This topic was a little bit like understanding football for me. Someone could explain it fully, I would get it (mostly) in that moment, and then it would become really blurry and confusing in my memory afterwards. I have been working with these materials for a few years now, and I finally understand them. So, now let me try to explain it all to you.
The most straightforward of this aromatic group of materials is the essential oils. There are several ways to make essential oils, the most common of which is steam distillation. This means that steam is passed through a hopper containing raw plant material, causing the plants to release their aromatic compounds, which rise with the steam. As the combined steam and plant compounds are cooled in another tube or chamber, they separate into water and oil. The water contains the water-soluble (hydrophilic) parts of the plant, which is why the remaining water is called a floral water or a hydrosol. The oil-soluble aromatic compounds rise to the top in a separate layer, which can be decanted off, and is then called the essential oil of that plant. This same procedure is used in hydro-distillation, where the plant material is actually submerged in water, and in hydro-diffusion, where steam is forced in from the top, rather than passed through the plants from below. Not all botanicals can withstand this high-heat method of extracting essentials oils, which is why the other methods exist.
Citrus oils, for example, are obtained by expression. This means that the peels are first pricked all over to puncture the cells containing the oils, or soaked in warm water, and then mechanically pressed to squeeze out the essential oil. Certain citrus peels are obtained in bulk as a byproduct of the citrus juice industry, which is why many of these essential oils (lemon, orange) are readily available and less expensive.
That was the easy part.
The rest of the aromatics are obtained using solvents: hexane, dimethyl ether, or supercritical carbon dioxide (CO2). Previously, benzene was used as a solvent in the fragrance industry. We are glad it is no longer used for two reasons – first, it is carcinogenic and, second, it leaves a fairly high residue in the final product. With hexane and ether, the residue is typically less than 10 ppm (parts per million), which is fairly negligible. the least toxic of these solvents is carbon dioxide – you know, the stuff you just exhaled.
The gas, CO2, is placed in a chamber with the plant material, the chamber is then put under extreme pressure (100 to 200 times normal atmospheric pressure), at approximately 85 degrees, and this combination of mildly increased temperature and hugely increased pressure puts the CO2 into a “supercritical” state. This is an uber-geeky concept, but basically it is a condition where the CO2 is part liquid, and part gas (like a dense fog), and allows the aromatic components of the plant to dissolve in the CO2 fog stuff. The supercritical CO2, now containing the extracted aromatic compounds, is then returned to normal atmospheric pressure, where it can only exist as a gas. When it changes back to gas, it leaves behind ONLY the extracted aromatic part of the plant. Such a cool trick, really.
You must have a concrete to make an absolute. A concrete is made by combining plant materials, the solvent (hexane, a liquid, or ether, a gas) and waxes or resins. Once combined, the solvent is removed by gentle heating in a vacuum (negative pressure environment, not an electrolux) and reused. The remaining waxy compound is the concrete. It comprises the wax, resins, and other high molecular weight (heavy) botanical components, as well as the low molecular weight (light) aromatic compounds. From here, the concrete gets washed with ethyl alcohol, which takes with it the light aromatics from the concrete. The ethyl alcohol mixture is then distilled again, which removes almost all of the alcohol (>95%), leaving behind the extracted aromatic compounds in the form of an absolute.
Huh. Well, I'm not sure how I did explaining all of that. I can only assume that if you have selected this article, you are seriously interested in the topic, and willing to read it a few times to comprehend it fully. And, if you don’t, no worries. As I said, it took me more than a few tries and the experience of working with the materials over a period of years to understand it all myself. And I’m the scientist!