The most common method for the production of volatile oils is that by steam distillation. This method not only produces the best yields and the purest oils, but also enables the operator to attain the end sought most cheaply and with the simplest apparatus. Furthermore, large quantities of oil can thus be produced without much human labor. In particular instances, however, distillation does not produce the desired result. This is particularly true when the high temperature of the steam destroys the unstable perfume substances, or when the amount of oil is relatively small, moreover if it is readily soluble in water. In such instances distillation may, under favorable circumstances, yield a fragrant water, which, even by cohobation, may not separate a volatile oil.
As a matter of fact, some of the most fragrant parts of plants, namely the flowers, are not suited to steam distillation. In order to prepare the odoriferous substances which impart to them their fragrant odor, other methods are employed. In such cases the volatile oils may be extracted either with volatile solvents or with fat, or they are allowed to be absorbed by fat.
Hence a distinction is made between:
B. Extraction with nonvolatile solvents, /'. e. fats.
1. Without the aid of heat: enfleurage.
2. With the aid of heat: maceration.
Which of these methods is best adapted to the extraction of the perfume of a particular flower has been determined in southern France by long years of experience.
Extraction with volatile solvents is applied to roses, Parma and Victoria violets, orange blossoms, jasmine, tuberose, jonquil, cassie, and migonette, occasionally to carnations, lily of the valley, heliotrope, elder flowers, stock, narcissus, and mimosas. In like manner a concrete oil is prepared from the so called Mousse de chine, the lichen Evernia prunastri.
Enfleurage yields the best results with jasmine, tuberose, jonquil, lily of the valley, and migonette.
Finally, the process of maceration is applied to rose, orange flower, violet, cassie, and lily of the valley.
As becomes apparent, some flowers e. g. rose, cassie and violet, can be treated equally well either with volatile solvents or with warm fats. It has been known for a long time that jasmin and tuberose when thus treated give an unsatisfactory yield, whereas the enfleurage process gives good returns.
Passy1) who first called attention to this fact, gives the following explanation for this difference in behavior of the several flowers.
"Flowers can be divided into two classes of which the one, to which rose and orange flower belong, contain their perfume ready made. The other, to which belong jasmin and tuberose, contain either no ready made volatile oil or but very little thereof. These, however, continually produce perfume as a part of the life process of the cells. If the flower be killed by petroleum, ether, or hot fat, the life process and with it the formation of volatile oil naturally ceases. If, however, the cut flower be placed over fat, it continues to live for a long time and to emanate odors which in the enfleurage process are absorbed by the fat."
When Hesse2) tested experimentally this hypothesis he arrived at the surprising result that in the case of jasmin the en-fleurage process yields ten times the amount of oil yielded by the extraction with volatile solvents. In the case of tuberose the proportion is even greater. Hence the conclusion is justified that in the case of the jasmin and tuberose the production of odoriferous substances continues during the process of enfleurage, being absorbed as it is formed. This also proves the superiority of the enfleurage process over that of the extraction method for certain flowers. For other flowers this does not hold true.
1) Compt. rend. 124 (1897), 783; Bull. Soc. chim. III. 17 (1897), 519. 2) Berl. Berichte 34 (1901), 293, 2928; 36 (1903), 1465.
Thus, according to Hesse and Zeitschel,1) 1000 ko. of orange blossoms yield only 100 grams of volatile oil when submitted to the enfleurage process, whereas maceration yields 400 grams and distillation with steam 1200 grams of oil, of which, however, about 400 grams are dissolved in the aqueous distillate.