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The Volatile Oils
A Textbook of Pharmacognosy.
By T.C. Denston. 3rd Edition 1939





Volatile or essential oils may be defined as oily liquids which are entirely, or almost entirely, volatile without decomposition. A few, e.g. oil of anise, are solid at 15.5� c., but melt to form a liquid at slightly higher temperatures. Those volatile oils prepared other than by distillation, e.g. oil of lemon, contain a small proportion of non-volatile matter.

All the official volatile oils are of vegetable origin. In most instances the volatile oil pre-exists in the plant and is usually contained in some special secretory tissue, e.g. the oil-ducts of umbelliferous fruits the 'oil-cells or oil-glands occurring in the sub-epidermal tissue of the lemon and orange, and the mesophyll of eucalyptus leaves. In some instances the volatile oil does not pre-exist, but is formed by the decomposition of a glycoside. For example,
whole black mustard seeds are odourless, but upon crushing the seeds and adding water a strong odour is evolved. This is due to allyl isothiocyanate (the principal constituent of essential oil of mustard) formed by decomposition of a glycoside, sinigrin, by an enzyme, myrosin. Glycoside and enzyme are contained in different cells- of the seed tissue and are unable to react until the seeds are crushed, with water present, so that the cell contents can intermingle.

The weight per ml. of volatile oils varies from 0.800 to 1.15 g., most of them being below 1.00 g. The official oils having a weight per ml. above 1.00 g. are oil of cinnamon and oil of clove.

All volatile oils are freely soluble in ether and in chloroform, and fairly soluble in alcohol; they are slightly soluble in water, sufficient to give it their characteristic odour and taste.

Smeared on paper, they give a translucent stain which is temporary only, disappearing as the oil volatilizes.

Methods of preparation
The majority of volatile oils are produced by distillation. With certain oils, chiefly those valued for their aroma (e.g. oil of lemon), a distillation process would promote oxidation or other changes in the constituents of the oil, and impair its flavouring power. For these oils a process not involving the use of heat is adopted.

Preparation by Distillation
The boiling-point of many volatile oils is above 200� a., and at this high temperature chemical changes, e.g. oxidation, would in some instances take place.

Distillation of a volatile oil with water follows the law governing the distillation of immiscible liquids, namely that distillation takes place when the sum of the vapour pressures is equal to the atmospheric pressure. Hence the boiling- point of such a "mixture" would be lower than that of the constituent with the lower boiling-point-in other words, below the boiling-point of water (100� a.). The process of distillation after admixture with water therefore provides a means of separating a volatile oil from the drug in which it occurs, at a much lower temperature than would be possible without the addition of water. Further, if water were not added, considerable carbonisation of the drug would take place, and the resultant volatile decomposition products would distil over with the oil and render it useless.

Direct heating of a mixture of drug and water by a naked flame would, similarly, cause partial carbonisation: of the layer of drug resting on the bottom of the still, and again volatile decomposition products would give an empyreumatic odour to the volatile oil, and impair its value. Consequently some forms of still for the production of volatile oils are provided with a false, perforated bottom, below which is placed the water. Upon heating, steam rises through the perforations, passes through the drug and on to the condenser, carrying with it the volatile oil.

In the more modern stills the admixture is heated by means of a steam coil. The principle is shown in
Fig. 144. A layer of water is run into the still, and the drug, suitably comminuted, is placed in the wire basket. Steam is admitted to the jacket of the still, and the water therein raised to boiling. At this point, steam is admitted into the still itself by the free-steam pipe, thus heating the drug and reducing the condensation of steam therein. The oil-vapour and steam are condensed in the worm condenser, and drip into the Florentine receiver. In this, most of the oil is mechanically separated from the water, the latter being returned to the still. by using this water for each subsequent distillation, the procedure adopted in Bulgaria, where the bulk of the oil is produced. In France, however, the production of Oil of Rose is not the primary objective, consequently the oil-saturated water, which contains a preponderance of odorous substances, is not returned to the still, but enters commerce as Rose Water.



Separation of Oil and Water .
The distillate is usually collected in a vessel called a Florentine receiver. The one shown at the top of Fig. 145 is a type used for oils lighter than water. The distillate collects in the receiver, and separates into an upper oily layer and a lower watery layer. As the receiver fills, the liquid also passes into the spout, whence it overflows upon reaching the upper bend. Because the spout leads from the bottom of the receiver, only water can overflow, and in this manner the bulk of the water is automatically separated from the oil. When the receiver becomes nearly filled with the separated oil it must be emptied, and the process restarted.

A form of Florentine receiver suitable for oils heavier than water {e.g. oil of clove), is also shown diagrammatically in Fig. 145. In this, the spout opens a little below the centre of the receiver and thus draws off the upper layer of water . The oil is removed from. time' to time by means of the tap seen on the right. .

Prevention of Loss due to Dissimilar Solubilities of Constituents of Oil in Water

It will be apparent that, as volatile oils are slightly soluble in water, the liquid which overflows will be a saturated solution of the oil. To prevent loss of this oil the liquid is usually returned to the still by means of a pipe, so that a larger yield of oil is obtained, and, furthermore, a smaller initial volume of water is required.

This procedure is very important in some instances. As will be seen later, volatile oils are rarely single chemical substances, but usually consist of several, differing widely in their solubility in water. Generally speaking, the valuable substances are fairly soluble, and the valueless practically insoluble. Consequently the water, which always forms the major portion of the distillate, may contain in solution only a negligible quantity of the valueless, but an appreciable amount of the valuable part of the oil. Oil of Rose is a good illustration of this distribution. It is estimated that the oil produced by distillation with fresh water contains only about one-third of the principal odorous substance (phenyl ethyl alcohol) of the oil as it occurs in nature, two-thirds passing into solution in the water. This initial reduction in the proportion of odorous sub- stances in the oil can, of course, be almost entirely eliminated.

Preparation by Scarification.
This method is used for the preparation of oil of lemon, oil of orange, and oil of bergamot. These oils are found in large oil-glands just below the surface in the peel of the fruit. The two principal methods of scarification are the "sponge," and the ecuelle a piquer. The " sponge " process is used almost exclusively for the production of oil of lemon and oil of bergamot in Sicily and Southern Italy, whence the bulk of the world's supply is obtained. The ecuelle a piquer method is used in Southern France for the preparation of oil of lemon.

(a) Sponge Process.
In the sponge process the lemons are halved transversely, or longitudinally when the peel is required for candied peel. The contents of the fruit are then removed, and after the peel has been immersed in water for a short time it is ready for expression. The operator takes a sponge in one hand, and with the other presses the softener! peel against the sponge, so that the oil-glands burst open and the sponge absorbs the exuded oil, which is transferred to a collecting vessel. The turbid liquid consisting of oil and water is allowed to stand for a short time, whereupon the oil separates as an upper layer which is poured off. Itstill contains a small proportion of water and suspended matter, which is removed by repeated decantation after suitable periods for sedimentation. The whole of the above process is carried out in cool darkened rooms in order to minimize the harmful effects of heat and light on the oil.

(b) Ecuelle a Piquer Process.

For the
ecuelle a piquer process the apparatus is made of pewter, and takes the form of a cup-shaped funnel about 25 cm. across. It is provided with a lip and "' short, closed stem, which serves both as a handle and as a reservoir for the exuded oil. Projecting from the inner surface of the 'Cup are numerous brass pins across which the softened peel is drawn with a rotary movement. The liquid exuding from the ruptured oil glands collects in the stem, whence it is transferred to a collecting vessel and treated as above.

The above two processes, though yielding an oil of high quality, involve a considerable amount of hand-labour which makes the cost of production high. Consequently machines ,of various types (utilizing the principle of scarification of the peel of whole lemons with a suitable arrangement for the collection of the exudation of oil and water) have been invented to replace preparation by hand, and are being increasingly used.

The only official volatile oil produced by expression is Oleum Limonis. The yield is about 0.8 grammes per lemon, and, as mentioned above, the principal producing areas are Sicily and Southern Italy, only a small proportion of the world's supply coming from Spain, France, and California. A small but increasing amount is also being produced in Algeria.

Other Methods of preparation:

By Solution.

This method is used for the extraction of volatile oil from certain flowers ; it is not used for the production of any of the official oils. The delicate odour of many flowers would be seriously impaired by a distillation process. Further, only a small proportion of oil is present in the flowers, and these floral oils, like other volatile oils, are slightly soluble in water. Consequently a distillation process would yield an aqueous liquid from which only a little oil would separate ; thus it is only in those instances where the oil-saturated portion of the distillate has a wide commercial use (e.g. triple rose water), that a distillation process is economically sound. Two types of solvent process are employed.

Extraction by Volatile Solvent.
As mentioned earlier, the boiling-point of most volatile oils is near to 200C., hence a low boiling-point solvent, e.g. light petroleum {45-60� a.) can be separated by fractional distillation. The flowers are extracted by the solvent, and the latter distilled off at a low temperature, leaving behind the volatile oil. In an alternative and newer method used at Grasse for the extraction of roses, the fresh petals are cut up, subjected to strong pressure, and the juice collected. From the latter the rose oil, admixed with other substances' is then extracted with light petroleum. The pressed mass is broken up, damped with water, allowed to stand for an hour or so, again subjected to strong pressure, and the expressed liquid similarly extracted. This method utilizes the long-known fact that enzyme action increases the yield of odorous substances, part of which occurs in the form of glycosides which undergo decomposition by the enzymes present in other cells, when the cell-contents are allowed to intermingle. Similar glycosidal decomposition takes place in enfleurage
(vide infra), with a resultant higher yield of perfume. In hot fat-extraction (vide infra), the temperature is prejudicial to enzyme action, and the yield of perfume is less, a fact which explains the continued use of the more tedious process of enfleurage.

Extraction by Non Volatile Solvent.
A non-volatile solvent, e.g. a fine quality of either lard or olive oil, is used in the other solvent process. After saturation with the floral oil the lard or olive oil is sometimes used as a base for the preparation of pomades, bril- liantines, etc., or converted to a "triple extract." In the latter instance the lard or oil is agitated with two or three successive portions of alcohol, which dissolve the odorous substances. The mixed alcoholic solutions so obtained constitute the "triple extract "
of commerce.

There are three chief methods of saturating the lard or oil with the per- fume of the flower, one called enfleurage, another maceration, the third being a spraying process.

(a) Enfleurage.
For use with lard the apparatus consists of a wooden frame enclosing a sheet of glass, both surfaces of which are thickly smeared with lard. The flowers are placed on the glass, and the frames stacked in a vertical column. In this way each layer of flowers is enclosed above and below by a layer of lard, which ensures solution of the odorous principles. The exhausted material is replaced by fresh flowers until a desirable concentration is reached, when the lard is removed from the glass, and a triple extract then prepared.

When oil is used as a solvent the flowers are placed on an oil-soaked cloth supported by a metal grid enclosed in a frame. These are stacked as described above, fresh flowers introduced as required, and finally the oil is expressed from the cloths. It may then be used as a perfumed oil, or extracted with alcohol to produce a triple extract.

(b) Maceration.
This process is used at Grasse to extract the flowers of the orange, rose, and violet. The lard or oil is heated over a water .bath, a charge of flowers added, and the mixture stirred continuously for some time. The exhausted flowers are removed, pressed, the expressed fluid returned to the hot fat, fresh flowers added, and the process continued until defined weights of flowers and solvent have been used. Again, a triple extract is prepared by extracting the perfumed lard or oil with alcohol.

(c) Spraying.
In this process a current of warm air is forced through a column of the flowers. The emerging oil-laden air is then sprayed with. oil or melted fat which absorbs and dissolves most of the perfume, the collected oil or fat being then extracted with alcohol as described above.

Rectification to Remove Non-Volatile Matter.

Oil of Turpentine. The oil initially produced from Turpentine, contains traces of resin carried over mechanically during steam distillation. This oil is quite suitable for most industrial purposes and is sold without further treatment under the name of Spirit of Turpentine. For medicinal purposes, this oil is redistilled, thereby producing an oil containing only a small proportion (official limit 0.5 per cent) of non-volatile matter, which is chiefly resin.



Oil of Cajuput.
The oil obtained initially is
green due to the presence of traces of copper dissolved from the still, or more usually from the containers used for transhipment. By re-distillation the oil is freed from copper and is then either colourless or yellow. In which case, re-distillation is not the only means of rectification the copper can be removed by shaking the oil with a solution of tartaric acid, separating this liquid, washing the oil with water and then drying.

Rectification of Volatile Oils 'the oil obtained by steam distillation of the oil-containing material does not, in certain instances, conform to the official standards and the product may ,therefore be subject to rectification by re-distillation or other means as described below. Rectification may be necessary in order to remove non volatile matter, or to adjust the proportion of a particular constituent or constituents to the official standard. 

Oil of Caraway, Oil of Eucalyptus.
As initially obtained, these oils contain excess of non-volatile matter, hence rectification is required, and is effected by re-distillation.

2. Rectification for Purposes of Adjustment Oil of Peppermint.
Genuine English Oil of Peppermint may exceptionally contain as little as 3 per cent of ester, or
as high as 15 per cent, the official range being 4.0 to 9.0 per cent. The proportion of ester in American oil is 5 to 9 per cent, i.e. within the official range, but the proportion of free menthol may be as low as 40 per cent, the official minimum being 45 per cent.

These variations are due to factors such as the amount of sunshine during the summer preceding collection of the herb, and other factors beyond control. It follows from the above figures that part of the oil produced does not comply with the official standards unless it is subject to rectification.

Rectification is effected according to the composition of the oil, often by blending different batches to produce an oil conforming to the official standards.

The bulk of Oil of Peppermint is consumed as a flavouring in the manufacture of confectionery, cordials, dentifrices and liqueurs. Oils are specially prepared for these purposes by blending natural oils, or by fractional distillation and admixture of selected fractions. These oils do not, in all cases, conform to the official standards, solubility in alcohol, odour and taste being the criteria by which they are assessed rather than chemical standards. Actually many of these special oils exceed the official standards.

Oil of Australian Sandalwood.
The British Pharmaceutical Codex had two oils of sandalwood, one known simply as Oil of Sandalwood or as East Indian Sandalwood Oil, the other being the Australian variety. The medicinal action of the East Indian Oil is due principally to certain alcohols, of which 92-98 per cent are present, and the Codex therefore fixed the reasonable standard of 90 per cent w/w. The alcohols present in the Australian Oil, though not identical chemically, possess the same medicinal action as those of the East Indian Oil. The oils are therefore compar- able medicinally, provided the same proportion of alcohols is present, consequently the same standard (not less than 90 per cent w/w of alcohols) is desirable for both oils. As produced initially, the Australian Oil does not contain 90 per cent of alcohols, hence rectification to raise the proportion is necessary. To do this, the oil is fractionally distilled whereby part of the terpenes are separated, and the alcohol content thereby raised to 90 per cent or more.

Rectifying Stills
These vary considerably in design according to the special purpose in view. When removal of non-volatile matter is the objective, the still is usually provided with a long head to minimize the possibility of non-volatile matter being carried over mechanically with the steam and oil vapours, as shown in Fig. 146.

Drying of Volatile Oils
The distilled or rectified oil, or that prepared by scarification contains a proportion of mechanically-admixed water. This may be separated in several ways, e.g. by setting the mixture aside until the water separates completely, by centrifuging the mixture whereby separation is quickly effected, or by the addition of a dehydrating agent, e.g. anhydrous sodium sulphate, calcium chloride.

Storage of Volatile Oils
For all volatile oils, three conditions of storage are required, namely;

1.Storage in Well-closed Containers

This is prescribed for three purposes, viz. to minimize (i) volatilization, (ii) exposure to air, and (iii) absorption of moisture. Various changes may ensue from exposure to air, usually due to oxidation, and, in some cases, these changes are accelerated in the presence of moisture. Examples are;

Purified Volatile Oil of Bitter Almonds.
This oil consists principally of benzaldehyde which, upon exposure to air, undergoes oxidation to benzoic acid which separates as crystals.

Oil of Anise.
This oil contains anethole, an alcohol, which upon exposure to air is slowly oxidized to anisic aldehyde, and this in turn to anisic acid.    

Oil of Caraway.
This oil becomes thicker and its weight per ml. increases upon exposure to air .

Oil of Clove, Oil of Cinnamon.
These oils become reddish-brown upon exposure to air.

Oil of Dill.
This becomes darker on exposure.

Oil of Turpentine.
Upon exposure to air and moisture oxidation occurs, the odour becomes. stronger (probably due to formation of an aldehyde), the oil becomes viscid, and develops a yellow colour, the weight per ml., boiling point, and solubility in alcohol increase, and the oil may develop an acid reaction, changes possibly accounted for, in part, by conversion of some of the terpene to resinous substances.

2.Protection of Oils from Light
This is conveniently effected by the use of amber-colored bottles, well-stoppered to meet the first-mentioned requirement. Light promotes many of the changes mentioned above, particularly those due to oxidation. Further, it accelerates hydrolysis of esters if moisture is persent: it may also cause polymerization of the constituents of certain oils.

3. Storage in a Cool Place
This requirement, of course, reduces the possibility of loss by volatilization, and minimizes oxidation and other changes.

Physical Examination of the Volatile Oils.

In the descriptions which follow, mention is made of chemical tests for the evaluation of many of the volatile oils, but for others no tests are mentioned. It should not however be concluded that no safeguards are imposed in these instances to confirm genuineness or detect adulteration. In all cases the Pharmacopoeia prescribes numerous physical requirements to which volatile oils must conform, and these either alone or in conjunction with other data constitute the standard of quality. In the brief summary which follows, the physical tests are enumerated, together with examples of their importance.

1. Odour.

This is important in distinguishing genuine from factitious oils which have been prepared to conform with official chemical, and other physical, requirements.

Fractions of Volatile Oils.

Odour is an important criterion of quality for oil of lemon and oil of lavender; for these oils the chemical and physical constants are not entirely relied upon as criteria of value.

2. Solubility.
The Pharmacopoeia sometimes prescribes solubility tests in alcohol, and in some instances these tests afford evidence of adulteration. In general, the solubility requirement serves to detect fixed oils and petroleum benzine, which are occasional adulterants of anise, sandal wood, and other oils-these adulterants reducing the solubility in alcohol.

3. Weight per mI.
This is an important standard in almost all cases, and in some it indicates quantitatively the proportion of valuable constituent. For example, the carvone content of oils of caraway and dill can be calculated approximately from the weight per ml. of the oils, provided adulteration has not been practised.

4. Melting-point.
Only one of the official oils is solid at just below room temperature, namely oil of anise; for this the Pharmacopoeia defines a minimum melting-point which is almost a criterion of purity.

5. Optical Rotation.
The official figure is the rotation of the plane of polarization produced by a column of oil 10 cm. long, at 20� a., using sodium light for the observations. The test is frequently very important in confirming genuineness or detecting adulteration or substitution.

The optical rotation of solid substances is determined by observation in alcoholic solution of stated strength. Menthol is optically active when pre pared from the natural sources described on page 582, but synthetic racemic menthol, which is also official, is optically inactive.

6. Refractive Index.
This term has the ordinary meaning associated with the measurement of the refraction of an oblique ray of light when passing from one medium into another of greater or less density-the sine of the angle of incidence divided by the sine of the angle of refraction giving the figure called the refractive index. This varies with the temperature at the time of determination, and when necessary must be corrected to the temperature officially indicated in the test.

As this test requires very little of the liquid, and is quickly performed, it is one of the first tests carrier out with volatile oils and many other substances. Deviation from the official range is
prima facie evidence of adulteration, and where it is merely a question of acceptance or rejection of the oil, there is no call for further detailed examination.

Official Volatile Oils

The two principal kinds of substance present in volatile oils are hydrocarbons and oxygenated compounds, the latter embracing a wide range of chemical groups, e.g. alcohols, esters, aldehydes, and ketones.
For description, the volatile oils will be classified according to the chemical character of the principal constituent or constituents.

Oils consisting chiefly Hydrocarbons
This term indicates the groups known as the terpenes, the sesquiterpenes, and the polyterpenes-all of which are com- pounds containing only hydrogen and carbon.

These hydrocarbons are usually devoid of aroma. Further, the terpenes particularly are subject to oxidation or other changes when exposed to moisture, light, and .air. These changes form new compounds, which frequently have an objectionable odour. A few volatile oils consist entirely of hydrocarbons, but most of them are valued for one or more aromatic or odorous substances which are usually accompanied by hydrocarbons. The latter are practically insoluble in water, and much less soluble in alcohol than the non-hydrocarbon constituents. It is, therefore, often desirable to eliminate the hydrocarbon portion of volatile oils for two reasons-

(a) to reduce the quantity of alcohol necessary to effect solution in the preparation of essences,

(b) to remove the portion likely to develop an objectionable odour . 

Oils from which terpenes have been removed in part or entirely are called terpeneless, and those from which sesquiterpenes have also been removed are called sesquiterpeneless oils. The method used is fractional distillation under reduced pressure. The boiling-point of terpenes is usually considerably below, and that of sesquiterpenes above, that of the non-hydrocarbon constituents of the oil. Hence rejection of the lower boiling- point fraction ensures a terpeneless oil.

From this, a sesqui-terpeneless oil may be produced by collecting only that portion which distils through a defined range of temperature, thus leaving the sesquiterpenes in the still. Terpeneless and sesquiterpeneless oils may thus be regarded as concentrated oils. The concentration necessarily varies for different oils, according to the proportion of hydrocarbons available for elimination. For example, oil of lemon contains about 92 per cent of valueless hydrocarbons, chiefly terpenes with a small proportion of sesquiterpenes. Elimination of the terpenes concentrates the valuable portion about 10 times, and the additional elimination of sesquiterpenes raises it to about 12. In this instance the
actual concentration exceeds the theoretical, because the flavouring power of the odorous portion is partly reduced when terpenes and sesquiterpenes are present. Oil of caraway consists of approximately equal proportions of terpene and aromatic constituents, and consequently the maxi- mum concentration for this oil is only 2.

These are isomeric substances of the formula C10H16  but they vary considerably in structure. In volatile oils almost all the terpenes, and similarly the sesquiterpenes, occur in an optically active form. In common with all substances containing an asymmetric carbon atom, they are able to rotate the plane of polarized light either to the right {dextro-rotatory), or to the left {Ievo-rotatory). The more important terpenes are,

Pinene.This is widely distributed in volatile oils, d-pinene being the principal constituent of American oil of turpentine, and l-pinene of French oil of turpentine.

The d-form {also known as citrene, and carvene) is the principal terpene in oils of lemon, caraway, and dill ; l-limonene is present in .small proportion in oil of lemon ; dl-Iimonene, a mixture containing equal proportions of both, and therefore optically inactive, occurs in oils of nutmeg, and Siberian fir. It is also known as dipentene. Terebene, an artificial terpene prepared from oil of turpentine, consists almost wholly of dipentene.

Phellandrene. This terpene occurs in oil of Siberian fir, and is present to a considerable extent in the oil obtained from species of Eucalyptus, e.g. Eucalyptus amygdalina. Its presence in substantial proportion in eucalyptus oil used for medicinal purposes is undesirable, and therefore the Pharmacopoeia prescribes a test  which excludes oils containing an undue proportion.

Camphene. This terpene is exceptional in being solid at room temperature. It occurs in small quantities in oils of rosemary and Siberian fir .


These also are isomeric, their general formula being C15H24. As earlier stated, they are usually the highest-boiling fraction of the oils in which they occur. The principal sesquiterpenes occurring in official volatile oils are
Oaryophyllene. This is the main constituent of oil of copaiba, and is present in oil of clove.
Oadinene. This is found in oils of cubebs, cade, and lemon. With the terpene pinene, it comprises almost the whole of oil of juniper. Other constituents, occurring in small proportions, are azulene, to which is due the blue tint of freshly distilled oil of chamomile, santalene, present in oil of sandal wood, and bisabolene, in oil of lemon.


These substances have the same empirical formula, C5H8, as the terpenes and sesquiterpenes, but they are more complex. The only official volatile oil which consists principally of terpenes is Oil of Turpentine

Extracted from:
A Textbook of Pharmacognosy
by T.C.Denston. 3rd Edition 1939             

See also ..  Essential Oils  .. Classification of Volatile Oils ..