In addition to distillation and expression methods, there are various extraction methods to obtain essential oils. Although extraction methods are more complicated than distillation, it is preferred for plants that are too delicate for distillation.

The product obtained by solvent extraction is called “absolute”. Absolutes are more viscous than essential oils because they contain heavier non-volatile molecules. In solvent extraction, plant material is extracted with an organic solvent such as petroleum ether, hexane or heptane. Along with the essential oil, other lipophilic substances such as fixed oils and waxes also pass into the solvent. This mixture is vacuum distilled to remove solvent to yield thick and waxy material called “concrete”. The concrete is further processed with alcohol and “absolute” is obtained after removing alcohol by vacuum distillation. Since trace amount of solvent may remain final product, absolutes are generally avoided in aromatherapy. However, they are widely used in perfume industry because the smell and the aroma of the absolute is much closer to authentic fragrance of the plant. Jasmine and vanilla absolutes are the best known examples.

Enfleurage

Enfleurage is one of the oldest and traditional methods used to obtain delicate essential oil from plants mostly flowers. Since, resulting product can contain both volatile and non-volatile components similar with the solvent extraction, it is considered as “absolute” and not preferred to use in aromatherapy.

In enfleurage, flower parts are placed on the layer of fat and its lipid soluble components transfer to fat layer. Plant parts are usually renewed every 24 hours until the fat reaches full saturation. This final product is called “pomade” and can be used directly in this form. Previously animal fats such as tallow or lard were used during the enfleurage process, today vegetable oils such as Shea butter have been preferred instead of animal fat. It is important to use odorless fat or vegetable oil to ensure high quality essential oils. After further extraction of pomade with alcohol, pure “absolute” is obtained. This method can be applied to delicate flowers such as jasmine, rose, lilac, mimosa, acacia and orange blossom in the past, it is used rarely in very special occasion today.

CO₂ Extraction:

Although CO₂ extraction is a relatively new method, it is now possible to see CO₂ extracts in the market today. The process is more complex and expensive than other methods. This method is essentially known as “Supercritical Fluid Extraction” and CO₂ is used as a supercritical fluid which becomes liquid under a certain temperature and pressure. Under the supercritical condition, CO₂ is non-flammable, odorless, tasteless, colorless liquid and a good solvent for essential oils. In this method plant material extracted with liquid form of CO₂ and resulting mixture is filtered to remove plant material. When the pressure and the temperature of the system is return to atmospheric condition, CO₂ in gases stage easily removed from the mixture. Since essential oils obtained by CO₂ extraction are not in the same composition as their distillation equivalents, they are called “CO₂ extracts”. There is no solvent residue in the product obtained in this method. CO₂ extracts can be used in aromatherapy, pharmaceutical and cosmetic industry.

References

Başer, K.H.C., Buchbauer, G., 2015. Handbook of Essential Oils: Science, Technology, and Applications, Second ed. CRC Press, Boca Raton, FL.

Buckle, J., 2015. Clinical aromatherapy – Essential Oils in Healthcare, Third ed. Elsevier Health Sciences,  London, UK.

Tisserand, R., Young, R., 2013. Essential Oil Safety: A Guide for Health Care Professionals. Elsevier Health Sciences, London, UK.