Essential oils can be produced in a number of different ways:
Most essential oils are produced by steam distillation (see picture montage on this page of Nana Mint production in Morocco). Steam distillation should ideally be by low or medium pressure without the use of chemical solvents. Unfortunately, the distiller or producer is often more concerned with profit than with the correct treatment of the plants. High steam pressure and quick distillation are more cost-effective, but rarely create a fine and precious product. This is why organic farmers distill their plants very carefully with the slower method of low pressure steam distillation when they create organic essential oils.
Many plants require a longer time to distill in order to extract the entire spectrum from head to tail of active substances (particularly the slow boiling sesquiterpenes) in the essential oil. These slow low pressure methods yield a richer, therapeutically more effective essential oil.
Most citrus essential oils are produced through the method of cold pressing. This involves gentle abrasion of the rind of the fruit, releasing the oil contained in tiny oil glands in the peel.
This is the usual method for the production of oils from very fragrant flower plants like Jasmine, Tuberose, Red Champaca (in fact many of the oils used in perfumery). Oils produced in this way are known as absolutes. The plant material is thoroughly mixed with a solvent such as hexane, which dissolves out the aromatic substances, and also waxes and other products. The solvent is then recovered, leaving a residue know as a concrete. The waxes and other unrequired ingredients are removed from the concrete with alcohol, leaving the oil.
CO2 extraction uses carbon dioxide to extract the aromatic substances from the plants. This method allows a low 'cold' treatment of the plants because the oil can be extracted at temperatures of about 30 degrees Celsius (90 degrees Fahrenheit). In many cases the end result is a very subtle, round aroma, (e.g. Jasmine CO2) particularly suitable for the flavour industry. However, the energy of these oils mostly does not show the same results as found in distilled products.
Hydrodiffusion is another variation of steam distillation. The steam is injected from the top of the still instead of being induced from the bottom. The distillation time is shorter and the process often allows for a better penetration of the steam into the plant material. Hydrodiffused oils sometimes tend to have a slightly subtler note.
Enfleurage could be compared to certain aspects employed in maceration, but is done in a slightly different way. Glass plates in a frame (called a chassis) are covered with highly purified and odorless vegetable or animal fat and the petals of the botanical matter that are being extracted are spread across it and pressed in. The flowers are normally freshly picked before so encased in their fatty bed. The petals remain in this greasy compound for a few days to allow the essence to disperse into the compound, after which the then depleted petals are removed and replaced with a fresh harvest of petals. This process is repeated until the greasy mix is saturated with the essence, and needs to be repeated a couple of times until saturation is achieved. When the mix has reached saturation point the flowers are removed and the enfleurage pomade - the fat and fragrant oil - then washed with alcohol to separate the extract from the remaining fat, which is then used to make soap. As soon as the alcohol evaporates from the mixture you are left with the essential oils. This is a very labor-intensive way of extraction, and needless to say a very costly way to obtain essential oil and is nowadays only sometimes used to extract essential oil from tuberoses and jasmine.