This guide to making essential oils tells you how plants create these oils and how we can harvest them.
This guide to making essential oils shares how plants create the oils humans want to harvest.
When people visit an herb garden, they touch and sniff the plants. It’s almost an involuntary reaction. People associate herbs—and spices, too—with fragrance, and the roots of this association go deep. Even the earliest botanical records show that people have been fascinated with fragrant plants.
But what is it about herbs and spices that makes them fragrant? When I first asked this question, I was simply told, “They contain essential oils.” As I nodded thoughtfully and tried not to appear ignorant, I wondered: What’s so essential about them?
After years of carrying around this unasked question, I discovered that the term “essential oil” was coined by the sixteenth-century alchemist Paracelsus because, in his mind, these substances contained the quintessence of plants. Centuries earlier, Greek philosophers had proposed that matter appears in four forms (air, earth, fire, and water), and Aristotle had proposed a fifth form—in Latin, the quinta essentia—which would represent the essence of things. To Paracelsus, the oils represented the most concentrated form of the individual characteristics of the plant—its essence. Today, I often get a blank stare when I tell people that I breed essential oil plants. However, like most people, I love to talk about my intense interests, and given time, comfortable surroundings, maybe a cup of tea, and a willing ear, I’m likely to launch into a discussion of one of my favorite subjects—essential oils.
An essential oil is a volatile material derived from a plant, and it usually bears the aroma or flavor of that plant. Although a few animal-derived aromatic products exist (mainly musk, civet, and ambergris), the ones of botanical origin are far more numerous. Like fixed oils (vegetable oil, motor oil), these substances are generally liquids, they won’t mix with water, and they are soluble in many organic solvents. Unlike fixed oils, however, essential oils are volatile: they evaporate rapidly at room temperature, whereas fixed oils will not. Chemically, an essential oil is a complex mixture of 30 to 100 or more compounds. Only with the advent of modern analytical techniques, particularly gas chromatography, have we fully appreciated the complexity of these mixtures. With gas chromatography, an oil is separated into its components, and the relative proportions of the components are represented graphically as a series of peaks—some large, some small. The area under each peak represents the proportion of each component in the oil, and by experience, structural analysis, and comparison of the chromatogram with others made with pure reference chemicals, we can identify many of the components. Next time you touch and sniff an herb, remember that your nose is being bombarded by a wide array of chemicals.
Essential oils are found in various plant parts. The oils of peppermint, patchouli, basil, and geranium come from the leaves and stems, clove oil comes from flower buds, and oils of jasmine, rose, and tuberose come from the open flowers. Oils are produced from the whole dried and crushed fruit of anise and coriander, the peels of citrus fruits, the seeds of cardamom, the wood of cedar, the bark of the cinnamon tree, the roots of vetiver grass, the needles of fir trees, the twigs of cypress trees, and the exuded resin of myrrh—in short, just about every anatomical structure. Some plants produce more than one type of oil. The flowers of bitter orange (Citrus aurantium) yield neroli oil; its leaves, petitgrain oil; and the fruit peel, orange oil. Cinnamon is just as versatile, supplying different oils from its leaf, bark, and root.
Essential oils are secreted and stored in specialized structures. Members of the mint family—sage, basil, thyme, rosemary—have two types of epidermal oil glands. The first is a two- or three-celled glandular hair, the second, a larger, ten-celled glandular scale. The latter can be seen with the aid of a hand lens, particularly on the calyx (the tube from which the flower emerges). The scanning electron microscope reveals a surrealistic landscape of pincushionlike glandular scales and teatlike glandular hairs.
Members of the laurel family, such as cinnamon and cassia, store aromatic substances in specialized oil cells. The fruits of caraway, dill, and other members of the parsley family hold the oil in intercellular canals or ducts; fruits of plants in the rue family, notably citrus, have oil reservoirs that are formed as the walls of secretory cells gradually disintegrate.
I have always been fascinated with the diverse fragrances of herbs and spices. I have come to understand something of their complex chemical nature and the anatomical parts where the oil is produced, yet I still don’t know why some plants make essential oils while others do not. I have trouble accepting the old belief that the oils represent metabolic waste products; this seems a rather elegant approach to disposal, akin to gift-wrapping garbage. More recent evidence indicates that essential oil constituents are made and then broken down again, which suggests that they may play some other role (as yet unknown) in plant metabolism. The antibacterial and antifungal properties of some essential oils could act as a defense mechanism against plant pathogens, or even deter insects or foraging animals.
Regardless of the function they serve in the plant, essential oils must be removed if we wish to use them, and the methods of removal depend on the plant material, the type of product to be removed, and the availability of equipment. The three basic methods of essential oil production are steam distillation, expression, and extraction; steam distillation is the most common and most economical.
In steam distillation, plant material is exposed to steam, whose heat causes the essential oil to evaporate. Subsequent cooling of the hot vapors causes condensation of both water vapor and oil. Because the oil and water will not mix, they are easy to separate at that stage.
Large-scale commercial distillation is a little more complex, and the first problem is growing the plants. For example, R. J. Reynolds Tobacco Company became interested in clary sage (Salvia sclarea) in the early 1960s as a source of a tobacco flavorant called sclareol. Years of in-house research and thousands of dollars were required to develop efficient ways of cultivating this plant on a large scale because, unlike corn or wheat, clary sage (and most other essential oil plants) is not a major agricultural crop. You can’t just go out and buy several thousand pounds of clary sage seed as you can seed corn, and there’s no army of researchers at the agricultural schools of major universities to inform you about optimal seed rate, row spacing, fertilizer rate, pest control measures, or high-yielding varieties.
Clary sage is considered a biennial, but it can be treated as a winter annual. In eastern North Carolina, where Reynolds has several thousand acres, if seeds are sown in the fall, the plants flower and are ready to harvest in June. If you’ve seen clary sage, you know that even a single plant is eye-catching—imagine the effect of more than 2000 acres of plants in full bloom!
To make the most of the plant material, the company steam-distills clary sage to extract its essential oil. Flowering plants are harvested with a forage chopper, which cuts and grinds the plant tops. In a feat of agricultural choreography, the chopped material is blown into a tub on a truck that drives alongside the harvester. These tubs are specially designed with perforated pipes in the bottom for the introduction of steam, and they form the body of the still. A loaded tub, containing perhaps three tons of chopped clary sage, is driven to the processing area, where a steam line is connected to it and a tight-fitting lid secured on top. The lid has an outlet pipe through which the steam and oil go to the condenser—a pipe surrounded by a jacket of cold water—where the steam and oil condense for collection in a receiving tank. Because the essential oil is lighter than water, it rises to the top of the receiver and is drawn off through a spout. A single tubful of clary sage produces about a gallon of oil, but yields vary with season and weather conditions.
Many other essential oils are produced by steam distillation of various plant parts. They are produced all over the world, wherever the plants grow naturally or can be cultivated—often in remote places and by primitive, labor-intensive methods. Lavender is now often mechanically harvested from large fields in France and Yugoslavia, but some farmers still harvest their lavender crop with sickles. Farmers may own their land, but the processing stills may belong to the community.
The epitome of a labor-intensive, steam-distilled crop is the Damascene rose. Bulgaria is the largest single area of rose oil production in the world, and during the 30-day blooming period, the flowers are picked individually, by hand. Collecting bags slung over their necks, whole families begin picking the flowers early in the day, before rising temperature causes oil to be lost. Distilling 2500 pounds of flowers produces about a pound of rose oil, which sells for about $2,300. At that price, I doubt whether much oil gets carelessly spilled!
Expression is another means of getting an essential oil out of plant material. This method is used exclusively for citrus oils—orange, lemon, lime, mandarin orange, grapefruit, and bergamot—because citrus oils are easily damaged by the heat of distilling essential oils. (Lime does produce a useful oil by distillation, but it is different from the expressed oil.)
The oil reservoirs of the citrus fruits are located in the outer, colored portion of the peel, called the flavedo. Expression is sometimes called cold-pressing because the oils are literally pressed out of the peel by hand or machine at ambient temperatures.
At one time, citrus oils were produced manually. In an early method called sponge-pressing, the ripe fruit was cut in half, a sharp-edged spoon was used to cut out the pulp, and the peel was soaked in water for several hours before pressing. The actual pressing required considerable and continued pressure. The peel was placed between two flat sponges and pressed against a wooden bar laid across the lip of a collecting bowl. Squeezing out the sponges every so often caused a mixture of oil and water to collect in the bowl. Sure sounds like a lot of work to me!
The labor-intensive and expensive sponge method has been supplanted by mechanical presses which remove the oil by squeezing the peels between rollers or abrading the flavedo, thus separating the oil from the rest of the fruit. When the fruit juice is desired, cleverly designed machines can be used that produce both juice and oil while keeping the two products separate.
The emulsion of oil, water, solids, and waxes that results from expression is centrifuged to separate the oil. A cold storage period then settles out the heavier, nonvolatile waxes.
As merely a by-product of a high-volume juice industry, orange and grapefruit oils are relatively cheap—about $15 a pound. Florida and California are major producers of these oils. The other citrus oils are only somewhat more expensive, except for bergamot oil, which costs about $100 a pound.
Essences Within an Essence
Gas chromatography is an analytical method of identifying the chemical components of an essential oil by separating them according to molecular weight. This gas chromatogram of thyme oil illustrates the relative proportions of the chemicals in the list below. (Only the major peaks are identified.) Determining which peak represents which chemical is the tricky part of the process, usually requiring comparison with chromatograms of other compounds of known composition.
The process of extraction with solvents is suitable for materials that contain only small quantities of volatile compounds, such as jasmine flowers, or those that contain a lot of compounds of high molecular weight, such as exuded resins.
Extraction works according to the principle of differential solubility. A plant is mostly water, in which oils are only sparingly soluble, so if the plant is exposed to an organic solvent in which oils are highly soluble, the oil will be drawn from the plant. The solvent is then evaporated, and the aromatic product remains. It is not an essential oil, which by definition is volatile; extraction removes not only volatile components but nonvolatile compounds, plant waxes, and pigments as well. The product that remains after the solvent is evaporated is called a concrète. When a resin, such as myrrh, is extracted, the product is called a resinoid. To obtain an alcohol-soluble product more useful in fragrance manufacture, the concrète is treated with warm alcohol, in which most waxes are not soluble. The alcohol is distilled off at reduced pressure to yield an absolute of the respective essential oil.
The production of clary sage by R. J. Reynolds also provides an example of the extraction process on a commercial scale. After clary sage oil has been distilled, the plant material is dumped into a pit, from which it is fed up a conveyor and into a rotocell, a countercurrent extractor as tall as a two-story building. Inside the rotocell are a series of baskets filled with plant material. Solvent is poured into each basket and allowed to percolate through the sage as through a giant drip coffee maker. Successive solvent washes ensure efficient extraction. Rather than evaporate the solvent to produce clary sage concrète, a second solvent is introduced which draws out sclareol. This aromatic compound has been used as a tobacco flavorant, but Reynolds now uses it as the starting material in the synthesis of a botanically derived substitute for ambergris, a rare fragrance product from the sperm whale.
An extraction process of primarily historical interest is enfleurage—extraction with lard. Still used on a limited basis in southern France for the extraction of jasmine and tuberose flowers, this labor-intensive process is impractical for large-scale use. In enfleurage, a glass plate mounted on a wooden frame is covered with a layer of lard, and the flower petals are pressed into the lard. Several such frames are stacked to form an airtight seal. After two days, the spent flowers are replaced with fresh ones. The process continues until the lard is saturated with the extracted oils. This aromatic fat, called a pomade, is further extracted in alcohol to produce an absolute.
Essential oil production methods don’t easily lend themselves to experimentation at home. Solvents are flammable or explosive, and distilling off the solvent can be dangerous, too. If you had a large and steady supply of flowers such as roses, you might try making a pomade, but I’m not sure what you’d do with it then—perhaps scent your hair as was once fashionable. After pumping iron at the gym for several months to get your arms and back in shape, and then buying up all the oranges at every supermarket for miles around (plus a couple of sponges), you might be able to express a thimbleful of oil—and be awash in orange juice.
Steam distillation offers the only really feasible means of small-scale essential oil production. If you’re willing to invest some money to set up a small still, you can produce a small vial of oil from material collected in your herb garden. Unless you want to distill oils regularly, I suggest that you first try to borrow equipment from a college chemistry lab; some of the pieces are in general use. Glassware is not cheap, and you may not want to buy it for just a few trial runs.
The distillation setup consists of a one-liter, two-neck, round-bottom distillation flask and a 300-mm Allihn or Liebig condenser. Oil is collected in a Clevenger trap that is designed for oils lighter than water. A heating mantle and variable voltage transformer provide a safe way to heat the flask, and a ring stand and three clamps provide support. Two 5-foot lengths of 3/8-inch plastic tubing circulate water through the condenser. Unless you’re in a chemistry lab, you’ll probably need an adapter to connect the small tubing to your household water supply. A large rubber or glass stopper is needed to plug the larger neck of the flask. The total investment in all this could be as much as $500. You’ll also need a source of cold water and electricity.
Flowering leafy herbs such as basil, thyme, or mint, or perhaps lavender flower spikes, are good for a start. Cut up the plant—scissors will do—and stuff the pieces into the larger neck of the flask. You’ll need about 200 grams of plant material. Fill the flask about half full of water and mash the plant bits down so that they are completely submerged. Set the flask in the heating mantle, supporting it with a clamp on the ring stand. Plug the larger neck of the flask and connect the heating mantle to the transformer. Fit the lower end of the Clevenger trap into the smaller neck of the flask, and support the upper end with a clamp. Fit the condenser into the top of the trap and clamp it in place so that it’s perpendicular to the work surface. Cold water goes into the lower inlet arm on the condenser, and another piece of tubing connected to the upper arm allows water to drain back to the sink.
Start the distillation by turning on the water to the condenser. Otherwise, the volatile oil will escape out of the top of the still. A steady trickle of water running back into the sink is evidence of water flow sufficient to keep the condenser cool. Heat the water in the flask to boiling, then reduce the heat to maintain a rate of flow into the trap of no more than two drops per second. Check the amount of oil in the trap periodically. When it appears that no more oil is accumulating, turn off the heat and let the system cool. Carefully drain the water out of the stopcock on the condenser, then collect the oil in a small glass vial, preferably one with a screw cap. After all this work, you may be a little disappointed to find you have scarcely a milliliter of oil, but you will certainly feel a kinship with those Bulgarian rose pickers.
Up to this point, I haven’t mentioned synthetic aroma chemicals. Many people today feel that oils from natural sources are better than those that have been synthesized. For an aromatherapist who believes that the life force of the plant is extracted in the essential oil, synthetic oils may have no value. To a perfumer, however, natural and synthetic fragrance chemicals are equally valid raw materials from which to build a fragrance. Nearly all fragrance compounds and many flavor compounds are blends of natural essential oils and synthetic aroma chemicals. This is a necessity: the supply of natural materials can satisfy only about one-third of the world’s demand, and climate, economics, and politics can greatly influence the availability of natural oils. For example, Yugoslavia is a major producer of lavender, rosemary, and caraway oils, so civil war there is bound to affect the supply of those oils.
Synthetic aroma chemicals were first used as inexpensive extenders of natural oils and are still used to stretch the limited supply of an essential oil. Produced from coal tar or petroleum, many synthetics are modeled after natural compounds and are identical in chemical structure (nature-identical). Organic synthesis has also produced fragrant compounds that do not exist in nature, such as hydroxycitronellal, which has an odor reminiscent of lily-of-the-valley and lilac.
Aroma chemicals are not always entirely synthetic. Natural essential oils are sometimes used as the source of a single chemical which is produced by physical and chemical separation methods. Eugenol is isolated from clove leaf oil or cinnamon leaf oil, anethole from anise oil, menthol and menthone from peppermint oil, and thujone from cedarleaf oil. These chemicals might be used to extend another natural oil, as part of a flavor blend, or as the starting point for organic synthesis of other nature-identical or novel aroma chemicals.
Despite their widespread use, synthetic aroma chemicals will never completely replace essential oils. Some oils such as wintergreen and bergamot are relatively easy to copy synthetically, but others such as patchouli and sandalwood have no satisfactory extender. Essential oils are complex mixtures of many chemicals, and compounds present in the highest proportion never totally represent an essential oil. It’s the diversity of compounds, some present only in trace amounts, that gives certain essential oils a unique richness and character that can’t be duplicated.
Commercially produced essential oils and aroma chemicals bombard us every day in the flavors and fragrances we mostly take for granted. We start the day with a shower—first the soap, some shampoo, maybe a little cream rinse. Dry off and reach for some body lotion or powder, then apply deodorant. Next comes a shave, followed by a little skin toner, or maybe a multistep make-up routine. Now for some mousse or setting gel and a touch of hair spray. Finally, just a dab of a favorite cologne. We might easily be exposed to a dozen fragrances before breakfast!
Personal care products are the beginning of our daily contact with essential oils. In this age of microwave dinners, frozen yogurt in dozens of exotic flavors, canned spaghetti sauce (“just like homemade”), Cool Ranch Doritos, Cherry Coke, Fruity Pebbles, Bavarian Mint coffee, Starburst candy, Juicy Juice, Flintstones vitamins, and cool mint toothpaste, it’s impossible not to ingest a wide array of essential oils. An extraordinary amount of food advertising and packaging proclaims that the products contain “no artificial flavors” because marketers have discovered the sales potential of appealing to our appreciation of naturally derived essential oils.
Air fresheners and light rings appeal directly to our appreciation of fragrances, but there is hardly a dish soap, floor polish, laundry detergent, furniture polish, rug shampoo, oven cleaner, fabric softener, or spray starch that doesn’t have a fragrance. Even if consumer publications say that Brand X is better or cheaper to use, I won’t buy it unless I like its scent. And I’m not the only one who buys by smell: I’ve seen other shoppers surreptitiously smelling the box of dishwashing powder, unscrewing the cap on liquid detergent, scratching open a corner of the plastic packaging around a triple-pack of bar soap, even spraying a touch of air freshener and then quickly sniffing the aerosol.
People no longer risk life and limb to acquire the plants that contain essential oils, as Columbus did when setting out to find a new route to the Spice Islands, but a lot of effort and expense goes into getting essential oils out of plants and into products. From all parts of the globe, with back-breaking manual labor or modern machinery, using ancient arts or high-tech instrumentation, these fragrant, flavorful oils continue to find their way to the skilled hands of the perfumer, flavorist, and aromatherapist.
Harriet Flannery Phillips likes her home in Bloomfield Hills, Michigan, enough to travel back and forth to work in North Carolina, where she breeds essential oil plants for R. J. Reynolds Tobacco Company.
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