There are many methods of extracting the precious life force of the bark, root, flower, leaves, stems & seeds of plants.  The most common being Distillation & Cold Expression.  Even these two basic and common methods of extraction break down in to further sub groups of methodology.

Essential Oil – Distillation or Cold

This has become the common term used for most aromatic plant extracts, but it can be misleading. In truth, essential oils are a product derived simply through distillation or the application of pressure.  In other words, organic matter (such as flowers, leaves, seeds and bark) are put through steam distillation treatments to separate out the volatile compounds. The non-water soluble oil is then separated from the water. Alternatively, essential oils can be created through expression, also known as cold pressing, where citrus rinds are perforated and squeezed to release the oil.

Concretes

In some cases, the aroma imbued in organic materials may be damaged by certain extraction methods, such as the heat from steam distillation. Solvent extraction uses hydrocarbons to extract the volatile compounds without degrading them in the process. Once the solvent is removed, the resulting product is known as a concrete. The extract contains the volatile aroma  molecules in addition to chlorophyll and other plant tissues, resulting in a highly coloured and viscous extract. Concretes also tend to be more stable and enduring than essential oils.

Resinoids

Resinoids are also produced via the same process used to produce concretes. However, they are derived from naturally resinous plant matter (such as balsams, gum resins, and natural oleoresins) that have been subjected to extraction with hydrocarbon solvents. The resulting product typically contains predominantly non-volatile, resinous compounds.

Absolutes/Solvent Extraction

Absolutes are a further derivation of concretes that result when concrete is subjected to a secondary solvent extraction process with ethanol. The solids (waxes, oils, fats, and so on) dissolve in the alcohol.  The alcohol is then removed through a process of evaporation. The remaining “essence” is called an absolute and it is the most concentrated form of fragrant material. Unsurprisingly, it tends to be one of the most expensive processes.

CO2 Extraction

Carbon Dioxide Extracts. Hypercritical carbon dioxide extraction is a relatively new process used to extract the essence of the plant. When a certain amount of pressure is applied to CO2 (carbon dioxide) this gas turns into liquid. This liquid CO2 can be used as a very inert, safe, “liquid solvent.”  Another expensive method of extraction that is said to remove the most volatile oils from the plant matter.  This is a relatively new method.

Effleurage

An age old art utilising a cool oil medium and super delicate handling of flowers to extract the most sensitive of oils (from Jasmine flowers for example).  This is a labour intensive process that I do not think happens in the world commercially any more.