Topic 7: Essential oils
7.1 Introduction
7.2 Classifying essential oils
7.3 Physical properties of essential oils
7.4 Chemical properties of essential oils
7.5 Obtaining essential oils
7.6 Main uses of essential oils
7.7. References
7.1 Introduction
Essential oils are compounds made up of several organic volatile substances. These may be
alcohols, acetones, cetones, ethers, aldehydes, and are produced and stored in the secretion
canals of plants.
At room temperature they are usually liquid. Given their volatility, they can be extracted using
steam distillation, though other methods exist. On the whole, they are responsible for the
aromas of plants.
According to AFNOR (1998), they are defined as:
Products obtained from raw vegetable matter either by steam dragging or by mechanised
processes (epicarpium of citrus fruits) or by dry distillation. The essential oil is later separated in
the aqueous phase, using physical methods in the first two cases. They are able to undergo
physical treatment without important changes in composition (re-distilling, airing…).
This definition clearly establishes the differences there are between medicinal essential oils and
other aromatic substances used in pharmacy and perfumery which are commonly known as
essences.
Essential oils are widely distributed in nature and are found in conifers (pine, fir), myrtaceae
(eucalyptus), rutaceae (citrus spp), compounds (camomile), although the majority of plants with
essential oils are found in the labiatae (mint, lavender, thyme, rosemary) and umbelliferous
(aniseed) families.
They are found in different organs: roots, ryzomes (ginger), wood (camphor), leaf (eucalyptus),
flowering parts (Labiatae family).
Composition depends on place of origin. The habitat where the plant grows (normally warm
climates have more essential oils), the moment of harvesting, extraction methods, etc… are
also important.
Among the main therapeutic properties of essential oils antiseptics stands out (for many years
these spices have been added to foodstuffs not just for flavouring but to help preserve them).
Other properties are: antispasmodic, expectorant, carminative, eupeptic…
We should bear in mind that certain essential oils, especially in high doses, may be toxic to the
central nervous system in particular. Others, such as rue or juniper have abortive properties.
Others may cause skin problems, rashes or allergies
In addition to having therapeutic properties, essential oils are widely used in the pharmaceutical,
food, and perfume (especially) industries.
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7.2 Classifying essential oils
Essential oils may be classified using different criteria: consistency, origin, and chemical nature
of the main components.
a. Consistency
Depending on their consistency, essential oils are classified as :
-essences
-balsams
-resins
Fluid essences are liquids which are volatile at room temperature.
Balsams are natural extracts obtained from a bush or tree. They usually have a high
benzoic and cynamic acid content with their corresponding esthers. They are thicker, not
very volatile, and less likely to react by polymerising. Examples of balsams are copaiba
balsam, Peruvian balsam, Banguy balsam, Tolu balsam, Liquid amber…
Within the resin group we find a number of possible combinations and mixes:
1. Resins. These are amorphous solid or semi-solid products of a complex chemical
nature. They are physiological or physio-pathological in origin. Colophony, for example,
is obtained by separating trementine an oleoresin. It contains abietic acid and derivates.
2. Oleoresins. These are homogeneous mixes of resins and essential oils. Trementine,
for example, is obtained by making incisions in the trunk of different pine species. It
contains resin (colophony) and essential oil (trementine essence) which are separated
by steam drag distillation.
The term oleoresin is also used to refer to vegetable extracts obtained using solvents,
which should be virtually free of said solvents. They are frequently used instead of
spices in foodstuffs and pharmacy because of their advantages (stability, microbiotic
and chemical uniformity, and easy to add). They have the aroma of the plant in
concentrated form and are highly viscous liquids or semi-solid substances (black
pepper, paprika oleoresin, cloves…).
3. Gum-resins. These are natural plant or tree extracts. They are a mix of gums and
resins.
b. Origin.
Depending on their origin, essential oils are classified as:
-natural
-artificial
-synthetic
Natural oils are obtained straight from the plant and are not modified physically or
chemically afterwards. However, they are expensive because of their limited yield.
Artificial oils are obtained using processes of enriching the essence with one or several of
its components. For example, essences of rose, geranium, and jasmine are enriched with
linalool, and aniseed essence with athenol.
Synthetic oils, as the name suggests, are usually produced by combining their chemically
synthesised components. These are the cheapest and are thus much more commonly used
as fragrance and taste enhancers (vanilla, lemon and strawberry essences…)
c. Chemical nature.
The total essential oil content of a plant is generally low (less than 1%). However, by
extraction we obtain a highly concentrated form which is used in industrial processes. Most
of these are highly complex chemical compounds. The proportion of these substances
varies depending on the oil, but also on season, time of day, growing conditions, and
genetics.
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The term chemotype refers to the variation in chemical composition of an essential oil,
even of the same species. A chemo-type is a distinct chemical entity, different from
secondary metabolites. Certain small variations in the environment, geographical location,
genes…) which have little or no effect on a morphological level can, however, produce big
changes in chemical phenotypes.
Thyme (Thymus vulgaris) is a typical example. It has 6 different chemo-types depending on
which is the main component of its essence (timol, carvacrol, linalool, geraniol, tuyanol -4,
or terpineol). When this is the case, the plant is named using the name of the species
followed by the main component of its chemo-type. For example, Thymus vulgaris linalool,
Thymus vulgaris timol.
7.3 Physical properties of essential oils.
Essential oils are volatile and become liquid at room temperature.
When distilled they are at first colourless or slightly yellowish.
They are less dense than water (sassafras essence and clove essence being exceptions).
They are nearly always rotational and have a high refractory index.
They are soluble in alcohol and in the usual organic solvents, such as ether or chloroform,
and also in high grade alcohol.
They are lipo-soluble and not very soluble in water, but can be dragged using steam.
7.4 Chemical properties of essential oils (terpenoids).
Essential oil components are divided into terpenoids and non-terpenoids.
i. Non-terpenoids. This group contains short-chain aliphatic substances,
aromatic substances, nitrogenated substances, and substances with
sulphur. They are less important than terpenoids in terms of uses and
applications.
ii. Terpenoids. These are more important commercially and in terms of their
properties.
Terpenes, as we saw in topic 10, derive from isoprene units (C5) bonded in a chain.
Terpenes are a type of chemical substance found in essential oils, resins, and other
aromatic plant substances, (pines, citrus fruits…). They are usually found in
monoterpene oils (C15) and diterpenes (C20). They may be aliphatic, cyclic, or
aromatic.
According to their function group they can be:
· Alcohols (menthol, bisabolol) and phenols (timol, carvacrol)
· Aldehydes (geranial, citral) and cetones (camphor, thuyone)
· Esthers (bornile acetate, linalile acetate, methyl salicilate, anti-inflammatory
compound similar to aspirin)
· Ethers (1.8 - cineol) and peroxides (ascaridol)
· Hydrocarbons (limonene, pinene α and β)
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Table one shows the functional groups for each category:
Compound Functional Group Example Properties
Alcohol Menthol, geraniol
Anti--microbe, antiseptic,
tonic, spasmodic
Aldehyde Citral, citronelal Spasmodic, sedative,
antiviral
Cetona Camphor, tuyona
Mucolitic, regenerator
cellular, neurotoxic
Esther Methyl salicilate
Spasmodic, sedative,
antifungal
Ethers -C – O – C - Cineol, ascaridol Expectorant, stimulant
Phenolic ether Ring – O – C Safrol, anetol, miristicine
diuretic, carminative,
stomach, expectorant
Phenol Timol, eugenol,
carvacrol
Antimicrobes
Irritant
Stimulant (imunological)
Hydrocarbons
Only contain C and
H
Pinene, limonene
Stimulant, decongestant
Antiviral, antitumoral
a. Monoterpenic hydrocarbons
These are the commonest compounds in essential oils, and precursors of the
more complex oxidised terpenes. Their names end in –ene.
Limonene, for example, is the precursor to the main components of mint
essences (Mentha spp, Lamiaceae Family) such as carvone and menthol.
Limonene is also found in citric plants and in dill (Anethum graveolens,
Apiaceae family).
Pinene α and β are also widely present in nature, especially in trementine
essence of the Pinus genre (Pinaceae family).
b. Alcohols
Alcohols have the hydroxile group (OH) bonded to a C10
skeleton. Their names end in –ol. They are highly sought after for their
aroma.
Linalool, for example, has two forms. R-linalool is found in roses and
lavender and is the main component of Mentha arvensis. S-linalool found in
lavender oil at > 5% indicates adulteration.
Linalool gives tea, thyme, and cardamom leaves their taste. Menthol,
another compound found in this group, is responsible for the smell and taste
of mint. Mint essence may contain up to 50% of this component.
Geraniol, from scented geraniums (Pelargonium spp), citronelol, from
roses (Rosa gallica), borneol from rosemary, and santalol from sandalwood
(Santalum album, Santalaceae family).
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c. Aldehydes
Aldehydes are highly reactive compounds. Their names end
in –al. Many of them, such as those found in citrus fruits,
match their respective alcohol. For example: geraniol –
geranial, and citronelol – citronelal.
They are found in abundance in citrus plants, and are
responsible for their characteristic smell, particularly the
isomers geranial (α citral) and neral (β citral) known as citral
in combination (see graphic).
In addition to its characteristic aroma, citral has anti-viral, antimicrobiotic,
and sedative properties. But many aldehydes,
including citral, cause irritation to the skin and can not be used
externally.
Another important group are the aromatic aldehydes, such as
benzaldehyde, main ingredient of bitter almond oil and cause
of their typical aroma.
d. Phenols
They are only found in a few species but are very powerful and
irritating.
The most important are timol and carvacrol, which are found
in thyme (Thymus) and oregano (Origanus), both of the
Labiatae family.
Another important phenol is eugenol, which is found in many
species, for example, clove essence. It is both a powerful
bactericide and also anaesthetic, and is used in dentistry.
e. Phenolic Ethers
These are the main components of species such as celery and
parsley (apiol), aniseed (anetol), basil (metilchavicol), and
estragon (estragol).
Safrol is a component which is used extensively in the perfume
industry and is found in the bark of the sassafras tree
(Sassafras albidum Lauraceae family).
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f. Ketones
These are produced by the oxidisation of alcohols and are fairly
stable molecules. They end in –one. Carvone is found in Mentha
spicata.
Tuyone -first isolated in Tuya- (Thuja occidentalis Cupressaceae
family) and pulegone are fairly toxic and should never be used
during pregnancy.
Tuyone is found in plants of the Artemisia genus (Artemisia
absinthium with which absinthe and vermouth are made), and in
salvia (Salvia officinalis).
Pulegona was first isolated in Mentha pulegium.
g. Ethers
Ethers or monoterpenic oxides are reactive and unstable. One
example is bisabolol oxide found in camomile (Matricaria
chamomilla).
Another common ether is 1.8 –cineol (also known as eucaliptol),
which is the main component of eucalyptus oil. It is an expectorant
and mucolitic, and the main component of cough medicines.
The aroma of eucalyptus oil varies depending on 1.8 –cineol
content: the oil with a high content (Eucalptus globulus) is used for
medicinal purposes, whereas that with a lower content (Eucaliptus
radiata) is used in aromatherapy.
h. Esthers
Most esthers are formed from a reaction of a terpenic alcohol with
an acetic acid. Their aroma is characteristic of the oils in which they
are found.
Lavender oil, for example, contains linalool in its esther, linalile
acetate. The relative abundance of both these components is a sign
of good quality.
Methyl salicilate, a derivate of salicylic acid and methanol, is an
anti-inflammatory compound similar to aspirin and is found in a certain
type of heather (Gaultheria procumbens Ericaceae family). It is used
externally in liniments.
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7.5 Industrial uses of essential oils. Applying industrial processes to essential oils.
These are processes which are applied to essential oils and other aromatic vegetable
extracts to separate and concentrate components ready for industrial use or simply to make
it easier to homogenise quality.
a. Obtaining essential oils.
Steam drag distillation. The plants are placed on a perforated base or sieve at a certain
distance over a distilling tank. The tank contains water at a level less than the depth of the
sieve. Heating is via saturated steam when the water is heated using an in-built heater. The
steam flows at a low pressure and penetrates the vegetable matter. The component parts
volatilise and are then condensed in a refrigeration tube and collected in a Florentine flask
where water and oil are separated because of difference in density.
Pericarpium squeezing. This consists of a tray with spikes on it and a channel underneath
to collect essential oils. It is usually used for citrus fruits.
Solution in fats (enfleurage). Oils are soluble in fats and high grade alcohols. A thin film of
fat is placed on a glass plate and flower petals are then spread over it. The essence passes
into the fat until saturated. Then the essential oil is extracted using 70% proof alcohol. It is
used for flowers with a low but prized essential oil content (roses, violets, jasmine, orange
blossom).
Extraction using organic solvents. These penetrate the vegetable matter and dissolve
substances, which are then evaporated and concentrated at low temperature. Then the
solvent is eliminated, leaving only the fraction we want.
When choosing a solvent we want it to dissolve all the ingredients quickly while dissolving
the minimum amount of inert matter. It should have a low and even boiling point so that it
can quickly be eliminated, though this should not cause ingredient losses. It should be
chemically inert, so as not to react with the components in the oils, non-flammable, and
cheap.
This ideal solvent does not exist, and petroleum ether (boiling point between 30 and 70
degrees, flammable, easy to evaporate), benzene (which also dissolves waxes and
pigments), and alcohol (soluble in water) are commonly used. Alcohol is used when there
are components with a high molecular weight but which are not volatile enough.
Extraction using gases in super-critical conditions. Gases (usually CO2) at a
temperature and pressure above their critical points) are used. In these conditions, yield is
good and we avoid changes in the components of the essential oils. The necessary infrastructure
is expensive, put has its advantages, such as the rapid elimination of the extractor
gas by decompression, the absence of solvent residue, and the fact that gases are not
expensive.
b. Rectification.
This is the most common process. It consists in fractioning in a rectification column so as to
obtain portions which are then analysed separately. Those of the same quality are mixed
together. On the whole, essential oils are fractioned into three parts:
-Top or light part.
-Heart or middle part.
-Heavy fraction.
c. Fractioning.
This is similar to the above but the split is more specific. Essential oils with a 60-70% citral
content are fractioned to try and eliminate other components so as to obtain 90-97% purity.
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d. Deterpening.
When we eliminate terpenes (if they do not have the organo-leptic properties we want) the
essential oil becomes more soluble in water, and smell and colour are concentrated.
e. Dewaxing.
When an essential oil is extracted by squeezing rather than steam distilling, it contains
compounds such as the wax from the epicarpia of the fruit (as well as volatile terpenic
fraction).
f. Filtering.
Raw essential oils are filtered using filtering soils or other materials which retain residual
water (anhydrous sodium sulphate, magnesium carbonate…). This eliminates impurities.
g. Chemical reactions.
To obtain new aromatic products of a better quality or value, with pleasanter sensations, we
can use:
-Estherification (cedar, vetiver, and mint).
-Hydrogenation (citronella).
-Hydration (trementine)
h. Discolouring.
For essences with bright colours.
-Patchouli.
-Bursera graveolens.
-Clove.
i. Washing.
We wash the oil with a 1% sodium hydroxide or 10% sodium carbonate solution. This
eliminates the unpleasant smell caused by the presence of acids and phenols.
j. Standardising.
This is not an industrial process in itself. It arises from the need to homogenise or normalise
the quality of a product, because of the many variables which modify its characteristics. It is
carried out to comply with industrial requirements: same characteristics whatever the origin,
time of year, time of harvest…
k. Isolating specific products.
Some essences are commercialised to isolate some of their main components, such as
eugenol (essence of clove) or cedrol (essence of cedar).
7.6 Uses of essential oils
Food industry.
They are used to season or condiment meats, dried and cured meats, soups, ice-cream,
cheese… the most commonly used essential oils are cilantrum, orange, and mint. They are
also used in the elaboration of alcoholic and soft drinks, especially the latter. We should
make specific mention here of the essences of orange, lemon, mint and fennel, which are
also used in the making of sweets and chocolates.
Pharmaceutical industry.
They are used in toothpastes (mint and fennel essences), analgesics, and decongestant
inhalers (eucalyptus). Eucalyptol is also widely used in dentistry. They are used in many
medicines to neutralise unpleasant tastes (essence of orange or mint, for example).
Cosmetic industry.
This industry uses essential oils to make cosmetics, soaps, scents, perfumes, and make-up.
We should mention geranium, lavender, roses and patchouli essences as common
examples.
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Veterinary product industry.
This industry uses the essential oil of the Chenopodium ambrosoides, which is highly prized
for its ascaridol (worm-killer) content. Limonene and menthol are also used to make
insecticides.
Industrial deodorants.
At present, the use of essences to disguise the unpleasant smell of industrial products like
rubber, plastic and paint is being developed. Paint manufacturers use limonene as a biodegradable
solvent. Toys are also scented. In the textile industry they are used to mask
unpleasant smells before and after dyeing. In paper manufacture, products such as
notebooks, toilet paper, and face wipes are scented.
Tobacco industry.
Requires menthol for mentholated cigarettes.
Biocides and insecticides.
There are certain substances such as thyme, cloves, salvia, mint, oregano, pine… with
bactericidal properties. Others are insecticides:
- Against ants: Mentha spicata (spearmint), Tanacetum y pennyroyal.
- Against aphids: garlic, other Allium, coriander, aniseed, basil.
- Against fleas: lavender, mints, lemongrass, etc.
- Against flies: rue, citronella, mint, etc.
- Against lice: Mentha spicata, basil, rue, etc.
- Against moths: mints, Hisopo, rosemary, dill, etc.
- Against coleoptera: Tanacetum, cumin, wormwood and thyme, etc.
- Against cockroaches: mint, wormwood, eucalyptus, laurel, etc.
- Against nematods: Tagetes, salvia, calendula, Asparagus, etc.
Table 2. Some species with commercially used essential oils.
Common name Scientific name Family
Ajedrea Satureja montana Labiateae
Basil Ocimun basilicum Labiatae
Salvia Salvia sclarea Labiatae
Artemisia Artemisia vulgaris Asteraceaeae
Cardamom Elettaria cardamomum Zingiberaceae
Juniper Juniperus communis Cupresaceae
Lavender Lavandula latifolia Labiatae
Estragon Artemisia dracunculus Asteraceae
Eucalyptus Eucalyptus globulus Myrtaceae
Hisopo Hyssopus officinalis Labiatae
Lavender Lavandula officinalis; L. angustifolia Labiatae
Lavandine L. latifolia x L. angustifolia Labiatae
Melissa Melissa officinalis Labiatae
Mint Mentha piperita; M. spicata Labiatae
Myrtle Myrtus communis Myrtaceae
Oregano Origanum vulgare; O. majoricum Labiatae
Rosemary Rosmarinus officinalis Labiatae
Salvia Salvia officinalis Labiatae
Spanish Salvia Salvia lavandulifolia Labiatae
Sandalwood Santalum album Santalaceae
Thyme Thymus vulgaris Labiatae
Ylang – Ylang Cananga odorata Annonaceae
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7.7. References
Bruneton, J. (2001). Farmacognosia. Fitoquímica. Plantas Medicinales. 2nd Ed. Zaragoza:
Acribia S. A.
Las Plantas de Extractos. Bases para un Plan de Desarrollo del Sector. Fundación Alfonso
Martín Escudero. Madrid, 1999.
Pengelly, A. (1996). The constituents of Medicinal Plants. 2nd Ed. Cabi Publishing, U. K.
Publicaciones de la Cátedra de Farmacognosia y productos naturales. Facultad de Química,
Universidad de la República Oriental del Uruguay.
Van Ginkel, A. (2003). Apuntes del Máster y Diplomatura de posgrado de la UAB “Plantas
Medicinales y Fitoterapia. Módulo 2. Cultivo de plantas medicinales. Tecnología y Producción.”