There are a variety of ways that essential oils can be extracted, although some of these are less commonly used today. The most common method currently in use is steam distillation, although there are various more efficient and economical processes being developed all the time.
The plant material is placed into a still which is similar to a very large pressure cooker. Steam under pressure is passed through the plant material. The heat causes the globules of oil in the plant to burst open and the oil will evaporate quickly.
The essential oil vapour and the steam then pass out of the top of the still into a water-cooled pipe. The vapours are condensed back to liquids and the essential oil separates from the water and floats on the top. These oils must be stored in dark-coloured bottles in cool place as they are adversely affected by heat, oxygen, light and moisture.
If stored under the correct conditions, as described, oils should last quite some time. However citrus oils such as Lemon and Orange are best kept for about six months only.
Some essential oils, notably Rose and Jasmine, are very expensive due to the fact that the petals of these flowers produce quite a low yield. It has been estimated that more than eight million jasmine flowers are required to produce one kilogram (2.2 lb) of Jasmine oil.
This process produces what is known as an 'infused oil' rather than an essential oil. Plant material is soaked in a vegetable oil, heated and strained. The oil mixture can be used for massage.
This method uses wooden frames with a plate glass top which is covered with warm lard. Flower petals are spread over the layer of grease and replaced at regular intervals until the lard is saturated with the essence. Alcohol is then used to wash the grease away and obtain the essence. Any remaining lard can be used in soap making. Although this method is useful for essences which tend to disappear in the process of distillation it is very rarely these days.
This process is used for obtaining essential oils from citrus rinds such as orange, lemon, grapefruit, and bergamot. The rinds are ground or chopped and then pressed. The resulting liquid is a mixture of essential oil and watery components which, if left, will separate. In the past the rinds were squeezed into sponges. Oils produced by cold pressing have a relatively short shelf life.
A hydrocarbon solvent is added to a drum containing plant material to help dissolve the essential oil. The solution is then filtered and concentrated by distillation and we are left with a substance containing resin known as a resinoid or a combination of wax and essential oil known as concrete. The oil is finally obtained by a process of extraction using pure alcohol.
The alcohol then evaporates and the residual solution is called absolute. Oils produced by this method are not ideal as the solvents are toxic substances and there is always a small residue left behind in the oil which could cause allergies and affect the immune system. Rose and Jasmine are extracted by this method.
This is a recently developed method. Carbon dioxide or butane, can extract the essential oil from the plant when liquefied under pressure. The resulting liquid is drained and allowed to depressurize and the carbon dioxide returns to a gas. The pure essential oil remains.
The same plant can sometimes be treated by different method to obtain different oils. For example, orange essence is produced form the skins of oranges by the method of cold pressing, while Neroli essential oil is produced from the orange blossom by solvent extraction.