ESSENTIAL OILS are those volatile aromatic constituents of certain flowers, fruits, seeds, etc., which contain their specific odors and flavors — that is, the properties which de light the senses of smell and taste. The object aimed at in the manufacture of these essences is that they may be transferred to other combi nations, through which the pleasure they afford may be enjoyed to a far greater extent. The delicacy of the methods to be employed may be better appreciated when it is remembered that these oils are products of the living plants, and that immediately upon harvesting the plant the essences begin to deteriorate, the loss vary ing with the period, which elapses between the time when the life of the plant is halted and the time when the essential oil is finally secured in a permanent form. Another point to be care fully attended to is that each essence is at its best at a certain time in the growth of the plant, and that it must be taken at that time neither immature nor past maturity. Other conditions are liable to affect the product injuriously, and heat is one of these. The delicate essence of the strawberry is quickly dissipated if the sun beats down hot upon it, and many other essen tial oils are as sensitive to heat. And this peculiarity of course prevents the use of the chief refining process of the chemist, that of distillation, for these particular oils. An ex ample of this condition is presented in winning the oil of lemon from the peel. Any attempt to obtain this oil by heat results in a product of low quality. The fine flavor has to be gained by cold pressing of the raspings of the surface. The banana, peach and pineapple are in the same class with the strawberry and the lemon in this respect. This difficulty is overcome in large measure by dissolving out the essential oils with alcohol, and distilling the alcoholic solution under vacuum — which so reduces the degree of heat needed that the delicate flavors are preserved. Another condition likely to in jure these sensitive substances is undue exposure of the plants or fruits to the air after harvest ing. Some oils quickly become rancid, and in the case of others fermentation of the source of the oil completely destroys it. The class of flavors and odors which are injured by fermen tation is practically the same as that which is supersensitive to heat. In the case of some other essences, as of the apple and cherry, fer mentation of the fruit serves to accentuate its particular flavor.
Volatile oils consist of two component groups, the taste-carriers and the terpenes. In addition
to these there may be varying proportions of waxy and resinous matters. The aim of the manufacturing processes•is to eliminate all but the taste-carriers' constituents, for it is solely upon these that its market value depends.
The methods employed in making essential oils are (1) expression ; (2) distillation ; (3) extraction. The first makes use of simple press ure; the second uses distillation with water or steam, and subsequent rectification to remove the water; the third is carried on by dissolving out the desired oil with a solvent, such as al cohol, chloroform, benzol, etc., these solvents being afterward distilled off at a low tempera ture under vacuum. The terpenes are removed from some kinds of oil by the vacuum process, being the first to pass over on the rise in tem perature. The sesquiterpenes follow. At a slightly higher degree the true flavor carrying oil comes over. When it is necessary to raise the temperature again, the fractions which then distil over are gathered separately, as not of the highest quality. Another method of remov ing the terpenes used with a class of oils which cannot be worked by the first process, is by alcoholic distillation. In this process one part of the oil to be treated is mixed with five parts of 43 per cent alcohol. Upon heating this mix ture vapors containing about 80 per cent alcohol and 20 per cent water, together with the vapors of the oil, pass over into a receiver where they are condensed. The terpenes separate as they are insoluble in alcohol of that strength. The essential oils remain in solution with the alcohoL The process is continued until the collecting terpenes cease to increase in quantity. The oil thus purified is dried by agitation with anhy drous sodium sulphate, and placed in lightproof and airproof bottles. From these essential oils are made the so-called °essences," tinctures, flavoring extracts, syrups for soda water, per fumes, cordials, liqueurs, etc. Many of the es sential oils are used as medicine, or in medicinal preparations, ointments and liniments.
The United States Census of Manufactures for 1914 reported 105 establishments engaged in the manufacture of essential oils, employing 435 persons, of whom 249 were wage-earners receiving annually $133,272 in wages. The capi tal invested totaled $1,616,682, and the year's output was valued at $2,313,606; of this, $748, 771 was the value added by manufacture. Two other establishments reported making essential oils as a subsidiary product, but the value of their production is not given. See OIL.