There are a number of ways to extract the volatile aromatic essence of the plant. Some of the methods used are steam and/or water distillation, solvent extraction, resin tapping, and cold pressing. These methods produce a variety of aromatic products or extracts including essential oils, concretes, resinoids and absolutes. The method used depend on the quality of the plant material and the type of aromatic product that is required.
We will cover the topic of extraction in the 2 part series. In the first part, here, we will discuss the distillation and cold pressing or expression methods. In the next part, we will discuss the solvent extraction method.
In the first part, we will briefly cover the following topics:
Distillation is one of the most popular and cost-effective methods for extraction of the aromatic materials from plants. Distillation is the process of separating the components or substances from a mixture by selective boiling and condensation. For essential oils usually, one of the following distillation methods is used.
Almost all constituents of essential oils are unstable at high temperatures. To obtain the best quality oil, extraction must be carried out at as low temperatures as possible. Distillation is not used in case of the plant material is very fragile or where the oils are extracted with great difficulty.
In the method, plant material is put into boiling water. The steam generated causes the release of the essential oils from the plant material. The essential oil, along with steam, is condensed in a condenser. Essential oil and water do not mix and; hence, are separated due to differences in the densities.
One disadvantage of water distillation is that high heat of boiling water may cause a change in the aroma quality. Great care has to be taken with the temperature and length of exposure of the heat to the plant material to prevent damage to essential oils. Specifically, plants with a high amount of esters react with water at high temperature to form acids and alcohols.
Another disadvantage of water distillation is that complete extraction is not possible. The water-soluble and high boiling components of oils can not be completely vaporized or they require large quantities of steam. Thus, the process becomes uneconomical.
Water distillation is a slower process and demands considerable experience and familiarity with the method.
For these reasons, water distillation is used only in cases in which the plant material by its very nature cannot be processed by water-steam distillation or steam distillation.
In this method, a combination of boiling water and steam is used to extract the essential oil. In comparison to water distillation, the method is faster and produces higher oil yield.
However, the method may also compromise the quality of essential oil since the high temperature of boiling water may lead to an alteration of the quality of essential oils. Still, this method is considered better than boiling water distillation since a combination of water and steam is used.
In this method, only steam is used for distillation. It is the most common method of essential oil extraction.
Water and essential oil are immiscible and, hence, they boil independent of each other. Boiling occurs when the sum of the pure vapor pressures equals the atmospheric pressure. Thus, the mixture of water and essential oil boils at a temperature lower than the normal boiling temperature. This means the essential oil will vapourize under very mild conditions.
The method is suitable for heat-sensitive plant materials, like the case of lavender, where high heat decomposes linalyl acetate into linalool and acetic acid.
However, one disadvantage is of the steam distillation method is that it requires a higher capital expenditure in comparison.
Hydro diffusion is a type of steam distillation method where the steam is introduced from the top onto the plant material, instead of from the bottom as in normal steam distillation. Condensation takes place below the area in which the plant material is held.
The method uses low-pressure steam and a lesser amount of steam. This method usually results in shorter processing time and higher oil yield. However, this method also results in the extraction of other non-volatiles and polar components with the essential oils.
Fractional distillation is a process by which components in a chemical mixture are separated into different parts, called fractions, according to their different boiling points. This method separates the volatile oil in different fractions or portions at various boiling points.
One of the most cited examples of using fractional distillation is ylang ylang essential oil, usually collected in four fractions. The first fraction, referred to as ylang ylang extra, contains the most volatile constituents. The following fractions are referred to as 1st, 2nd grade, and 3rd grade, each one containing lesser amount of volatile constituents, respectively.
Another example is camphor oil, which produces four colors of camphor oil (white, blue, yellow, brown) through fractional distillation.
During the distillation process, there may be few desirable compounds that may dissolve in water distillate and few undesirable compounds that may be distilled with essential oil. The following are a few ways to improve the quality of the essential oils.
Cohobation is a procedure to re-distill the distillate water from the water distillation or water-steam distillation to extract water-soluble components and add them back to the essential oils.
There are few oxygenated components, particularly phenols, which dissolve to some extent in the distillate water. For most oils, the level of oil loss through distillate water is less than 0.2%; whereas, in the case of oils with phenol-rich components, the level of oil loss in distillate water may be between 02.% to 0.7%.
The practice of cohobation is not recommended unless the temperature to which oxygenated constituents in the distillate are exposed is no higher than 100° C.
Cohobation is mainly used to extract rose oil. Phenyl ethyl alcohol is soluble in water and is distilled from the water distillate after water distillation and then mixed with the decanted oil in the correct proportion to create Rose Otto.
Rectification is a fractional distillation process used for the removal of unwanted constituents in the essential oil. The essential oils redistilled either with steam or in a vacuum. Sometimes the oil will be labeled as double distilled.
Some examples of rectification in practice are eucalyptus oil, bitter almond oil, and peppermint oil. In the case of bitter almond oil, highly toxic hydrocyanic acid (cyanide) is removed. In peppermint, minor sulfur constituents are removed as they impart a slightly unpleasant taste, and most peppermint oils are used in food flavoring.
Please understand that it is the essential oil that is re-distilled and not the plant material. Heat or chemical refining are methods used to standardized the quality of oil and are different from the rectification process.
Hydrosol is the by-product of the distillation process. It is the residual aromatic water from the process after the separation of essential oils. It is also known as floral water, herbal distillate, herbal waters or essential waters.
Hydrosol contains water-soluble components of the plant material. Hydrosol is used for therapeutic purposes and creating cosmetics.
Some examples of floral waters are rosewater, lavender water, orange water, neroli, and roman chamomile.
Expression, also known as cold pressing, is an extraction method that is primarily used in processing citrus essential oils such as lemon, lime, bergamot, tangerine, and sweet orange. In this method, the rind of the fruit is placed in a rotating container with spikes in order to puncture the peel. Upon puncturing, the rind releases the essential oil which is then collected in a small container below the expresser.
This method leads to no decomposition of essential oil constituents since it does not involve any solvents or heat of any kind.
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. Although many citrus oils are also produced by steam distillation, they seem to lack the vibrancy of the cold-pressed oils.
One of the earliest methods to extract essential oils using the expression method was the sponge extraction process. in this method, oil was removed from the peel either by pressing the peel against a hard object of baked clay which was placed under a large natural sponge or by bending the peel into the sponge. The oil emulsion absorbed by the sponge was removed by squeezing it into some other container.
It is an extremely slow and labor-intensive process and hence is not used these days.
To summarise, the extraction of essential oil is a mix of art and science. The method used depends on the quality of the plant material and the type of aromatic product that is required. Distillation, specifically steam distillation, is the most widely used method for the extraction of essential oils. The expression is the preferred method for the extraction of essential oils from citrus fruits.
In the next part of the article, we will discuss the solvent extraction methods.