Essential oil is an unfortunate name. An essential oil is not necessarily an oil in the chemical sense, and may not even feel oily at all. Nor is it “essential” in the sense of being indispensable.
Aromatherapy and cosmetic flacks take advantage of this misunderstanding by touting the essential oils in their products as if they were somehow imperatives for health and beauty. The adjective essential in the name means simply that the substance is the aromatic essence, the concentrated spirit, if you will, of the plant.
Essential oils can be obtained in pure form by steam distillation (boiling the crushed plant material in water and condensing the mixed oil and water vapors), or by extraction into cold fat (enf leurage), hot fat (maceration), or volatile organic solvents that can be evaporated away.
If an essential oil is to affect our senses as a flavor or fragrance, it must consist of small, light molecules (with molecular weights of less than about 300 or 400) that can float through the air and reach our noses. These airborne molecules can enter our upper nasal passages, either directly through the nose or through the back of the mouth when we eat the spice.
In the upper nasal passages the molecules lock onto olfactory receptors, which fire nerve cells to generate a smell signal. These signals are interpreted in the cortex of the brain, along with taste signals received from our taste buds, to produce the overall sensation of what we call flavor. Although we habitually localize flavor in the mouth, anywhere between 70 and 85 percent of the flavor of our foods is contributed by our sense of smell.
Many essential oils are chemicals called terpenes, a class of unsaturated hydrocarbons. Some examples are menthol in oil of peppermint, limonene in orange and lemon oil, and zingerone, which (no kidding) puts the zing in ginger.
A funny coincidence? No. Our English name ginger comes to us via a tortuous path from singivera in Pali, the religious language of Buddhism, to the Greek zingiberi, the Latin zingiber, and the Old English gingifer. Hence its species name Zingiber officinale and the name of one of its main pungent components, zingerone. Our slang word zing, meaning zest, may have a consequential origin.
And before you ask, the distilled alcoholic beverage we call gin has an entirely different origin. Its name comes from its predominant flavoring agent, the juniper berry, called genever in Dutch. The beverage was invented “for medicinal purposes” (wink) by a seventeenth century professor of medicine at the University of Leiden in Holland.