Terpenes are ubiquitously found in plants, including cannabis plants. For example, they are found in hops, peppercorns, lavender, juniper, rose, citrus fruits, and even wine grapes. And what do these examples have in common? Well, they all have robust, distinct aromas. Terpenes are the molecules that impart the characteristic odors associated with these various examples.
On cannabis plants, terpenes are predominantly synthesized in epidermal protrusions known as trichomes that are part of the female flower. Trichomes in this area of the plant are often referred to as “goodie bags” because they also contain concentrated levels of cannabinoids, like CBD and THC. Interestingly, cannabinoids have been found to be toxic to their own plant cells, so they are actually contained within a storage cavity or secretory reservoir. The figure above is a cartoon representation of the trichome head structure and its contents.
The compounds produced and contained within trichomes serve an important protective function for cannabis plants. They act as deterrents for harmful insects while encouraging pollination. Additionally, anecdotal evidence and some pre-clinical research suggest terpenes play an important role in the pharmacological efficacy of cannabinoid products. This is commonly known as the “entourage effect”.
Isoprene molecules make up the basic chemical building blocks of terpenes. The number of isoprene molecules concatenated together is the primary differentiating feature between terpenes. Which chemical functional group connected to the joined isoprene molecules is the second differentiating feature. Think of terpenes like tadpoles. Each terpene has a head and a tail. The head is its functional group and the tail is its isoprene units. It is the differences in types of heads and length of tails that give a particular terpene its unique fragrance.
Terpenes can also be classified as lipids, which explains two important chemical properties that affect their purification from other components in hemp. Common examples of lipids include cholesterol, oils, and detergents. And we all know what happens when oil is added to water -- the two liquids stay separated. They don’t mix together because all lipids share the properties of being hydrophobic and nonpolar, meaning they do not mix with polar water molecules. The semi-good news is that lipids are soluble with nonpolar solvents, such as chloroform, hexane, or carbon dioxide. However, these solvents tend to be much more physically and environmentally dangerous than water.
Volatility is another chemical property of terpenes that adds complexity to their isolation. Terpenes from cannabis plants are predominately monoterpenes. Monoterpenes consist of only two joined isoprene units. Volatility and the number of concatenated isoprene units are roughly inversely proportional, so molecules with fewer isoprene units are more volatile. Therefore, the majority of cannabis terpenes are especially volatile and tend to dissipate close to room temperature. Experimental evidence has shown at least a fifty percent loss in volatile terpenes after 3 months under drying conditions. As a result of their high volatility, special consideration and equipment are required to successfully trap these terpenes.
Historically, terpenes have been purified using hydrodistillation (i.e. steam distillation) and solvent extraction both within and outside the cannabis industry. Newer methods for extraction include emission trapping and microwave-assisted extraction. In both of these methods, whole cannabis plants are enclosed in a sealed structure and either gas or microwaves are passed through the chamber, releasing and collecting the terpenes.
One technique that is currently gaining momentum within the cannabis industry is carbon dioxide (CO2) solvent extraction. The technique is similar to supercritical CO2 extraction of cannabinoids, but the temperature and pressure ranges required are lower than with cannabinoids. There are several reasons why this technique has been gaining popularity. Firstly, CO2 is considered a “green” solvent, meaning that it is not toxic to people or the environment. Secondly, gaseous CO2 is readily removed from extraction products making them solventless or lacking in residual solvents. Additionally, CO2 is a nonpolar compound like terpenes, so it has increased solubility with terpenes compared to other solvents. Finally, the physicochemical properties of CO2 make it more tunable for terpene separation than other solvents.
The high volatility of terpenes previously mentioned also affects the appropriate method required to accurately identify and analyze terpene profiles within a laboratory setting. Currently, the two main techniques adopted by the cannabis industry are 1) gas chromatography mass spectrometry (GC-MS) and 2) gas chromatography with flame ionization detector (GC-FID). In both techniques, gas chromatography is used to separate out similar compounds and mass spectrometry or a flame ionization detector is used to identify the compounds and measure their quantities. Both techniques require sophisticated, highly calibrated instrumentation that has long been utilized in the research community.
Written by: Joanna Dunn, Ph.D.
- Andre CM, Hausman JF, Guerriero G. Cannabis sativa: The Plant of the Thousand and One Molecules. Front Plant Sci. 2016.
- Hartsel J, Eades J, Hickory B, Makriyannis A. Chapter Title: Cannabis sativa and Hemp. 2016.
- Jiang Z, Kempinski C, Chappell J. Extraction and Analysis of Terpenes/Terpenoids. Curr Protoc Plant Biol. 2016.
- Kawka E. A Systematic Approach to Developing Terpene Extraction Conditions Utilising Supercritical Carbon Dioxide. Chromatography Today. 2018.