Peppermint

From Chempedia

Cooling Effect of Menthol in Peppermint

Have you ever wondered why peppermint makes your mouth feel cool? Many people experience this sensation but never take the time to investigate the chemistry behind it. This article will help explain the science behind the cooling taste of peppermint.

Image:peppermint.jpg

Menthol in Peppermint

Peppermint (Mentha piperata) is the result of cross-species fertilization. When water mint (M. aquatica) and spearmint (M. spicata) are crossed, the result is a sterile peppermint plant. An analogous example in animals would be the mule, which is the sterile offspring of a horse and a donkey.[1]

Noted for cooling and soothing, peppermint is often used as an ingredient in candy, tea and other food items. However, the peppermint leaf itself does not cause the cooling and soothing effects.

Rather, menthol, a compound that occurs naturally in peppermint oil, is the key factor in peppermint’s cooling and soothing effects. Menthol can be obtained in two ways: by synthesizing the molecule or by extracting it from the oils of plants like peppermint. Like other oils, peppermint oil can be extracted by steam-distilling the plant’s leaves and stems. The plants should be gathered just before the peppermint plant’s light purple flowers appear in the summertime. [2]

Even though peppermint has been used as an herbal medicine in Japan for more than two millennia, the soothing effects of menthol are relatively new to the western world. Since 1771, when menthol was first introduced to North America, "menthol has graduated from the realms of herbal medicine into the field of molecular pharmacology".[3] Today, menthol is used in a plethora of medicinal products including lozenges, nasal sprays, vapor-rubs, inhalers and cough syrups.

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Menthol: the Molecule

Menthol is the common name for 2-(2-Propyl)-5-methyl- 1-cyclohexanol. At first glance, it is easy to see why the common name menthol was coined. The chemical formula of menthol is C10H20O. However, this chemical formula is not unique to menthol. Rather, menthol has seven stereoisomers. The menthol present in nature is always the stereoisomer on the bottom left of the picture.

Vector of Menthol Cooling

To have an effect on the body, the body must have some type of receptor or method of stimulus. This is exactly the case for menthol. The receptor for menthol is named cold- and menthol-sensitive receptor-1 (CMR1). [4]

Normally, the CMR1 responds only to temperatures between 8 and 28 degrees Celsius. The sensor then turns "on" and activates a cold sensation in the body. However, in addition to low temperatures, menthol can trigger the receptor. Thus, in a way, menthol "tricks" your body into feeling a cold sensation. [5]

The CMR1 Receptor: a Closer Look

But how exactly does the receptor for menthol make you feel cooler? Recent studies indicate that menthol can directly act on presynaptic stores of calcium ions in sensory neurons. This stimulation of the CMR1 receptor releases Ca2+ ions, resulting in the release of more glutamate, which increases the signals across the neural synapses.[6] Through these interactions with the neural and sensory systems, the human body thinks it is "cold."

Conclusion

In closing, let’s review what we’ve discovered about the chemistry behind peppermint. We have learned that peppermint isn’t directly responsible for causing cooling and soothing effects. Rather, these effects are caused by menthol, a compound naturally present in peppermint oil. Menthol triggers the CMR1 receptor, a receptor that turns "on" for colder temperatures and menthol. Via this receptor, menthol is able to "trick" your body into feeling a cooling and soothing sensation. That’s a whole lot of chemistry in one small piece of gum!


Written by: Shannon Stanley, Jenna Stenger, Greg Strand, Cory Szczepanik and Greg Williams

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Footnotes

  1. ^ http://en.wikipedia.org/wiki/Peppermint
  2. ^ http://www.wholehealthmd.com/refshelf/substances_view/1,1525,812,00.html
  3. ^ http://www.biomedcentral.com/1529-7322/3/210/abstract
  4. ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14736862&dopt=Abstract
  5. ^ http://www.yesmag.bc.ca/Questions/peppermint.html
  6. ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14736862&dopt=Abstract

Peppermint image: http://home.comcast.net/%7Eholachapulin/Nutrition.html

Stereoisomers of menthol image: http://en.wikipedia.org/wiki/Menthol

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