Element 4. Describe the various extraction processes used for essential oils
Essential Oil Distillation
Since essential oils are derived from different parts of the plant, the methods of removing these essences are not always the same. The methods used for preparation and extraction of essences from plants include; distillation, solvent extraction, percolation, carbon dioxide (CO2) extraction, enfleurage, expression, and maceration.
There are various methods of distillation, but steam is by far the most common, this method having been used for thousands of years. With certain flowers and leaves the essential oil evaporates quite quickly, which means that the oils need to be extracted almost immediately after harvesting. Most oils are distilled in their country of origin, others that do not evaporate so readily, may be exported as raw materials to other countries where they can be stored until ready for extraction.
The plant matter is placed into a deep vat and hot steam is passed through the plant material from below, either in a vacuum or under deep pressure. Steam softens the plant tissue to allow the essential oil to be released and vaporised. The essential oil will condense once the steam is cooled, and because the oil is not soluble in water it will either float on top of the water, or sink to the bottom, depending on the density of the oil. This makes collection quite easy.
Prior to distillation, gums, roots, or wood have to chipped or powdered, and often after the first distillation, many crude oils need to be redistilled because a portion of non-volatile matter is present. To remove these unwanted substances, the crude oil is purified by either filtering, or by a process known as rectification.
This method is similar to distillation, and is used to extract the essential oil from woody or tough materials. Percolation is a much faster method of extraction and so produces a better quality of oil, as it involves a shorter period of time that the plant materials are in contact with the steam. The difference is the steam is passed through the plant material from above.
Enfleurage is the traditional method that was used for extracting the essences from delicate flower petals. This method is not used commercially very often today, although in the south of France in the Grasse region as well as some parts of India this method is still employed. The labour intensity of this process creates work for many during the flower seasons. The process is quite tedious and time consuming but the oils that are produced tend to be of a very high quality compared to steam distillation of the petals .
The Process of Enfleurage
The process of enfleurage is limited to flowers that continue to exude an essence after harvesting. Oils include jasmine and tuberose, as well as flowers whose oils are destroyed by the steam extraction process. Fresh blossoms are spread across a layer of cold fat that has been coated over a rectangular wooden-framed glass plate called a chassis. The odourless fat usually consists of a prepared mixture of purified lard and beef suet. The petals are left for many hours, being replaced regularly with fresh blossoms, until the fat is completely saturated with the fragrant oils. This perfumed fat is called pomade. To extract the essences from the pomade, alcohol is used, and once the alcohol has evaporated, the concentrated essences known as absolutes, remain. As many as 36 separate additions of fresh flower petals are involved in the process. For absolutes to become totally volatile they need to be processed further by molecular distillation which removes all traces of non-volatile matter. Occasionally at room temperature, these absolutes solidify. However, they will liquefy once being slightly warmed, even by being held in the hands for a short while.
Expression is the process used to extract oil from citrus fruits such as orange, lemon, grapefruit etc. The citrus oil is produced from oil glands found in the outer rind of the fruits. The underlying pith and pulp must be removed before extraction. Various methods can be used to extract the oil. Originally this was carried out manually, but due to the labour involved this is not commercially viable. Therefore, most of the oils are now produced commercially using machines. Whatever system is used, the oils separated from the juice and peel by allowing the oil to settle on the surface. Those citrus oils extracted by hand are still by far the best quality. No heat is involved in this process because many of the important constituents would be destroyed. This is why they are sometimes referred to as cold pressed oils. Lime oil, is generally extracted by distillation as this method produces finer oil with a superior odour.
To produce your own citrus oil at home. Peel the rind from an orange or lemon and squeeze the oil through a garlic press. It is preferable to by a new garlic press, so no traces of garlic are present. Ensure that the fruit has not been sprayed or dyed.
Solvent extraction, is an alternative to distillation, it is mainly used for gums, resins and flowers. It is also used in those cases where the essential oil is destroyed by distillation and is often used instead of distillation if a different composition and odour are required (rose). The solvents used to extract the oils from flowers are either petroleum or benzene, with acetone usually being used for resins and gums. Solvents can be highly toxic and have been known to cause allergic reactions and even weaken the immune system. More recently, liquid carbon dioxide and liquid butane have been used and they produce extremely fine oils. Carbon dioxide has the advantage of not being toxic or flammable, thereby making it environmentally safe to use. The only problem is the cost of the equipment.
In solvent extraction the flowers and other materials are placed in a vessel with sufficient solvent to dissolve the essential oil from the plant matter. The solvent is then distilled off, leaving a dark coloured semi solid matter with a wax-like consistency. The residue is regarded as concrete. The resins and gums are usually dissolved in acetone, with the resultant extracted substance from this being known as a resinoid. To prolong the effects of the fragrance of perfumes, resinoids are often used as a fixative. Incorporated within these concretes and resinoid extracts are large amounts of non-volatile matter, which is why they are therefore not classified as true essential oils. Frankincense, myrrh, and benzoin are examples of resins. However, essential oils of frankincense and myrrh that are used in aromatherapy are produced by steam distillation.
Maceration involves placing the flowers into hot fat. As cells of the plant soften, the essential oils are released into the fat. The flowers are strained, removed, and replaced with fresh flowers. The fragrance of the saturated fat derived from this process is the same as that of enfleurage, and also known as a pomade. To separate the essential oil from the fat, it is treated in the same way as with enfleurage. Calendula and st john’s wort (hypericum) are macerated oils. You can use this method at home, as long as you have a sufficient supply of suitable herbs or flowers. Simply place leaves or flowers into a clean jar, filling it one-third of the way then fill to the top with quality carrier oil such as sweet almond.