For years, 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 intimately with these materials for a five years now, and I finally understand them. So, now let me try to explain it all to you.
The most straightforward category in this aromatic group 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 (like a steaming basket in the kitchen) containing raw plant material, causing the plants to release their aromatic compounds, which rise with the steam into a cooling system. 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.
Hydrosols are used frequently in skin care. They are very mild, and have gentle benefits to the skin. A properly made hydrosol is sterile - as it undergoes distillation, a process where the liquid is turned to gas and then liquid again, all organisms are left behind. Hydrosols make excellent skin toners, or can become part of the water phase of a cream or lotion.
The oil-soluble aromatic compounds rise to the top of the hydrosol in a separate layer, which can be decanted off, and is then called the essential oil of that plant. Essential oils have been used for centuries in aromatherapy, perfumery, medicine, and skin care. Most essential oils are too acidic to be applied directly to the skin, with a few exceptions. One exception is lavender essential oil, which is one of the more well-studied essential oils. There is clear evidence to support the use of lavender essential oil as an analgesic (pain-reliever), anti-inflammatory, and topical antibiotic. Applied to a fresh burn, lavender essential oil can alleviate discomfort and decrease the inflammatory response. Another essential oil with increasingly documented efficacy is thyme - it is the natural component in many natural hand sanitizers and wipes, capable of killing most common bacteria.
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 (also known as cold-pressed or expeller-pressed, the former being a temperature controlled process). This means that the peels are first pricked all over to puncture the cells containing the oils, and/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 than things like geranium or lavender. Caution must be used with citrus oils in skin care - a somewhat alarming rash called a phototoxicity reaction (or phytophotodermatitis) can occur when citrus oils on the skin are exposed to sunlight. Too many skin care companies use citrus oils in leave-on products, so be sure to look at the ingredients carefully, especially if you have a history of a rash from certain lotions or oils. (One exception is bergaptene-free bergamot essential oil, which has had the component responsible for photosensitivity removed, and does not increase the likelihood of a reaction. Also, distilled citrus oils, rather than expressed citrus oils, have a lower incidence of phototoxicity.)
How are you doing so far? I hope you're still with me, because that was the easy part.
The rest of the aromatics are obtained using solvents: hexane, dimethyl ether, or supercritical carbon dioxide. 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 (CO2) – you know, the stuff you just exhaled.
In this method, which is steadily gaining popularity, the solvent, 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 Fahrenheit, 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 separated from the remaining, raw plant material, and then returned to normal atmospheric pressure, where carbon dioxide can only exist as a gas. When it changes back to gas, it leaves behind ONLY the extracted aromatic part of the plant. There is literally ZERO residue of the solvent in the remaining CO2 extract, because it has changed physically from a supercritical state to a pure gas. Such a cool trick, really.
CONCRETES & ABSOLUTES
These are more traditional fragrance compound extraction methods, used in the world of perfumery for many years. Long ago, people used a technique called enfleurage - flower parts were combined with animal fat and pressed between pieces of glass. After a few days, the flowers were removed, and then replaced with fresh flowers - this process was repeated for several rounds until the fat was as full of the desired aroma as possible. This technique is used much less frequently now, as most people do not want their personal care products to be derived from killing animals. In most instances, concretes and absolutes have taken the place of enfleurage.
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.
Your eyes are crossed and you're drooling. It's okay, mine were too at first. But, I 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!