Distillation and cold pressing, as discussed in the previous article, are the most common methods used in the extraction of essential oils. However, in a growing number of cases, the aromatic content of the plant is now being distilled using solvents extraction. The products from solvent extraction are not to be confused with essential oils. Solvent extraction results in products with high aromatic content but they have other constituents also.
In the second part of the articles about how essential oils are extracted, we will discuss the following topics:
Solvent extraction is a method to separate a compound into its parts based on the solubility of its parts. For essential oil extraction, the plant material is mixed with the solvent. Some of the commonly used solvents are petroleum ether, methanol, ethanol, or hexane.
The solvent extraction method is used in cases where raw plant material is either delicate like flowers or where water or steam alone will not release their therapeutic essences. Some of the plants for which solvent extraction is used are mimosa, carnation, tuberose, gardenia, jasmine, narcissus among others.
The method is simple and efficient. However, there are few disadvantages like long extraction period, high solvent requirement and unsatisfactory reproducibility.
It is important to choose a good solvent since a residue of solvent could be present in the finished product. Because of this, some people believe that it should not be used for aromatherapy oils. However, some solvents, like, hexane leave residue of about 10 ppm (parts per million), which is extremely low. European Union standards are for less than 10 parts per million solvent residues in a finished absolute.
Absolute extraction is a multi-step process where raw plant material is macerated under vacuum into a solvent. After this, the solvent is removed through evaporation and what is left behind is called ‘concrete‘. It is semi-solid to solid and extremely fragrant with a large amount of pigment and waxes.
Important: If resinous plant material is used to extract, what is left behind is called a ‘resinoid’ instead of a ‘concrete’.
Concrete is used for making solid perfumes and is soluble in both carrier oils and alcohol after filtering any insoluble waxes/solid materials remaining.
Further, concrete is melted by warming and diluted with ethyl alcohol. Essential oil, some waxes, fixed oils, and fats then dissolve in the alcohol. Then the mixture is steam distilled under reduced pressure to separate alcoholic solution and the aromatic substance. This aromatic substance is called ‘absolute’.
Absolute is highly concentrated with the true aroma of the plant. Absolute mostly contains essential oils and some pigments, waxes and other compounds. Absolute is costly to produced and is mostly used in the perfume industry.
This is a solvent extraction method in which carbon dioxide gas under pressure is used as a solvent. The use of carbon dioxide extraction is a fairly new way to extract essential oils from plant material. Carbon dioxide is used since it is inert in nature and is non-toxic and odorless.
To use carbon dioxide as a solvent, it is needed to be pressurized to 200 atmospheres. This results in carbon dioxide changing state to liquid. At the end of extraction, the pressure is reduced to normal and the carbon dioxide simply evaporates. To achieve such high pressure, heavy-duty stainless steel equipment is required, and this makes an expensive way to extract essential oils.
The method is useful in cases of lipophilic compounds that have a high vapor pressure and a low molecular weight. Such compounds are ethers, esters, terpene, hydrocarbons, alcohols, ketones, etc. – compounds found in essential oils. The method is used for the extraction of calendula, German chamomile, frankincense, ginger, kava kava, marjoram, melissa, and rosemary.
CO2 extracts contain more plant constituents than the steam distillation extract of the same plant. In comparison to extraction using other solvents like butane, carbon dioxide extraction offer few advantages:
Enfleurage is an ancient method of extraction based on the principle that fat, especially animal fats, have a higher power of absorption. This method is used for flowers that keep giving off their aroma even after harvesting, for example, jasmine and tuberose. Under the method, flowers are strewn over the surface of a specially prepared fat base.
The fat absorbs the perfume emitted from the flowers over a certain period of time – 24 hours in the case of jasmine and longer in the case of tuberose. Once the flowers have released all of its essences, they are replaced with a fresh batch of flower petals. The whole process continues for the entire period of harvest, which lasts for eight to ten weeks. The fat used must be relatively stable against rancidity.
At the end of the process, the oil is extracted with the help of alcohol from the fat.
Enfleurage has the advantage that even the most delicate components of the flower oils are preserved. However, enfleurage is a very labor-intensive way of extraction and an expensive process. Therefore it is rarely used today.
The enfleurage process can be either cold or hot – where heat may be used.
Maceration is a method used for the manufacturing of infused oils. Infused oils are carrier oils used as a solvent for the extraction of essential oils. When a carrier oil is used as a solvent for the extraction of essential oil, it retains some of the therapeutic properties of the plant. Some examples of infused oils are calendula and carrot.
Solvent extraction is gaining a lot of popularity these days for the extraction of aromatic extracts of the plant. In addition to distillation and cold pressing, solvent extraction may be used to extract different kinds of products from plant material. It is always advisable for customers to enquire about the extraction method since for extraction since properties and usage may differ for each kind of product.