-by: T.C.Denston

Introduction

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.