menthol

Menthol is carried by two entries in this library's pantry that are really one plant twice: fresh mint and dried mint, both Mentha × piperita, both listed at 33.37 percent of peppermint oil. One controlled study measured fresh peppermint leaf oil closer to forty-four percent menthol and fifteen percent menthone, a reminder that a single percentage on a plant profile is one measurement, not a constant. The eighty percent figure that circulates for menthol content usually describes Mentha arvensis, corn mint, the industrial source of crystalline menthol and a different plant from the one in these teas.

Menthol, C₁₀H₂₀O, is a cyclohexane ring carrying a hydroxyl, a methyl and an isopropyl group, and three of its carbons are chiral centers, which allows eight possible stereoisomers: menthol, neomenthol, isomenthol and neoisomenthol, each as a mirrored pair. Plants make essentially one of the eight. (−)-menthol, configuration 1R,2S,5R, is also the most potent of the set at the human receptor, with the lowest cooling threshold, which is either a coincidence or evidence that receptors evolve inside a world already full of the terpenes plants happen to build. Drying shifts the mixture further: composition studies show it moving with oxidation and hydrolysis and the loss of the more water-loving volatiles, and kitchens that use both fresh and dried mint, from a Persian ash to a Levantine bowl of cucumber and yogurt, treat them as two ingredients rather than one and a weaker substitute.

Hold a mint leaf and breathe in and the cold feels completely real, but a thermometer in the mouth shows nothing has moved. What happened instead took until 2002 to explain, when two labs independently identified TRPM8, the cold receptor, and showed that menthol binds it and shifts its temperature-activation curve toward ordinary mouth and skin temperature: a channel that would need real cold to open now opens on its own, and the nerve reports what it always reports, cold. The same receptor's expression in the gut tracks with pain scores in irritable bowel syndrome, and peppermint oil trials there are real, ten trials and over a thousand patients giving a number needed to treat of four on evidence graded very low, but every one of them used an enteric-coated oil capsule built to survive the stomach, not leaves steeped in a cup. In this library the leaves are there for the cold on the tongue in ten recipes, from a two-kiwi smoothie to a wine, and the therapeutic trials belong to a different product entirely.