Selection of the Solvent (Petroleum Ether, Benzene, Alcohol) EXTRACTION WITH VOLATILE SOLVENTS

(a) Selection of the Solvent.

The most important factor for the success of the extraction process is the quality of the solvent employed. The ideal solvent should possess several properties:

1. It should completely and quickly dissolve all the odoriferous principles of the flower, yet as little as possible of such inert matter as waxes, pigments, albuminous compounds, etc. In other words, the solvent should be selective.

2. It should possess a sufficiently low boiling point to permit its being easily removed (distilled off), without resorting to higher temperatures; yet, the boiling point should not be too low, as this would involve considerable solvent loss by evaporation in the warm climate of Southern France.

3. The solvent must not dissolve water since 'the water present in the flowers would dissolve and accumulate in the solvent.

4. The solvent must be chemically inert, i.e., not react with the constituents of the flower oil.

5. The solvent should have a uniform boiling point; when evaporated it must not leave any residue. The slightest traces of high boiling compounds, upon evaporation of the solvent, would accumulate and remain in the flower oil and completely spoil its odor. It should be borne in mind that the yield of flower oil is generally very small, and that large quantities of solvent are required to cover the flower material in the extractors. In the case of petroleum ether, for instance, even traces of high boiling impurities are apt to impart to the concretes and absolutes an objectionable off-odor of kerosene, which cannot be eliminated without doing considerable harm to the delicate flower oil.

6. The solvent should be low-priced and, if possible, nonflammable. The ideal solvent which would fulfill all these requirements does not exist. Considering every feature, highly purified petroleum ether appears to be the most suitable one, with benzene (benzol) ranking next. Mixed solvents form a fascinating problem which so far htis been little touched, but which promises quite interesting results. As compared with straight solvents, mixed solvents can either reduce or increase their dissolvin power. Much experimental work along these lines has still to 1x3 undertaken.

Petroleum Ether.

Crude petroleum on fractional distillation yields a number of hydrocarbon fractions of different boiling ranges which find certain'' industrial applications. The fractions, boiling range 30^o-70^o, commercially called petroleum ether, consist of saturated paraffins, viz., mainly pentane and hexane. Because of their chemical inertness and complete volatility, these fractions are particularly suited for flower extraction. A further advantage lies in their selective power of dissolving: they yield products which contain relatively little wax, albuminous and coloring matter, but correspondingly more of the odoriferous compounds. Certain American petroleums are best suited for our purpose, because they consist mostly of inert, saturated paraffins, whereas Galician, Rumanian, Russian or "cranked petroleum" contain derivatives of benzene and naphthene, as well as unsaturated olefinic compounds, the latter being chemically active and liable to polymerization. They may thus form high boiling compounds of objectionable kerosene odor, especially on prolonged use of the solvent.

The petroleum ether must be free from sulfur and nitrogenous compounds. It is purified by washing in turn with strong sulfuric acid, water, hot dilute sodium hydroxide solution, water, and then drying. Castille and Henri³⁸ recommended repeated washing with sulfuric acid monohydrate, followed by washing with alkaline potassium permanganate solution and drying. They found the hexane fraction of petroleum ether, boiling range 65^o-70^o, of great advantage in extraction work because solutes remain in their normal molecular state; unstable compounds stay unchanged and no addition compounds are formed.

------------------

8 Bull. we. chim, biol. 6 (1924), 299.

When testing petroleum ether for use in extraction work, special attention must be paid to the presence (absence!) of a nonvolatile residue. For this purpose a sample of 50 cc. should be evaporated in a glass or porcelain dish at a temperature not exceeding 40. After complete evaporation the glass dish should show no residual odor whatsoever, but especially no odor indicating the presence of kerosene or sulfur compounds. A similar test can be carried out by permitting the solvent to evaporate on a clean filter paper at room temperature. More detailed methods of testing petroleum ether for purity are described in the "United States Pharmacopoeia," Thirteenth Revision.

In the extraction plants of Southern France the petroleum ether is usually prepared by submitting petroleum fractions to slow and repeated rectification in special stills provided with high fractionation columns and dephlegmators. As a rule, a small quantity (about 5 per cent) of odorless paraffin or fat is added to the gasoline in the still so that higher boiling compounds are retained and prevented from distilling over. According to the quality of the gasoline employed, 20 to 40 per cent remains as residue in the still, while 60 to 80 per cent represents the final cceur (heart) of petroleum ether suitable for extraction. Its boiling point should not be higher than 75.

Although petroleum ether is the best solvent found so far for flower extraction, it possesses some inherent disadvantages for example, relatively high solvent losses in the course of the extraction process. These losses are due primarily to evaporation of the low boiling, almost gaseous, fractions. Furthermore, petroleum ether is readily inflammable and dangerous to work with.

Benzene (Benzol).

Benzene ranks next to petroleum ether as a solvent for the extraction of flowers. It is a coal-tar product made by treating and purifying coal-tar naphtha with sulfuric acid and subsequently with sodium hydroxide. The fractions below 130^o contain the lower benzene hydrocarbons which are composed mainly of benzene (C₆H₆), toluene, and other homologucs. The industrial "benzol" often contains pyridine, carbon disulfide and thiophene which must be removed by treatment with concentrated sulfuric acid, water and caustic soda solutions. Further fractionation eliminates most of the higher boiling homologues but complete purification is obtained only by repeated crystallization. Thus, pure benzene melting point 5.5^o, is obtained, the higher homologues remaining liquid and being separated by vacuum filtration, or other methods.

Crystallizable benzene is of such purity that 95 to 98 per cent of it distills within 1 per cent of the theoretical boiling point 80.1^o. This uniform boiling point is of great advantage in extraction work, also because solvent losses are reduced. Yet, 80.1^o is a relatively high boiling point, which makes it rather difficult to remove the last traces of solvent from the concentrated flower oil.

A further drawback of benzene in flower extraction work lies in its high dissolving power. It dissolves not only the odoriferous principles but also much wax, albuminous, and coloring matter, so that the final flower oils extracted with benzene are dark, highly viscous, often almost solid masses, which can be purified only under considerable difficulties and by special processes.

Compared with petroleum ether, benzene usually gives much higher yields of concretes, due to the higher amount of inert wax, albuminous, and coloring matter present. As far as the actual odoriferous principles are concerned, the yields obtained by benzene or petroleum ether are usually quite similar.

Summarizing, it can be stated that petroleum ether is preferred for extracting the more expensive flowers, while benzene serves in the case of lower priced plant material such as oak moss and labdanum, where the presence of coloring matter is not considered of too great a disadvantage.

Alcohol.

Alcohol cannot be used for the extraction of fresh flowers because it dissolves the water contained in them and becomes increasingly more dilute. With some flowers (tuberoses, for example) alcohol developsa most disagreeable odor; from others (jasmine, for example) it extracts dark, solid masses which possess an odor similar to molasses.

High-proof alcohol in some instances dilute alcohol is widely employed, however, for the extraction of dried plant materials, leaves, barks, roots, and especially gums, from which alcoholic tinctures are obtained. These tinctures find wide application in pharmacy and perfumery.

Concentration of these tinctures, usually by driving off the alcohol in a vacuum still, results in the so-called oleoresins and resinoids. These products are usually viscous, often almost solid, masses of dark color, representing the concentrated odoriferous principles, plus the alcohol soluble resins, coloring matter, etc., contained in the original plant material.

Resinoids of olibanum, myrrh, opopanax, benzoin, etc., are widely employed in perfumery, oleoresins of vanilla, ginger root, capsicum, celery seed, etc., in the flavoring of all kinds of food products and beverages.