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From percolation and infusion to CO2, sifting and pressing: An Extract Primer


With the steamrolling legalization effort of Cannabis sativa, there have been a lot of discussion toward the production of extracts. Here is a little primer of what’s what in the marvelous world of extract production. Each of those will be the focus of a small post with more information.

Solvent extraction

First some basic information. Solvents have a solvation capacity which is usually directly proportional to their energy, or temperature. This means that the higher the temperature of a solvent is, the higher the capacity of the solvent to extract compounds. Secondly, solvents have a polarity, which we will explained in detail here, but to refresh your memory: like likes like, the closer the polarity of a compound is to a solvent the better it will dissolve in it.
Liquid-solid extraction

Maceration is the extraction of a plant material in a solvent, may it be water, hexane, vegetable oil, etc. to extract the compound. This is done at room temperature during a long period to be sure that the compound has time to diffuse.

Closely related there is the infusion where the plant is put in contact with a heated solvent. Camelia sinensis or more simply, tea, is usually consumed in an infusion.

There is also the percolation, which differ from the infusion in the sense that the solvent is passed through the finely divided plant material instead of being left in contact with. Coffea sp. seed powder is more often than not extracted using this method before consumption of the percolate under the name of coffee.

Liquid solid extraction with a gaseous phase

The simplest method that uses a gas as a tool is reflux extraction. In this extraction the solvent that is boiling, hence with its solvation capacity at maximum (the liquid cannot get hotter… or can it…). The vapor are recondensed into the mixture so there is no loss of liquid. This allows the extraction to go on for a very long time without going dry.

There is also a close variant called Soxhlet extraction where the plant material is not directly in the solvent, but in a small basket over it. The condensed vapor are routed through the small basked where it will slowly fill it. Once full the solvent will be flushed back into the distiller part where it will repeat the cycle. This has the advantage of extracting a lot with very little solvent since at every cycle the solvent solvation power is at it’s maximum.

Also very similar to the reflux is the hydrodistillation. This is done with boiling water as the solvent, the steam is then recondensed but not returned back into the distiller it is condensed outside. This technique is pretty useful for non-polar volatile component since they are carried by the steam and since they are non-soluble in water they will produce two phase once condensed.

Liquid-solid extraction under exotic condition

A solvent cannot stay liquid over it’s boiling point, so for example water can’t go over 100 °C (212 °F); its maximum solvatization potential is at this point and can’t go farther. … not! This is only true at atmospheric pressure. If we play a bit with a phase diagram you will notice that if we increase the pressure the boiling point increase accordingly. This bring us to pressurized solvent extraction in which the extraction system is put under pressure to be able to increase the energy in the solvent without turning it into it’s gaseous form, hence increasing it solvatization potential.

By putting the system under enough pressure, you end up able to turn CO2 into a liquid. This is called subcritical CO2 extraction. Carbon dioxide has the characteristic of being a non-polar solvent, thus very good for the extraction of non-polar compounds, and gaseous at room temperature. This makes the solvent removal part of the process very easy since a simple depressurization of the system is enough to remove the solvent. Furthermore, the low temperature of this method is very useful for compounds which would degrade when heated.

If you heat a little bit the liquid CO2 under pressure it will turn supercritical. This is a state of the matter where it can act partly like a solid and partly like a liquid. This is to say that it has the viscosity of a gas but the solvating properties of a liquid. This is very interesting since it allow us to extract a lot more easily. The fact that the matter act as a gas allows it to enter more easily through the nooks and crannies of the material while still being able to extract compounds. Furthermore, as it is the case with subcritical CO2, a simple depressurization will get rid of the solvent in the extract.

Solventless extraction

Solventless extraction uses mechanical or physical principle to remove the component from the plant. While not technically extraction in the strict sense of the term, it is in the larger sense since they separate constituents from the raw material.

Mechanical separation

Mechanical separation, as the name implies, is using mechanical means to remove the compound of interest from the raw material. Pressing is an example of this kind of separation in its simplest form. This usually works best with oily material such as seed which are put under intense pressure. This breaks the cell and liberates the liquid that it contains. This is used a lot in the food industry and is one of the main methods of producing cooking oil such as olive oil or rapeseed oil.

Sifting would be another example of mechanical separation. Some plants, and particularly Cannabis sativa, produce trichomes. Trichomes are small hairs on the surface of the plant. When those hair are glandular they gorge themselves with compounds produced by the plant. These are often visible as shiny, water droplet-like spot on the plant. These “hairs” are brittle and will break away from the plant when rubbed. Sifting them through smalls filters that allow for smalls particles to pass through while keeping bigger one out will concentrate the trichome-containing the product of interest. This is one of the methods used to make hash.

Physical separation

By physical separation we mean using the physical properties of the component to separate them from the plant material. For example, we can use microwave to heat the cells, this heat will in turn vaporize the compounds which can then be re-condensed and recuperated.

Hydrodiffusion is another method in which we apply heat to volatilize the compounds of interest before re-condensing them. In this case hot steam is passed through the plant material. The heat combined with the water solvation capacity carry the compounds with the steam and once the steam is re-condensed there will usually be two non-mixing layer. This is the traditional method used to produce essential oils.

 Secondary processing

Secondary processing is not really about extraction by itself. These are technics used to refine and purify the extracts until we have the wanted composition. While this could be a very long section by itself, I will keep it short by briefly explaining the three most common ones. Each of these three methods have a multitude of sub-methods which allow us to fine tune them to suit our needs.

An example of open column chromatography

An example of open column chromatography

First: Chromatography. We already have touched the topic of chromatography in other posts, so I wont go back in details, but here is the big picture – Chromatography separates compounds by affinity with a substrate and there is a myriad of chromatographic substrate that allow for almost endless possibilities. Using this we can produce crude extract to pure compounds.

Membrane filtration is another method that is gaining a lot of steam in recent years. This method separates compounds according to their size and can be very useful for the separation of smaller compound from oligomer or proteins.

The last one that I want to present is Liquid-liquid extraction. This method, similar in concept to chromatography but in liquid form, uses the difference between the affinity of two liquid to separate compound. Usually this allows for crude, first-step processing. However, there are some technology that use liquid-liquid extraction for further refining, usually those technologies are a form of chromatography.

This (not so) short post should cover all the basic of extraction and what it implies. As times goes, we will try to elaborate a bit more so you can see the advantages and the inconvenience of each of them. Remember that depending on your goal, a combination of technic is usually the way to go.

Feature image of reflux extraction (left) and soxhlet extraction (right).

Hubert Marceau is a chemist and co-founder of Laboratoire PhytoChemia. Laboratoire PhytoChemia is an analytical laboratory located in Québec who offer analytical services in the field of natural product chemistry, including cannabis analysis, and consultancy service in products and extract development. Hubert is a registered member of the Ordre des Chimistes in Québec and is finishing a master degree on natural products extracts.

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September 22, 2015
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