Solvent Extraction in general is a broad term and it does not include just chemical solvents like hexane but other forms like solid oils, fats and CO2 as well.
We use the process of solvent extraction majorly for botanical material that yield essential oil in exceptionally low quantity. It is also used when the botanical matter contains resinous components because it delivers a much fine fragrance than what can be achieved in normal distillation.
However, one must remember that solvent extraction often extracts the non-volatile components of the botanical material as well, which is why another filtration is required to separate them out from the extracted matter.
Most commonly, the process is used to extract absolutes from flowers as they are quite fragile and would not survive the process of steam distillation. These floral absolutes are different from essential oils as they comprise of both the aromatic and non-aromatic chemical elements.
Choosing a good solvent is the primary step. Often, less polar compounds like hexane and pentane or alcohols like methanol and ethanol are utilized as solvents. One must acknowledge that the type of solvent utilized is capable of altering the absolute and therefore one must consider which solvent to use to minimize the alteration.
The plant material is placed in a shallow tray and washed with a layer of the selected solvent to dissolve out the fragrant compounds. In order to allow the solvent to penetrate the plant material thoroughly, this process may involve physically breaking up the plant material or spinning it in a rotating drum. The resulting mixture is then filtered to remove the plant material and vacuum distilled to remove excess solvent. The yield of this process is a thick, waxy material called a “concrete.” Concretes contain all of the potent fragrant compounds of the plant as well as a variety of other lipid soluble compounds. The concrete is further processed with a second solvent (usually an alcohol like ethanol). The absolute is soluble in this solvent but the other lipid components are not. After another round of vacuum distillation to remove the solvent, a pure mixture of only absolute remains. Only 1–5 percent of the solvent will remain in the final product. Common absolutes extracted by this method are jasmine and vanilla. Solvent processing is known for producing absolutes with rich, authentic fragrances.