Essential oils are either distilled or expressed (Arctander 1960). Distillation can mean water distillation, water-and-steam distillation, steam distillation, or steam-and-vacuum distillation (Arctander 1960). To give some idea of yield, 200 kg of Lavandula angustifolia flowers will produce 1 kg of essential oil. However, between 2 and 5 metric tons of rose petals are needed to produce the same amount of rose oil.

Distillation (Steam)
The design of the distillation plants varies from region to region. Traditional and sometimes rather simple methods are still being used in some developing countries. Industrialized nations tend to use more technologically advanced equipment that is computer-controlled with software that can monitor the product throughout the distillation process. Nevertheless, excellent quality oils can be obtained even with a basic distillation apparatus. The aromatic plant material is placed on a grid through which steam passes, usually at a temperature not above 100° C (Bowles 2000). See Figure 3-3 for more details. The water boils at temperatures between 88° C and 100° C depending on the altitude at the distillation site.

(Reproduced from Price S. Practical Aromatherapy, with the kind permission of Harper Collins, London. 1983.)
High-pressure steam is the fastest way of distilling essential oils with high-boiling constituents like vetiver, sandalwood, and clove. The steam loosens the volatile nonpolar constituents of the plant and they pass, with the steam, into a condenser that cools the mixture. Steam also alters some of the components within an essential oil, for example, turning matricin to chamazulene. Some of the polar components from the plant dissolve in the water, producing floral water. The mixture of floral water and essential oil is cooled and becomes liquid. As essential oils and floral water do not mix, they quickly separate—the majority of essential oils float above the floral water, but some sink, depending on their specific gravity.

The degree of heat and the amount of time are vital parts of the distillation process as some components of plants are very sensitive to heat and others take much longer to distill (Guenther 1974). The distillation process for Lavandula angustifolia is approximately one hour, but it is considerably longer for sandalwood or vetiver. The length of the distillation process will affect the chemical composition of the essential oil (Guenther 1976). Steam distillation is suitable for the highly volatile components such as the monoterpenes, but heavier molecules like di or sesquiterpenes take longer. Some floral waters (hydrolats) can also be used therapeutically. Portable distillation equipment is simple to make and can be used for small quantities of plants when essential oils must be distilled on site (Alkaire & Simon 1992).

Variations of Steam Distillation

Fractional distillation: The essential oil is distilled at specific temperatures for specific lengths of time to collect different factions (or functional groups) within the essential oil. For example, peppermint contains menthol (a monoterpene) that is less volatile than other peppermint monoterpenes (e.g., α-pinene, β-pinene, sabinene and limonene) and other low boiling hydrocarbons. Monoterpenes evaporate at different temperatures, but many of the peppermint monoterpenes evaporate below 150° to 185° C, so this temperature can be used to “get rid” of lower boiling monoterpenes in order to obtain a high-quality peppermint flavor (Gafner 2014). Nowadays, fractional distillation is often done at reduced pressure in order to lower the boiling temperature.

Rectification: This aims to separate the volatile and nonvolatile components of an essential oil. If an essential oil contains impurities, it can be purified by redistillation. This process is called rectification. Sometimes peppermint and caraway seed oils can take on an unpleasant odor if they have been in contact with the wall of a hot still. This aroma can be removed through rectification (Guenther 1974).

Steam-plus-vacuum distillation: This method uses steam with partial vacuum (Arctander 1960). The pressure used is typically 100 to 200 mm Hg. The advantage of this method is speed. The disadvantage is a fast method of cooling is required.

In expression, the production of essential oils (by mechanically pressing the product) involves mainly the peel of citrus plants, like oranges, lemons, or grapefruit. The peel of the fruit is racked or abraded by mechanical scrapers, and the essence collected by centrifugal separation. Sometimes the whole fruit is crushed before the essential oil is separated from the juice and peel. Expressed oils will naturally contain a proportion of waxes, and citrus oils may include other components such as bergapten, a coumarin that can cause phototoxicity (Tisserand & Young 2013).

Methods for Producing Aromatic Extracts (not essential oils)
There are other methods of extraction that produce compounds called absolutes. These are mainly used by the fragrance and cosmetics industry. Rose absolute is yellow, viscous, and sticky. Rose essential oil is clear and solid at room temperature. The essential oil is ten times the price of the absolute. Residual solvents in extracts can produce adverse reactions.

This method was used on fragile blossoms like jasmine and tuberose. It is rarely used today as it is very labor intensive. I watched this process in Grasse, France. Animal fat is pounded until soft, then glass plates are coated with the fat. Each fat-covered glass plate is called a chassis. Fresh blossoms are placed close together on the chassis and left until the fat becomes saturated with essential oil. The chassis are constantly being replenished with fresh blossoms, and the old blossoms discarded. This process can take days or even weeks. The resulting oil/fat mixture is called a pomade. The pomade is washed with alcohol to remove the fat. The remaining extract is called an absolute. However, 99% of jasmine and tuberose extract is now produced by solvent extraction.

Supercritical Fluid Extraction
Supercritical fluid extraction (SFE) with carbon dioxide has been available since the 1980s. When the temperature of CO2 is maintained at approximately 31° C, under pressure, it acts like a fluid and dissolves the CO2-soluble constituents of an herb. The interest in SFE has increased in recent years because of legal limitations of solvent residues and solvents (Vagi et al 2005). However, the chemistry of the resulting extract is different to a steam-distilled essential oil because (a) less heat is used and (b) some different components are absorbed by CO2 extraction (Guan Hou et al 2007). For example, steam-distilled German chamomile (Matricaria recutita) essential oil is dark blue because heat converts matricine (found in the plant) to chamazulene—a sesquiterpene. However, matricine is present in the SFE extract. SFE extracts are not available for every plant used in aromatherapy but there is no reason not to use them, provided the chemistry is known and the components are relevant to the clinical goal being targeted. Volatile oils can also be extracted with other solvents. The use of benzene has declined but it is still used (Tisserand & Young 2013). Hexane is commonly used [cyclohexane and n-hexane (Tisserand & Young 2013)]. Such extracts are called “volatile oils” to differentiate them from essential oils (Coelho et al 2012).

Resinoids are obtained from resins (amber and mastic), balsams (benzoin) or gumlike exudates (frankincense and myrrh). Frequently resinoids are extracted by solvents. However, frankincense and myrrh can also be obtained by steam distillation or SFE. Resins are soluble in alcohol but not in water. Gums are soluble in water but not in alcohol.