The Essence of Essential Oils

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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.

What Are 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.

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