Welcome to part 2 of our guide to An Introduction to Aromatherapy. This time we are focusing on how essential oils are produced and extracted from the plant material.
There are a number of extraction processes used including the three principal methods:
- Solvent extraction
There are 2 types of distillation – steam and water.
Most of the essential oils that we use are produced by a process known as distillation.
Fresh, or sometimes dried, botanical material is placed in a closed container of the still, and pressurised steam is generated which enters the container and circulates through the plant material. The heat of the steam forces the pockets that hold the essential oils to open and release them. Tiny droplets of essential oil evaporate and attach to the steam, which then travels up a long tube surrounded by a cold water bath. The cold forces the steam to cool and condense back into water. Essential oils do not mix well with water so it forms a film on the water’s surface. To separate the essential oil from the water, the film is then decanted or skimmed off the top into a collection vial and the water into a large vat.
Most modern stills are made of copper and are heated with open wood first from below. During the process of distillation there is a large amount of the chemicals of the oils which is absorbed into the distillation water, this is known as the ‘first water’. This absorbed oil must be recovered from this water to produce an acceptable yield, and this is achieved by skilfully re-distilling the water to separate the oil using a process known as cohobation.
This is similar to steam distillation, except the plant material is covered up with water and then heated in the vacuum-sealed container.
This process is much slower and considered to not be as good as steam distillation. This is due to the fact that some of the very delicate chemical components of the essential oils may be damaged by heat.
Expression is used to obtain essential oils from the peels and seeds of citrus fruit, such as bergamot, grapefruit, lemon, lime, mandarin, orange, and tangerine oils. Citrus fruit store their essential oils close to the surface, usually in the peel itself. In this process, the fruit rolls over a trough with sharp projections that penetrate the peel which pierce the tiny pouches containing the essential oil. Then the whole fruit is pressed to squeeze the juice from the pulp and to release the essential oil from the pouches. The essential oil rises to the surface of the juice and is separated from the juice by centrifugation.
This process is quick, easy and cheap in comparison to many of the other extraction processes.
This process uses very little heat so it’s ideal to produce the essential oils which are very delicate and normally damaged by the heating process of distillation. It is a good method of extraction of the oils from flowers, which are the most delicate plant materials – and especially from the flowers which contain very little essential oil.
- Advantage – gentle method
- Disadvantage – essential oil contains non-volatile waxes and plant dyes as well as the essential oil
- Solvent used – hexane
- Extraction process produces a concrete
- Concrete is refined into an absolute
Enfleurage is an expensive process and is rarely used today except in a few places in France and India. Some flowers, such as jasmine or tuberose, have very low contents of essential oil and are extremely delicate, heating them would destroy the blossoms before releasing the essential oils. In such cases, enfleurage is sometimes used to remove the essential oils.
Nitrogen & CO2 Extraction
Carbon dioxide is a fairly new method of extraction using carbon dioxide gas which is kept under high pressure at a constant temperature. Plants are placed in a stainless steel tank and, as carbon dioxide is injected into the tank, pressure inside the tank builds. Under high pressure, the carbon dioxide turns into a liquid and acts as a solvent to extract the essential oils from the plants. When the pressure is decreased, the carbon dioxide returns to a gaseous state, leaving no residues behind.
Some basic facts:
- Roses cannot be distilled by directly injecting steam, because the petals compact to form a large mass that the steam cannot penetrate
- Rosa damascena will yield 1 kilo of oil per 4,000 kilos of flowers using modern distillation processes