Extraction can be simply defined as the removal of soluble material from an insoluble residue, either liquid or solid, by treatment with a liquid solvent. The extraction process itself is well defined, documented and relatively straightforward. In fact, plant extraction has been around for centuries and methods have been developed to “remove” and separate certain key elements from the plant. However, with cannabis, this “simple” process can become quite complex indeed.
I like to use an analogy of a bicycle ride. As the crow flies, your bicycle journey from point A to point B may seem like a straight, unobstructed path from start to finish, passing by clearly defined check points along the way. The reality, however, is that once you embark upon your journey, you may find steep hills you hadn’t anticipated, come across a ravine you must circumnavigate, or get bogged down by an extreme turn in the weather. That’s how it is with extracting cannabis. It can be a very bumpy ride.
Sticking with the bicycle analogy, you can also choose which type of bicycle you need for your journey. Perhaps you have singular, clear cut goal and choose a sleek, lightweight road bike built for speed. Or, you may opt for flexibility and want to gear up for a more arduous journey and choose a mountain-style bicycle to be able to address any obstacle in your path.
In solvent based extractions, the cannabis (trichome heads) dissolves in the liquid solvent which is separated from the plant material and evaporated, leaving a pure resin extract with as little residual solvent as possible. Ethanol is a well-documented alcohol-based extraction solvent. Butane and propane are also used as extraction solvents, each with pros and cons based on the component being extracted and the starting material.
There are also some very basic solvent-free extraction methods, sometimes referred to as mechanical extractions, including hash, dry sift, bubble hash and rosin. For example, kief can be separated from cannabis buds simply by grinding and sieving. With traditional hash, the goal is to separate the trichromes from the plant material by either taking frozen cannabis buds and breaking them into smaller and smaller parts over a screen or using ice water to separate the trichromes from the bud. After drying they can then be pressed in a hash block.
What is a supercritical fluid?
A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be “fine-tuned”. Carbon dioxide and water are the most commonly used supercritical fluids, while ethane, propane, ethylene, propylene, methanol, ethanol and acetone possess supercritical properties as well.
Ideally, an extraction method should be rapid, simple and inexpensive; should give quantitative recovery of analytes without loss or degradation; should yield a solution of the analyte that is sufficiently concentrated to permit the final measurement to be made without the need for concentration; and should generate little or no laboratory wastes that have to be disposed of. Carbon dioxide in supercritical mode fits the requirements nicely. CO2 is easy to remove simply by reducing the pressure, leaving almost no trace. It is environmentally benign ̶ non-toxic, non-flammable, odorless, tasteless, inert, and inexpensive. CO2 has been commonly used in decaffeination processes and more recently shown to be an effective supercritical solvent for extracting botanicals.
Finding the sweet spot (aka the optimum extraction method) that will deliver the highest yield of pure compound with the most efficient extraction process, whichever compound you are focusing on, is the holy grail. Most importantly, the extraction process must be performed correctly to produce a safe product for human use, and the results should be confirmed independently with proper analytical testing. In future blogs, I’ll spend more time on the supercritical extraction process itself.