Tuesday, December 13, 2011
Essential oils: Processing techniques
INFUSION: Soaking the plants in vegetable oil, water, alcohol, or other solvent. This is done for products like calendula, St John's Wort, and arnica. (This is something we can do at home, but I do recommend you learn from a skilled practitioner.) This can also be called the maceration method.
ENFLEURAGE: The plant matter is put on a fat layered screen and the fat absorbs the fragrance. They take the saturated fat off the screen and heated with alcohol to create an absolute. This isn't a common process, although jasmine and tuberose are still done this way (which might explain the extreme price of both!). As a note, many sources I've read say that the fats are generally animal fats - tallow or lard - so if you're a vegan, you might want to make sure your essential oils are not processed in this way.
COLD PRESSING: With something like a citrus fruit, the skin is pierced and the fruit is pressed to release the juices. The oils float to the top and are removed from the juice. This can also be called scarification (but put in "essential oils" with this word if you're doing a search because eek!).
WATER BASED METHODS
STEAM DISTILLATION: The plant matter is steamed and the essential oil removed. The water soluble bits are left behind, which we called hydrosols. The plant matter is put into a still and pressurized steam circulates through the material. "Tiny droplets of essential oil evaporate and attach to the steam. The steam which then contains the essential oil, is passed through a cooling system to condense the steam, which forms a liquid from which the essential oil and water is then separated by decantation. The oil forms a layer on the water surface as it does not dissolves in water and hence is separated easily."
WATER DISTILLATION: The plant material is placed in water, then boiled. The water is condensed and cooled down, then the oil - which rises to the top of the container because oil is lighter than water - is removed. The water left over can be called a floral water or hydrosols.
METHODS REQUIRING SOLVENTS
CONCRETE: (This method has replaced enfleurage for the most part.) The plant material is mixed with a solvent - usually hexane - removing waxes, pigments, and aromatic molecules. The solution is filtered, then distilled, and we end up with some waxes and an essential oil - this is the concrete - or resin.
ABSOLUTE: The concrete is treated with alcohol to remove wax, then its vacuum distilled to remove the water.
OLEORESINS: This term can refer to two things. Resin from trees like the myrrh tree or the resin that comes out of the concrete process. I'm referring it to the latter. The resin from the concrete process is treated with alcohol or acetone, and the oils extracted.
Supercritical carbon dioxide at around 31.1˚C and 72.9 atm behaves like a liquid and a gas...sort of. It expands "to fill its container like a gas but with a density like that of a liquid." (Remember that STP or standard temperature and pressure is 25˚C and 1 atm.) So they use this as the solvent instead of water, alcohol, hexane, and so on.
AFTER PROCESSING: RECTIFICATION
Rectification is a redistillation of crude oils intended to remove something unpleasant. In the case of peppermint, it can be done under a vacuum to remove sulfur compounds. For something like eucalyptus, it might be done to increase the amount of something like eucalyptol. For something like patchouli, it's to make the colour less dark, which is apparently offensive to some perfumers.
Which way is the best way to extract oils? I don't know. I know the concept of cold pressed sounds really lovely, but it can result in more furocoumarins in citrus essential oils, which can make them more phototoxic, and a lot of wasted fruits. The heated processing methods don't work for things that can't take the heat, so solvent extraction might be the only way we can get some of our lovelier oils. I am kinda surprised about the enfleurage in a way - vegans really need to be aware of this processing method - and I think the supercritical CO2 is awesome!