Essential oils can be extracted using a variety of methods, although some are not commonly used today. Currently, the most popular method for extraction is steam distillation, but as technological advances are made more efficient and economical methods are being developed. Here are a small sample of them, see the individual articles on extration methods for more information:
To extract the essential oil, the plant material is placed into a still (very similar to a pressure cooker) where pressurized steam passes through the plant material.
The heat from the steam causes globules of oil in the plant to burst and the oil then evaporates. The essential oil vapor and the steam then pass out the top of the still into a water cooled pipe where the vapors are condensed back to liquids. At this point, the essential oil separates from the water and floats to the top.
Now, this doesn't sound like a particularly complicated process but it takes more than 8 million Jasmine flowers to produce just 2 pounds of jasmine oil! No wonder pure essential oils are expensive!
Maceration actually creates more of an "infused oil" rather than an "essential oil". The plant matter is soaked in vegetable oil, heated and strained at which point it can be used for massage.
Cold pressing is used to extract the essential oils from citrus rinds such as orange, lemon, grapefruit and bergamot. The rinds are separated from the fruit, are ground or chopped and are then pressed. The result is a watery mixture of essential oil and liquid which will separate given time.
It is important to note that oils extracted using this method have a relatively short shelf life, so make or purchase only what you will be using within the next six months.
Solvent Extraction Method 1:
A hydrocarbon solvent is added to the plant material to help dissolve the essential oil. When the solution is filtered and concentrated by distillation, a substance containing resin (resinoid), or a combination of wax and essential oil (known as concrete) remains.
From the concentrate, pure alcohol is used to extract the oil. When the alcohol evaporates, the oil is left behind.
This is not considered the best method for extraction as the solvents can leave a small amount of residue behind which could cause allergies and effect the immune system.
Solvent Extraction Method 2:
Only recently developed, this method uses Carbon Dioxide to extract the essential oil from the plant when liquefied under pressure. Once the liquid depressurizes, the carbon dioxide returns to a gaseous state, and only pure essential oil remains.
Some of the finest flower absolutes are produced by means of solvent extraction. This method was first tried in the 1830s, and began to be used on a commercial scale in the 1890s.
Flowers are placed on perforated racks in hermetically sealed containers, which may be connected to each other in a series. At one end is a lank containing liquid solvent, and at the other a vacuum still. The liquid solvent is allowed to flow slowly over the flowers, dissolving the essential oils. The solvent is then distilled off and returned to its tank to be re-used, leaving a semi-solid perfume material known as a 'concrete'. This contains the aromatic material from the plant, together with natural plant waxes.
Twenty-five grams of this concrete is equivalent to a kilo of best quality pommade obtained by enfleurage. Like the pommade, the concrete is then shaken in alcohol to remove the plant waxes, leaving a very high quality flower oil, or absolute.
The solvent originally used in the nineteenth century was petroleum ether, and later benzole was introduced. Modern extraction processes may use liquid butane, or liquid carbon dioxide, which produce very fine oils without damaging the most delicate aromas.