An essential oil is a natural oil that carries the distinctive scent, or essence, of the plant (flowers, leaves, wood, bark, roots, seeds, or peel) from which it is derived. Essential oils are created through the steam distillation or expression process.
Most common essential oils are distilled — lavender, black pepper, cedarwood, etc. Raw plant material is placed in a distillation apparatus over water, then the water is heated. As the steam passes through the plant material it vaporizes the volatile (read: fragrant) compounds, carrying them with the steam through a cooling coil where the steam is condensed back into liquid. This liquid is our scented, essential oil — et viola!
In this method, different fractions distilled from a material can be selectively excluded to manipulate the scent of the final product. The product is more expensive, but this method is desirable because it can remove unpleasant or undesirable scents of a material and gives the perfumer more control over their fragrance formula.
Before distillation was discovered, all essential oils were extracted by pressing. Today, it is still used for most citrus oils. This method does not involve heat but rather the raw materials are pressed, squeezed or compressed and the oils are collected. The oils extracted contain water, but this water will eventually evaporate, leaving just the essential oils.
Many natural materials, for example flowers, are too delicate to undergo the high heat of the distillation process. In this case, the raw plant material is submerged and agitated in a solvent that can dissolve the desired (read: fragrant) aromatic compounds. The resulting extract, called the concrete, is a mixture of essential oil, waxes and resins. To remove the non-fragrant waxes and resins from the concrete, another solvent is used to extract only the fragrant oil from the concrete. The solvent is then removed by evaporation leaving behind the absolute. Absolutes are essentially highly concentrated essential oils.
Supercritical Fluid Extraction
This is a relatively new technique in which high pressure carbon dioxide gas is used as a solvent. Like solvent extraction, the CO2 extraction takes place at a low temperature, extracts a wide range of compounds, and leaves the aromatics unaltered by heat, resulting in an essence that closely resembles the original odor of the raw material. CO2 also leaves no trace of itself in the final product, allowing one to get the absolute directly without having to deal with a concrete. The results of this process are called CO2 extracts.