The word "essential oils" undoubtedly originates from the Latin "quinta essentia" (quintes- sence), the fifth element, the essence of things. Al- chemists believed essential, ethereal, or volatile oils to be the most condensed form of the individual character- istics of the plant or drug.

Since antiquity man has been mystified and intrigued by essential oils and has made great efforts to obtain them. Today as a result of many years of research these oils have become truly indispensable and essential to our way of life. During the course of a single day, from the first day of existence to the last, every living person in this country uses or consumes many of these substances. They have a variety of applications in al- most all industrial and consumer products, from rubber baby pants, food and toilet preparations to embalming fluids. Essential oils are used mostly for their sensory quality, but often also for their antibacterial and anti- fungicidal action. They are mainly of botanical origin, composed of the organoleptic principles which contrib- ute to the odor and flavor of the aromatic plant mate- rial; there are only very few exceptions of animal origin, such as musk, civet, and ambergris.

The term "oil" probably originated from observing that some of the glands or intercellular spaces in plant tissues are filled with oily droplets, similar to fatty or fixed oils. Essential oils and fixed oils have in common the properties that they are nonmiscible with (and usually lighter than) water, and soluble in ether and many organic solvents. On the other hand, the compo- sition and properties of these volatile oils differ greatly from fixed or fatty oils, the former consisting of various mixtures of different types of compounds, the latter being for the most part glycerides of fatty acids or paraffin oils. Essential oils are generally liquid (some- times semisolid or even solid) and, a t ordinary tem- perature, volatile. If a drop of essential oil and one of fatty acid are put on blotting paper, both a t first will leave an "oily" spot. The fatty-oil spot will remain transparent to light, but the essential-oil spot will be- come almost invisible in a short time, retaining only the non-volatile constituents which have an odor pecul- iar to the particular ethereal oil applied. There exist in scientific literature many references to essential oils, perfumes, and flavoring materials (1-10). Table 1 gives a list of the most important essential oils used today. For the botanical classification of essential-oil plants, a list of plant families and genera is available (11).

Frances 5. Sterrett Hofstra College

Hempstead, New York

This ia the firat of a two-part paper on essential oils. I n the next issue chemical constituents and analysis will be reported. Ae- knowledgment is gratefully made to Dr. Ernest Guenther for valuable suggestions.

The Nature of Essential Oils I. Production

The term production here refen to the various methods of separation of essential oils from the aromatic plant materials. These volatile oils are formed mostly in the secretory cells or reservoirs, internal or external glands of flowers, leaves, bark, wood, root, or fruit of an aromatic plant. Essential oils originate in practically every part of the world. Many are produced in remote geographical areas under exceptionally primitive con- ditions as an enterprise to supplement the family in- come.

Table 1. Important Essential Oils

Abies albh (pine) cone leaf (needle)

Acacia flower Ajowan Ailsnioe Almond (bitter) Ambretti seed' Angdioa root Angelica seed Anyustors bark Anise seed Arnica root Balm Balsam (different typed B a d (different tyge.) Bayberry Bay laurel Bay leaf norgarnot mint Bergamot Birch (sweet) niroi, tar Bois de rose Boineoeamphor Buohu leaf Cabreuva Cajunut Calhmus Cslnphor Carnnhor leaf

Car&& seed Carm.tion flower Carrot seed Cassia Catni" Cedar leaf Cedarwood Celery seed Cbhmomille (German and Roman) Cirenopodiurn (American wormaeed

oil) Cherry laurel Cinnamon bark Cinnamon leaf Citronella CLsry sage Clove (bud) Clove l e d Clove stem Coriander seed costus root Cllmin seed C Y P ~ ~ S P Dill herb Dill seed Elomi Ertragon Eucalyptus (different types) Fennel seed Fir (Siberian) Gariio Geranium Gingergrass Ginper root Gramfruit Hemlock Honeysuckle (concrete snd absolute) Hops Horsemint EIorseradish Jasmine flower Jonquil flower Juniper berry

Labdanum Leaf Labdanum Laurel Leaf Lavandin Lavender Lemonerass Lemon"' Lily (concrete and absolute) Lime (distilled and oddpressed1 Linaloe seed Lim.10e wood Lovage root xr""" ...- uv

Mandarin Marjoram (a?&) Marjoram (wrld) Mimosa Rawer Mint [different tvoesl . . M".k;oot Mustard seed M"rrh . . , . . . . Myrtle Ncroli bigarade Nutmeg oak moss (ooncrete and abeolute) Olibanum Onion 0pophne.x Orsnge (bitter) blossom

(Neroli bisarade oil) , Orange (bitter) leaf (Petltsrain) orange (bitter) or&"gc b e e t ) origsnum orris root Paimarosa Parsley herb Parsley seed Patehouly Pennyroyal Pepper P,,,,,,i"t bemy (different type4

prtitzrain biaarade Pin&ta ber& pirnenta leaf Pine needle (different types1 Pine root pinetar Rose (distilled, also oonorete and

absolute) Roeemary Rosewood Rue Sage Sandalwood

Volume 39, Number 4, April 1962 / 203

Methods of Production

The methods of production are complicated and practices vary, depending on plant material, the type of essential oil to be obtained, and availability of equip- ment. There are three basic methods of production: steam distillation, expression and extraction.

Steam distillation. Proper preparation of the plant material is of great importance, so that more rapid dis- tillation, higher yields, and better quality oils will result, and lower fuel consumption will be achieved as well (12-14). Since the essential oil is not distributed evenly throughout the plant material, it is often neces- sary to mechanically rupture as many plant cell walls as possible to render the volatile oil accessible to the passing steam. The exact degree of comminution (the mechanical breaking up) of the particular plant mate- rial varies. Flower petals and most leaves (which are relatively thin) can be distilled without comminution, seeds must be crushed, root stalks and woody material must he cut into small pieces. The comminuted mate- rial must be distilled as soon as possible to minimize evaporation of oil and possible chemical changes of con- stituents. For some oils the plant material may be used for distillation immediately after picking; sometimes the plant must be air dried or sernidried.

Even after the plant material is well comminuted, those plant cells which are not damaged have a tend- ency to retain some oil, and thus only the essential oil present on the surface is available for the passing steam. For this reason the plant tissues are often permitted to come in contact with the boiling water during distil- lation. The oil then diffuses through the surrounding membrane, bringing more volatile oil in contact with the steam and in this way improving the yield. On the other hand, large amounts of boiling water in con- tact with comparatively small amounts of essential oil may upset the equilibrium condition (water + ester + alcohol + acid) of the essential oil constituents. Hydrolysis of the esters present in the original oil may occur; the acid thus formed could possibly cause alter- ation of some of the other constituents present in the oil. However, since these hydrolysis reactions have a comparatively slow rate, a minimum contact with boiling water may in certain cases be preferred, using direct steam distillation, with the steam generated in a separate still. Since practically all essential oil con- stituents are unstable at high temperature (many isomerize, polymerize, and oxidize readily), the ideal dis- tillation therefore is one in which the diffusion rate is high, while hydrolysis and thermal decomposition are kept at a minimum. In general it is desirable to main- tain a low temperature during distillation and, if neces- sary, to employ high temperatures for as short a time as possible. Steam distillation may also be modified by changing the pressure in the distilling equipment to either reduced or excess pressure.

Expression. Essential oils which are contained in oil sacs in the peels of citrus fruit are obtained by "cold pressing" (15, 16). Basically the process of expression consists of mechanically rupturing these sacs and liher- ating the oil, which is collected together with some cell water and cell debris. The oil then is clarified and sep- arated by decanting, centrifuging, filtering, etc. Depending on the local facilities, the cold pressing is

done by hand or by machine. In Florida and Califor- nia (9, 10) modern, elaborate and ingenious machines keep the peel separated from the juice, thus producing the highest quality of "cold pressed" oils, such as sweet and bitter orange, lemon, grapefruit, mandarin, tange- rine, bergamot, and lime. The oil produced by this method at a yield of 1-5 kilogram per ton of fruit con- tains large amounts of waxes. The waste peel (after expression) still holds substantial quantities of oil as only 10-2070 of the oil contained in the peel is recov- ered by expression. To obtain the balance of the oil, the peel (after expression) is submitted to steam distil- lation, which process yields the so-called distilled citrus oils; their quality is inferior to that of the cold-pressed citrus oils.

Extraction. Most essential oils are isolated by steam distillation. However, some of the more delicate flow- ers (such as jasmine, tuberose, violet, jonquil, narcissus, mimosa, acacia, gardenia, hyacinth, etc.) when distilled yield either practically no oil or oil of inferior quality. The action of steam or boiling water on the flower material apparently causes the constituents of the essen- tial oils to hydrolyze, thermo-decompose, polymerize, resinify, etc., and the somewhat water-soluble constit- uents (such as phenyl ethyl alcohol or eugenol) are lost in the distilling water, thus rendering such oils actually useless since they do not represent the natural oil as it occurs in the flower. Therefore, the more delicate natural flower oils must be extracted. There are es- sentially three methods of extraction: enfleuwge (cold maceration), hot maceration, and extraction with vol- atile solvents.

Enfleurage and hot maceration (8, 17) are based on the property of fats (such as lard and tallow) to absorb the volatile odoriferous constituents of flowers. This was recognized empirically hundreds of years ago when flowers were treated by hot or cold maceration to yield fragrant pomades. Today these methods of extraction with fat are obsolete. Since they are complicated and involve much manual labor (which is costly) only in the region of Grasse, Southern France, are they still used and there on a very limited scale.

Hot and cold maceration have been almost com- pletely replaced by extraction with volatile solvents (18). This method is based on the fact that volatile solvents such as petroleum ether penetrate the petals and dissolve all of the natural perfume, also the waxes and coloring matters. The fresh flowers are extracted several times with a carefully purified solvent, which is subsequently removed, usually by vacuum distillation. The remaining product is a semisolid concentrated flower oil known as "concrete." These concrete flower oils contain considerable amounts of plant waxes, albuminous material, and color pigments, and are only partly soluble in alcohol. To obtain alcohol-soluble products the concrete must be converted into the so- called L'absolute" (19, 20). For this purpose the con- crete flower oils are treated several times with warm absolute alcohol in which most waxes are not soluble. The alcoholic extracts are then refrigerated to precipi- tate as much as possible of the remaining waxes which are filtered in the cold. The alcoholic tinctures thus obtained often find application in fine perfumes. In most cases, however, the alcohol is distilled off at re- duced pressure, to yield the absolute of the respective

204 / Journol of Chemicol Education

flower oil. The extracted natural flower oils are among the most highly priced products in the industry. (A pound of absolute of jasmine for example costs up to $500.) Separating the essential oils from the plant material by the method of cold pressing or extraction permits the operating temperatures to be kept low (under SOT) during most of the processes, thus pro- ducing an oil with a "true to nature" odor and flavor superior to the steam-distilled oils.

The yield of essential oil, disregarding the different methods of isolation, ranges on the average from about O.O1yo to roughly 5y0, by weight, of the aromatic plant material. It should also be kept in mind that the qual- ity of the same type of oil may vary greatly, depending 011 the availability of equipment and on geographic location and the climatic and soil conditions in which the aromatic plant grew.

Many natural oils are imported. However, during World War I and I1 when most imports were cut off, the United States developed domestic oils ($1). Today this country has become the world's largest producer of such important essential oils as citms oils from Cali- fornia and Florida, turpentine, peppermint, spearmint, dill, cedarwood, and sassafras oils; substantial quan- tities of these oils are exported every year. Citrus oils are now by-products in the manufacture of citrus juices (canned, straight, concentrated, frozen, etc.) and the machinery used in this billion-dollar industry is of the most modern engineering design.

Synthetic Oils

Prices of natural essential oils vary greatly, from less than 91.00 per lb to hundreds of dollars per lb. In order to get less expensive products, chemists for many years have endeavored to develop "synthetic" essential oils as substitutes for the natural products. This search made great advances particularly during World War I1 when pure essential oils were no longer avail- able from the sources abroad. Some of the artificial oils can be modified in grade and price depending upon the intended use. Many low-cost synthetic oils are used for covering objectionable odors, e.g., of artificial leather, synthetic rubber, etc.

The artificial oils consist primarily of two groups of aromatic chemicals. One group includes those syn- thesized economically from petroleum or coal tar de- rivatives plus some synthetics not known in nature. There is a constant demand and search for new chem- icals possessing interesting odor and flavor character- istics (88-$4). The other group, the L'isolates," are specific fractions from the distillation of the natural oils containing generally one chemical compound in more or less isomeric mixture, e.g., citral from lemon- grass oil, pinene from turpentine, linalool from hois de rose oil, geraniol from palmarosa or citronella oil, citronella1 from citronella oil, safrol from sassafras or camphor oil, eugenol from clove oil. These "isolates" may also be used as basic raw material for the manu- facture of other aromatic chemicals, e.g., citral con- verted chemically into ionones, eugenol into vanillin, beta-pineue into geraniol, citronellol, nerol, and lina- loo1 ("The Glidden process," $3), etc.

Another refinement in the manufacture of natural essential oils is the preparation of "concentrated,"

" terpeneless," "terpeneless and sesquiterpeneless oils" (25, $6). These contain mainly the oxygenated com- pounds concentrated from carefully selected pure nat- ural oils. They are more stable and stronger in odor and flavor, and are comparatively highly soluble in alcohol; hence they are more useful. Most essential oils consist of a mixture of hydrocarbons (terpenes, sesquiterpenes, etc.), oxygenated compounds (alcohols, esters, ethers, aldehydes, ketones, lactones, phenols, phenolethers, oxides, etc.) and a small percentage of viscid or solid non-volatile residues (paraffins, waxes, and products of resinification). Of these the oxygen- ated compounds are the principal odor and flavor car- riers, although the terpenes and sesquiterpenes con- tribute to some degree to the total odor and flavor char- acter of the oil. The oxygenated substances possess the advantage of better solubility in dilute alcohol and (with the exception of some aldehydes) of greater sta- bility against oxidizing and resinifying influences. The terpenes and sesquiterpenes, chemically unsaturated, oxidize and resinify readily in light and air; odor and flavor will be spoiled under improper storage conditions. The "concentrated oils" have only part of the low boiling hydrocarbons removed; this is done by simple frac- tional distillation in vacuo (1-5 mm pressure). "Ter- peneless oils" are those from which most of the terpenes and waxes have been removed by first distilling in vacuo and then extracting with dilute alcohol or other solvents in which the oxygenated compounds are soluble. Some newer methods of fractionation especially applied to essential oils are countercurrent distribution (87-29) and a recently reported continuous distillation for fine separation at low-flow rates (SO). The "terpeneless and sesquiterpeneless oils" have most of the terpenes and the higher boiling fraction of sesquiterpenes as well as waxes removed by a combination of fractionation in vacuo and extraction with dilute alcohol in which terpenes, sesquiterpenes and the waxes are almost in- soluble. The terpeneless and sesquiterpeneless oils represent the highest possible concentration of a nat- ural oil. The degree of concentration is automatically limited by the amount of oxygenated compounds pres- ent in the natural oil: for example, orange oil consists of 2% oxygenated compounds and 98% terpenes, ses- quiterpenes and waxes and therefore may be concen- trated fifty times. Bergamot oil, on the other hand, contains 50% hydrocarbons and 50% esters, alcohols, lactones, etc., hence can only be concentrated to double strength. These concentrated oils are more stable and soluble, but they do not have the freshness and bouquet of the original natural oil. They are used in many cosmetic preparations and for flavoring, e.g., gelatin desserts and candies.

Literature Cited

(1) GUENTAER, E., "The Essential Oils," Vols. 1-6, D. Van Nostrand Co., Inc., Princeton, N. J., 1948-52.

(2) GILDEMEIBTER, E., AND HOFFMANN, F., "Die Atherisehen Me," Vols. 1-3, 3rd ed., Miltite-Leipzig, 1928-1931. Also 4th ed., Vols. 1-4, Akademie-Verlag, Berlin, 1956.

(3) POUCHER, W. A,, "Perfumes, Cosmetics, and Soaps," Vols. 1-3, D. Van Nostrand Go., Inc., Princetan, N. J., 195960.

(4) BEDOUKIAN, P. Z., "Perfumery Synthetics and Isolates," D. Van Nostrand Co., Inc., Princeton, N. J., 1951.

(5) B E D O ~ I A N , P. Z., Am. Pmf. Armat., 75 (3) 51 and (4) 49 (1960), a review of progress in perfumery.

Volume 39, Number 4, April 1962 / 205

(6) SAGABIN, E., "The Science and Art of Perfumery," McGraw- Hill Book Go., Inc., New York, 1945.

(7) NAVEB, Y. R., AND M A X ~ E R , G., "Natural Perfume Mate- rids," translated by SAGARIN, E. (from "Les Perfuma Naturels," Gauthier-Villars, 1939), Reinhold Publishing Corp., New York, 1947.

(8) GRIBOU, H., Am. Pwf . Armat., 75, (6) 87 (1960), history of perfumes.

(9) BROWNE, C. A,, J. CHEM. EDUC., 11,131(1934), essentialoil industry of foreign lands.

(10) WHITE, L. J., AND EISERLE, R. L., Ind. and Eng. Chem., 53, No. 6, 421 (1961).

(11) GUENTEER, E., "The Essential Oils," Vol. 6: HAAS, T. P., "Botanical Classification of Essential Oil Plants," 391 (1952).

(12) GUENTHER, E., "The Essential Oils," Vol. 1, 1949, p. 104: practice of distillation.

(13) VON RECHENBERG, C., "Einfache und Fraktionierte Destill* tion in Theorie und Praxis," Schimmel & Co., Miltitz- Leipzig, 1923.

(14) BLOOMFIELD, L. A,, Pevfumew Essent. Oil Record, 27, 404, 443, 483 (1936); 28, 24, 59 (1937).

(15) GUENTHER, E., "The Essential Oils," Vol. 3, 1949, pp. 681. (16) GLENN, H. J., & e n . Botany, 1, 415 (1947). (17) GUENTBER, E., "The Essential Oils," Vol. 1, 1948, 188-200. (18) C r r r ~ ~ s , A., Perfumery Bssent. Oil Record, 12, 197 (1921). (19) GUENTHER, E., "The Essential Oils," T'ol. 1, 1948, 200-218. (20) NAYES, Y . R., Riechstoff lnd., 11, 135, 151, 176,212 (1936);

12, 23, 50, 137 (1937). (21) HALL, M. D., J. CHEM. EDUC., 25, 45 (1948). (22) HACKFORTH-JONES, J., Am. P q f . Avomat., 75 , No. 6, 90

(1960). ~ ~~~, (23) MAX, E., Perfumery Essmt. Oil Record, 51, 541 (1960). (24) NATES, Y. R., Bull. Soc. Chim. F~ance, 1754 (1959). (25) NAVES, Y. R., Mfg. Chemist, 18, No. 4, 173 (1947).

., Riv. ital. essaze profumi, 2I (27) SORM, F., ETAL., Coll. czeek. chem. Commun., 15,373 (1950). (28) MURRAY, J , Perfumery Easent. Oil Record, 43, 148 (1952). (29) SUTHERLAND. M D.. Perfumeru Essent. Oil Rccord. 43. 453 . . . . . .

(1952). (30) BAROUT, R., Parfi~ms Cosmet. Savons, 4, No. 2, 55 (1961).

206 / Journal of Chemical Education