It is important to know something about how essential oils are extracted because oils from the same plant extracted in different ways can result in very different products.
Essential oils are generally extracted by distillation, often by using steam. Other processes include expression, solvent extraction, and cold pressing.

Steam Distillation

Most common essential oils such as geranium, tagette, basil, lavender, peppermint, tea tree oil, patchouli, eucalyptus etc. are distilled. In this method, steam is directed through the plant material (flowers, leaves, wood, bark, roots, seeds). The steam vaporizes the lighter chemicals contained within the plant material. The steam is then condensed through a cooling process. This process generates two products: the essential oil, which contains oil-soluble molecules, and a hydrolat or hydrosol or the plant water essence, which contains water-soluble molecules. Hydrosols include rose water, lavender water, lemon balm, clary sage and orange blossom water. Most oils are distilled in a single process. One exception is ylang ylang (Cananga odorata) which is purified through a fractional distillation.

Expression

Expression is used to extract essential oils from citrus fruits. Expression is the process of grating or scraping the peel of a citrus fruit to release the oils. For example, when zesting a lemon, the scent of lemon rises into the air because the volatile oils have been released from sacs found in the peel.
In the process of essential oil expression, care is taken to capture the oil. Expression does not involve heating; thus, the chemistry of citrus essential oils is not heat-altered and citrus oils smell very similar to the fruits from which they come.

Solvent Extraction

Most flowers contain too little volatile oil to undergo expression, but their chemical components are too delicate and easily denatured by the high heat used in steam distillation. Instead, a solvent such as hexane or supercritical carbon dioxide is used to extract the oils. Extracts from hexane and other hydrophobic solvents are called concretes, which are a mixture of essential oil, waxes, resins, and other lipophilic (oil-soluble) plant material.
Although highly fragrant, concretes contain large quantities of non-fragrant waxes and resins. Often, another solvent, such as ethyl alcohol, is used to extract the fragrant oil from the concrete. The alcohol solution is chilled to −18 °C (0 °F) for more than 48 hours which causes the waxes and lipids to precipitate out. The precipitates are then filtered out and the ethanol is removed from the remaining solution by evaporation, vacuum purge, or both, leaving behind the absolute.

Supercritical C02 Extraction

Essential oils derived from the supercritical CO2 extraction of herbs are similar to the oils produced through distillation in that they can be used in aromatherapy and natural perfumery.
Oils derived from steam distillation vary in their qualities depending on the temperatures, pressures, and length of time applied for the process. The CO2 extraction process might thus produce higher quality oils that have not been altered by the application of high heat, unlike the steam distillation process. In CO2 extraction, none of the constituents of the oil are damaged by heat.
Thus, the difference between traditional distillation and supercritical extraction is that instead of heated water or steam, CO2 is used as a solvent in the latter method. The supercritical extraction process operates at temperatures between 95 to 100 degrees F whereas steam distillation operates at temperatures between 140 to 212 degrees F.
In steam distillation, the molecular composition of both the plant matter and the essential oil are changed due to the temperature applied. On the other hand, a CO2 extract is closer in chemical composition to the original plant from which it is derived, as it contains a wider range of the plant’s constituents.
CO2 extracts are usually thicker than their essential oil counterparts and often give off more of the aroma of the natural herb, spice, or plant than a distilled essential oil. CO2 extracts have been said to contain more plant constituents than the amount extracted from the same plant using steam distillation.

THE CO2 EXTRACTION PROCESS: Pressurized carbon dioxide becomes liquid while remaining in a gaseous state, which means it is now “supercritical.” In this state, it is pumped into a chamber filled with plant matter. Because of the liquid properties of the gas, the CO2 functions as a solvent on the natural plant matter, pulling the oils and other substances such as pigment and resin from the plant matter. The essential oil content then dissolves into the liquid CO2. The CO2 is brought back to natural pressure and evaporates back into its gaseous state, while what is left is the resulting oil.
C02 is colorless, odorless, and can be easily and completely removed by releasing the pressure in the extraction chamber. It is what we exhale and is needed by plants in order for them to thrive, which illustrates its harmlessness when employed in the extraction process. This absence of potentially harmful solvents in C02 extraction means neither the human body nor the environment is polluted.