification process was tedious, which is why supercritical CO2 and hydrocarbon extraction were the most popular choices for the last decade.
Now, however, the story is beginning to change. Thanks to technological advancements, post-processing for ethanol extractions has become much more manageable. As long as you have the right extraction equipment from the beginning, it is possible to get a clean, full-spectrum distillate from an ethanol wash with limited post-processing needs.
As with other solvents, however, the required amount of post-processing will vary depending on what kind of technology you have in the lab. Further filtration, distillation, or other processing methods may be necessary with some ethanol extraction equipment. Additional processing may also be needed if you intend to create an isolate rather than a distillate—the latter contains a greater diversity of cannabinoids.
Supercritical CO2 Extraction
Supercritical CO2 extraction is one of the most popular types of extraction in the cannabis industry. It’s also a form of extraction used widely in other industries. Coffee, for example, is decaffeinated via supercritical CO2 extraction. Dry cleaners sometimes use the same process; in the supercritical phase, CO2 can pass through clothing to remove stains without wetting the material.
This type of extraction takes advantage of some unique properties of carbon dioxide: it changes into a supercritical fluid at low temperatures and pressures relative to other substances. Typically, chemical substances can exist as solids, liquids, or gasses. When the pressure and temperatures are high enough, however, substances can move into a “supercritical phase,” which is akin to a half-way point between a liquid and a gas.
A supercritical fluid behaves like a gas and a liquid simultaneously. It can pass through liquids like a gas, but still retains some of the solvent properties of a liquid. Thus, supercritical fluids often make better solvents than pure liquid or “subcritical” forms of CO2.
Technicians perform supercritical CO2 extraction in a closed-loop system, meaning that the CO2 is recaptured and recycled after each pass, meaning that excess CO2 does not escape into the environment. For this reason, supercritical CO2 is considered a green technology.
Why Choose Supercritical CO2 Extraction?
Like ethanol, supercritical CO2 extraction is one of the greenest types of extraction available. Since CO2 is recaptured in a closed-loop system, there is little waste, so extractions can run again and again using the same solvent.
Unlike hydrocarbons and ethanol, CO2 does not need to be “purged” to remove residual solvent. For this reason, many consumers view CO2 as a safe option compared to other solvents. Although the extraction process itself removes and recaptures most of the solvent used in the wash.
Further, CO2 is still substantially less expensive than hydrocarbon extraction, which uses potentially explosive solvents like butane. Hydrocarbon extraction facilities require special safety precautions, like an explosion-proof room. Overall, CO2 is safer for workers and consumers alike.
Cons of Supercritical CO2 Extraction
Perhaps the biggest drawback of supercritical CO2 is the lack of chemical diversity in the end concentrate. In a 2017 study, researchers from Washington state tested six samples of various cannabis cultivars before and after extraction using supercritical CO2 technology. They found that the chemical composition of the concentrate was significantly different than the chemical composition of the original flower.
The supercritical fluid was more likely to extract sesquiterpenes, like alpha-humulene and beta-caryophyllene. Monoterpenes, like myrcene and pinene, fared differently; CO2 technologies did not extract these compounds well. As such, it can be difficult to achieve an honest full-spectrum effect with this particular solvent. Terpenes, however, can be added back into the final product to make it more similar to the original botanical.
Supercritical CO2 is also pressurized, which can be dangerous for extraction professionals. Anytime pressure is involved, the greater the safety regulations. Manufacturers of CO2 extraction technology must comply with safety standards appropriate for the pressurized fluid. Supercritical CO2 labs may also require different licensing than ethanol extraction labs due to the higher pressures.
Supercritical CO2 Post-Processing
Supercritical CO2 may be one of the most common solvents in the industry, but it is also a fairly limited solvent. CO2 is a fairly weak non-polar solvent, meaning that it will capture non-polar components like waxes and lipids along with cannabinoids. The end result is a cloudy mixture that is undesirable to most consumers. As such, most CO2 concentrates need to be further processed to separate active chemical compounds away from undesired waxes, fats, and other undesirable molecules.
Because supercritical CO2 is a weak solvent compared to, say, hydrocarbons, more post-processing may be necessary. So, technicians will often use a second solvent to further separate the desired cannabinoids and terpenes from fats. Ethanol is the solvent of choice for this second extraction.
After the initial round of supercritical CO2 extraction, the resulting mixture is then mixed with ethanol and chilled to very low temperatures. After several hours, the mixture will begin to separate into a layer of fats and waxes and a layer of oil. From there, extraction professionals will use a filter to separate the cannabinoid-rich extract from the fatty byproducts. The resulting extract would be as full-spectrum as this particular process allows.
If you are aiming for a distillate or cannabinoid isolate, you will need to continue processing. Distillation takes the purification process one step further. During fractional distillation, extraction professionals use specially designed distillation tubes to further separate chemical compounds with different boiling points.
As the extract is heated, compounds with lower boiling points will evaporate first. Then, technicians collect their vapors into another container. At different temperatures, different substances can be filtered and collected. This allows for the continued purification of different compounds. Fractional distillation is also used to further purify ethanol and hydrocarbon extractions. So, this technology is not limited to supercritical extractions.
Hydrocarbons are no strangers to the botanical extraction industry. Petroleum products like butane, propane, and hexane are commonly used in all types of botanical extraction. Hexane, for example, is often used to extract oils from seeds, making products like canola and soybean oil.
Unlike ethanol, hydrocarbon molecules are non-polar, consisting of hydrogen and carbon atoms. This makes hydrocarbons fairly common choices for dissolving cannabinoids, which are non-polar themselves. As the old adage goes, “like dissolves like.” Both cannabinoids and hydrocarbon solvents are non-polar, which makes the two prime partners when it comes to extraction.
Common hydrocarbons used in the cannabis industry include:
Butane, of course, was the first solvent of choice among early industry professionals. Butane extraction, believe it or not, is what ultimately inspired the demand for butane hash oil (BHO). While the first butane extractions were quite crude, professional extractors now use closed-loop systems to ensure that the majority of the solvent used is recaptured during the concentration process.
Why Choose Butane or Propane Extraction?
Hydrocarbon solvents are quite versatile and are often preferred by small-scale, artisanal extractors. Hydrocarbon solvents and technology is often considered the best at extracting both cannabinoids and the aromatic molecules in cannabis. Unlike other forms of extraction, it’s also exceptional for extracting resin from fresh, flash-frozen flower.
Both n-butane and propane have low boiling points, and thus perform extractions at low temperatures. Butane boils at below the freezing point of water, a mere 30.2 F (-1 C). Propane boils even colder, at -43.6 F (-42 C). These cold temperatures prevent heat-related degeneration of cannabinoids and terpenes. As such, butane and propane extraction will preserve much of the cannabinoid and terpene concentrations in the final product. Other hydrocarbons, like hexane, have a high boiling point and risk denaturing heat-sensitive cannabis compounds.
The Cons of Propane and Butane Extraction
Hydrocarbon extraction is expensive and precarious. Hydrocarbons are highly flammable, which means that hydrocarbon extraction facilities must comply with different safety regulations than those required for other solvents. In some places, like Canada, hydrocarbon extraction is banned due to safety concerns.
As petroleum products, hydrocarbon solvents are considered the least sustainable of all solvents common to cannabis extraction. Hydrocarbon solvents are byproducts of mining and refining oil, which is the single greatest contributor to anthropogenic (human-made) greenhouse gasses.
By the end of the first processing round, excess solvent needs to be removed from the concentrate. But, the type of purging required depends on the designed end product. Whipping the concentrate after collection, for example, will release excess hydrocarbon over a period of a few hours.
This whipped product will produce a softer concentrate that’s often called a “wax” or a “budder.” Extraction professionals hoping to create shatter, however, may prefer to “purge” residual solvent from the concentrate by placing the end product in a vacuum oven. The vacuum will pull away any solvent that may remain in the concentrate.
Hydrocarbon solvents pull out fewer fats and oils than supercritical CO2 and are not winterized as frequently. It is possible, however, to further process butane and propane extracts to create products with higher cannabinoid or terpene purity.
*Some professionals refer to supercritical CO2 as “solventless.” But, this is not the case. In chemistry, a “solvent” is defined as whichever solid, liquid, or gas is present in the highest amounts in a mixture.