Essential Oils

Essential oils are volatile, generally aroma-contributing liquids produced by plants. The term “essential oil” is derived from “essence,” as it carries the distinctive scent or essence of the plant material. Because they are volatile, they are also called volatile oils. Specific essential oils are named after the plant from which they are extracted, for example, ginger oil, nutmeg oil, and orange oil. As a group they are also called spice oils and citrus oils.

An essential oil is a concentrate of lipophilic and hydrophobic chemical compounds, which are volatile. Plants also produce fixed oils or fatty oils, which are not volatile. Fixed oils are fatty acid esters of glycerol called triglycerides. Triglycerides, which are also present in the animal kingdom, are viscous materials. They can be used as cooking oil and are nutritionally significant. The role of essential oils in food is to provide aroma and flavor. In some cases, they have a medicinal role. Essential oils are extensively used in the fragrance industry. In recent times, the popularity of aromatherapy has given a boost to essential oils. Aromatherapy is an alternative medicine wherein it is believed that specific notes of odor, contributed by essential oils, have curative powers.

An essential oil is composed of terpenes. More than 100 years ago it was seen that the hydrocarbons found in essential oils were C10, C15, and occasionally C20 compounds. A C10 hydrocarbon typically has the formula C10H16, and can be considered to be derived from two isoprene units of C5H8. Similarly, C15 and C20 compounds are derived from three and four isoprene units, respectively. Thus, C10, C15, and C20 compounds are called monoterpene, sesquiterpene, and diterpene, respectively. Monoterpenes can be: acyclic, as in a-pinene; monocyclic, as in limonene and p-cymene; dicyclic, as in camphene; and even tricyclic. A C10H16 terpene acyclic hydrocarbon has three double bonds, while a monocyclic has two, a dicyclic has one, and a tricyclic has no double bonds. With the addition of hydrogens or other derivatives, this rule for the level of unsaturation may be different.

C15H24 hydrocarbons are sesquiterpenes and can be considered to be made up of three isoprene units. Here, an acyclic compound will have four double bonds, a monocyclic has three, a dicyclic has two, and a tricyclic has one double bond. Sesquiterpenes will have a higher boiling point than monoterpenes, usually above 250 °C. In general, significant aroma-contributing terpenes will be derived from a monoterpene or a sesquiterpene structure. Diterpenes, being less volatile, are not commonly seen as aroma- contributing compounds in most essential oils.

The terpene and sesquiterpene compounds occur as oxygenated derivatives. Monoterpenes can be alcohols (citronellol, geraniol, menthol), aldehydes (citral, cinnamaldehyde), ketones (menthone, carvone), phenols (thymol, eugenol), esters (acetyl derivatives of alcohols), and oxides (cineole). There are also acids, lactones, and coumarins. Sesquiterpenes also occur as oxygenated derivatives, as already indicated.

Since volatility reduces with increased molecular size, sesquiterpenes are less volatile than monoterpenes. Oxygenated derivatives also have lower volatility than hydrocarbons. Despite this, sesquiterpenes and high boiling oxygenated derivatives are very important in food flavoring. While chewing, the food containing the flavor is kept for some time in the mouth, which is very close to the olfactory organ. In such cases, the molecules do not have to travel a long distance. Spices are generally used in savory foods that are eaten after warming: heating will increase the volatility. Very volatile molecules, such as a monoterpene hydrocarbon, will be very harsh on the olfactory system if held very close to the nose for a long duration or if heated.

It should be borne in mind that in many salable spice oils, the quality is measured by the content of a marker compound represented by a heavy oxygenative terpene or sesquiterpene (Table 4.1). A higher percentage of high boiler markers ensures that it has sufficient oxygenated terpenes and sesquiterpenes and is therefore considered superior.

A higher level of sesquiterpenes and oxygenated derivatives in salable spice oil is obtained by resorting to fractional steam distillation. In India, an improved two-stage extraction procedure is employed for making oleoresin. In this process, spice oil is first steam-distilled. In the second stage, deoiled spice is extracted with solvent. As mentioned earlier, in the first stage of steam distillation, the oil is collected into two fractions. The first fraction will have more of the low boilers, such as monoterpenes, and the second will have more of the high boilers, such as sesquiterpenes and oxygenated derivatives (Table 4.2). The first fraction, with stronger top notes, will be very good for blending with the solvent-extracted nonvolatile resin to produce a good oleoresin. The second fraction, with enriched high boilers, will be a good salable spice oil. Again, the way to further increase the sesquiterpene and oxygenated derivatives is by using prolonged steam distillation.

Some of the simple hydrocarbons can undergo oxidation during storage. This happens in limonene, which is the major monoterpene hydrocarbon in citrus oils. On oxidation, limonene gives off an unpleasant camphoraceous odor and flavor. An excessive presence of hydrocarbon not only dilutes the desirable flavor given by oxygenated compounds, but also limits the oil’s dispersibility in water and dilute alcohol when used in beverages. To avoid these problems, terpene hydrocarbons are removed to obtain terpeneless citrus oils. The hydrocarbons, being more volatile, can be separated by fractional vacuum distillation. Liquid partitioning using aqueous alcohol and hexane

Table 4.1. Quality marker constituents of major spice oils

Spice Oil

Quality Marker

Chemical Nature

Nutmeg oil

Myristicine

Dicyclic oxygenated terpene

Celery seed oil

Selinene

Dicyclic oxygenated sesquiterpene

Ginger oil

Zingiberine, яг-curcumene

Sesquiterpenes

Pepper oil

^-Caryophyllene

Dicyclic sesquiterpene

Table 4.2. Distribution of high boiler marker compounds in the first and second fractions during fractional steam distillation

Spice Oil

First Fraction

Second Fraction

% of the Total Oil

High Boiler Marker

% of the Marker in Oil

% of the Total Oil

High Boiler Marker

% of the Marker in Oil

Nutmeg oil (Sri Lankan)

25

Myristicine

0.2-0.5

75

Myristicine

2.5-3.5

Celery seed oil

40

Selinene

3-5

60

Selinene

16-18

Pepper oil

50

^-Caryophyllene

4-6

50

^-Caryophyllene

25-27

Ginger oil

10

Zingiberene

8-12

90

Zingiberene

42-44

is also possible, but residual solvent can be a problem. Oxygenated compounds, being more polar, are in the aqueous alcoholic fraction, while nonpolar hydrocarbons are in the hexane. Since residual hexane is unwelcome in a refreshing beverage, generally a single liquid fractionation with appropriately dilute alcohol is recommended. It must be stressed, however, that fractional vacuum distillation is the preferred procedure as it is more efficient.

Although old, the exhaustive six-volume books of Ernest Guenther (1948-1952) are still a valuable reference source for essential oils. For more detail, it would be worthwhile to look into the more recent book by Baser et al. (2010).

References

Baser, H.; Can, K.; and Buchbauer, G. 2010. Handbook of Essential Oils, Science, Technology and Application. Boca Raton, FL: CRC Press, Taylor and Francis Group.

Guenther, E. 1948-1952. The Essential Oils. 6 vols. Malabar, FL: Robert E. Krieger.

 
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