VOLATILE OILS AND FRACTIONS OF VOLATILE OILS :
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.