When my family moved to Vancouver from Hong Kong, I was at the awkward age of 14. In addition to language barrier and social adjustment, most of my cultural shock came inevitably from high school. One thing that impressed me the most was how popular gum-chewing was (and still is). All these cool kids were hanging out by their lockers, exchanging colourful and minty gums, and engaging in a perpetual chewing contest throughout classes. You see, students in Asia would never dream of eating/chewing/drinking/whining/talking in class. You can tell how fascinated I was at these rebellious classmates; they may not shower everyday and they shared one slurpee with ten people, but their breath was surely minty fresh all the time.
It didn’t take me long to realize that everybody is just paranoid about bad breath. It is socially embarrassing and simply unacceptable to speak without checking your breath. Mints are offered before/during/after meals, coffee, smoking, classes etc. The most common conversation openers are: “dude, you got some gum?” or “you want some gum?” As if that is not enough, they even have mints for dogs (note 1)! It is the trend, the necessity, the social etiquette to fight the eternal battle with unpleasant breath.
Of course, the smart business people have taken action to provide the goods and further promote them. You can feast your eyes on the different brands, flavours and forms of mints available, and the commercials on TV run non-stop…you need icy fresh breath, you need fiery cool breath (I thought I’d never see oxymoron again after English 10, note 2), you need fresh breath to attract that hot chick/dude, you need fresh breath for a good kiss…Excel, Trident, Extra, Eclipses, Mentos, Tic Tacs, Dentyne, peppermint, spearmint, winterfresh, cinnamon (!?!), strips, tablets, films, chewy, hard, etc (note 3). I think we have easily surpassed the world of Willy Wonka.
To be honest, I never liked mint. I remember only with the fear of being un-cool, did I accept that first strip of Excel peppermint gum from my friend. But in doing so, I had almost burst out in tears as the acrid taste shot up my nose, through my cerebrum, into my skull, and lingering at the tips of my hair. I don’t remember much afterwards; I must have spat it out somehow. Anyway, after this traumatizing experience, I am even now only brave enough to take restaurant mint candies and tic tacs.
With this popular trend engulfing every one of us, I started to wonder: what is mint anyway? Why does it taste and feel cool (or pungent for someone like me), in fact so cool that some people can’t be caught without it?
Most of us know “mint” comes from mint leaves/stems/flowers, primarily from peppermint or spearmint plants (note 4). These plants are all from the Mentha genus, and from them we are able to extract essential oils, where all the tasty and aromatic properties come from. Mint has been used for hundreds of years as herbal medicine, culinary herb, tea, and more[1, 2]. As well, peppermint oil is still widely used in cosmetics, candies, bath and oral hygiene products for flavour and fragrance. The magic component of peppermint oil is menthol, which gives the mint its smell and taste, and is also believed to be responsible for the therapeutic actions. Depending on the plant extraction process, peppermint oil can contain between 30% to 60% menthol [2, 5, 6].
In addition to the above, menthol is also used in cigarettes and medicinal products such as vapor-rub, cough syrup, and nasal decongestants. The most noticeable action of menthol is the cooling and soothing sensation when applied to skin and the oral cavity. Even as a herb, peppermint is used to alleviate itching, nausea and digestive disorders [2-4]. However, these pleasant outcomes are generally limited to a low concentration of menthol (<2%); in fact, once the concentration is above 5%, it becomes an irritant and causes a burning sensation .
So what is menthol then?
The chemical name of menthol is cyclohexanol-5-methyl-2-(1-methylethyl) . Although this is a mouthful and sounds impressive, menthol is actually a very simple organic molecule. It is chemically classified as an alcohol (don’t get too excited just yet), which only means that it has an OH group in the structure (figure 1). There are 8 forms, or 4 pairs, of menthol, each differing slightly in the orientation of its 3D chemical structure. The most abundant and important one in nature is (-)-menthol (note 5) [5, 7].
So how do we feel this cold, and sometimes burning, sensation? In a nutshell, our body is mapped by a network of sensory neurons, which are cells that send messages to our brain telling us about our environment. These signals are very important for our well-being, telling us if we have stepped on a nail (ouch!), whether the stove is too hot, and so on. These signals relay like dominos: once something triggers the neurons (pushing on the first domino), an electrical signal is transmitted from neuron to neuron, until it reaches the brain. Our brain then interprets the signal, making us feel, and instructs actions if necessary. Most stimulants act on receptors on the neurons to trigger the signal and subsequent transmission. This is like fitting a key into a lock, which in turn opens a door. For example, the smelly guy is giving off these sulfur-containing molecules (the key); the odour receptors (the lock) in your nose recognize these molecules, transmit the signal to your brain as “bad smell”, and your brain will make a mental note to put him in your “Do not date and avoid” list. We feel temperature with the same mechanism, except the locks and keys are different.
Now back to menthol. It has long been suggested that menthol inflicts a cool sensation by stimulating “cold” receptors, however, the actual receptor that responds to menthol wasn’t identified until 2002 by two separate groups of scientists [5, 8, 9]. As a result, two names were given for this receptor: CMR1 and TRPM8 (note 6) [8,9]. This receptor is found all over our body, and its obvious function is to monitor environment temperature. CMR1/TRPM8 is sensitive to a range of temperatures in the vicinity of 10°C – 30°C as well as compounds like menthol, eucalyptol (from eucalyptus), and icilin [8-13]. I should also mention that many other temperature-sensitive receptors had been identified long before the menthol receptor. For example, TRPV1 is a “hot” receptor sensitive to capsaicin (what makes chili peppers hot) and temperatures >43°C. In fact, it was the research on “hot” receptors and chili peppers that eventually led to the discovery of “cold” receptors [10, 12].
The burning pain associated with high-concentration menthol is probably caused by another receptor system – the pain receptors. However, although seemingly contradictory, high-concentration menthol stimulation if continued for a long time, will lead to local anesthesia [5, 6, 14]. Of course, the intensity of the cool/burn feeling differs from person to person, depending on the amount and sensitivity of the receptors, the amount and sensitivity of the neurons, and so on .
Now we understand how menthol feels cool, but what about the sensation of taste? Those who remember their high school biology would know that we cannot taste “cool”, since the four basic tastes are sweet, salty, sour and bitter. However, our tongues also sense temperature, like chili peppers are “hot”, and mint “cool”. More importantly, “the smell is half the taste”, meaninging that the mint “taste” is mainly in association with the minty smell and the cool feeling in our oral and nasal cavities. Interestingly, although the taste of “hot” and “cool” is a misnomer, temperature can evoke a sense of taste (known as “thermal taste”), and this response differs greatly from person to person . If pressed, menthol and capsaicin (from chili peppers) both induce a taste of bitterness. And a recent study on the taste of capsaicin and menthol in different areas of the human tongue shows that the bitterness and burn of menthol are strongest at the back (inner part) of the tongue, while the tip, front and side of the tongue mainly feel coolness . This would suggest that menthol and capsaicin do indeed stimulate taste receptors, and not just temperature receptors. Bottom line is, I don’t feel so bad now that I hate mint (and chili peppers!); I only have sensitive receptors.
In addition to touch and taste, and as mentioned before, menthol has many other alleged functions that prompt its use in medicinal products. These functions include nasal decongestion, anti-coughing, thirst quenching, arousal, relief of indigestion, and antibacterial properties. Although there are only a few research reports in these areas, it appears clear that some of these actions are miscontrued. For example, studies show that although people feel decongested after using menthol vapour, there is no actual improvement in the nasal airway when air flow is measured objectively. In other words, the decongestion is a totally subjective perception, although menthol does reduce discomfort and the urge to breathe. This is why cigarettes use menthol to reduce the harshness of the smoke for a smoother “taste” (note 7) [7, 17]. The better breathing is probably due to the stimulation of cold receptors lining the nasal area and throat, as menthol has no effect on the lungs or chest muscles themselves [5, 7, 18, 19].
There is some evidence that antibacterial and antifungal effects exist when using menthol and mint oils. After all, menthol and peppermint oil both stop some bacteria and fungi from growing, especially in the context of an infected mint plant. So in some respects, this may seem to justify the use of mint/gums in fighting bad breath – but let’s not jump to conclusions. While it is true that bad breath is caused by bacteria and menthol does kill some bugs, our filthy mouths are a very complicated ecosystem. Some bacteria are bad, whereas some are good. We really don’t know what impact menthol has on that type of mixed population of bugs . Incidentally, mint oil is way more effective than menthol itself in killing bugs [4, 5, 21]. One thing is for sure though, menthol is pretty good at masking bad smells .
Out of curiosity, I checked the ingredients of some popular gums/mints to see how much menthol they actually contained. To my great disappointment, my little survey came up with negative results. Maybe it’s trade secret, or conspiracy, but I only find “natural and artificial flavours” stated as the flavoring agent (what a waste of print! Why don’t they just say “flavours”?). What can I say…somehow my gut feeling tells me that “natural and artificial flavours” is just a bunch of chemicals that will soon be declared carcinogenic. Apparently all-natural mints are for dogs only (note 1).
Anyhow, the use of mint, whether for fashion or medicinal purposes, seems to be ubiquitous. Despite it being clear that menthol has multiple effects on several systems, there is still much to learn about this simple compound. To which it’s only fitting that I say, “Tic tac?”
1. I choked on my coffee when I saw the little tin boxes of “dog mints”. Dogs are the true rulers of this world. By the way, some dog mints don’t have mint at all. They mostly use parsley, dill and rosemary as the good-smell ingredients (e.g. YipYap Breath Fresheners for Dogs). All natural ingredients too!
2. Ah, the joy of oxymoron. It is a figure of speech that jams two contradicting words together; first introduced (to me) in Grade 10 English, Shakespeare’s Romeo and Juliet: “feather of lead, bright smoke, cold fire, sick health…” (Act I, scene I). If you didn’t know already, Romeo and Juliet die at the end. Other examples of oxymoron: fiery cool, freezer burn, dynamic equilibrium, Microsoft Works.
3. If you want to learn more about the history, development, and social impact of chewing/bubble gum, there is an interesting book called Chewing Gum, the Fortunes of Taste by Michael Redclift (New York: Routledge; 2004) that gives a nice discussion on these topics. Also, an article by T. Imfeld, titled Chewing gum – facts and fiction: a review of gum-chewing and oral health (Critical Reviews in Oral Biology and Medicine; 1999; 10(3): 405 – 419.) provides a good review.
4. The scientific names of peppermint and spearmint plants are Mentha x piperita and Mentha spicata, respectively. There are many other mint plants from the Mentha genus. Not that we care really, maybe just the botanists.
5. For those familiar with the chemistry lingo, I am referring to different isomers of menthol. Isomers may look similar, but they are very different molecules with different properties. There are 4 pairs because of enantiomers, which are mirror images of each other, like your left and right hand (they look the same, but are actually different). The menthol isomers are: (+) and (-)-menthol; (+) and (-)-neomenthol; (+) and (-)-isomenthol; (+) and (-)-isoneomenthol. In our case, (-)-menthol is “the mint” that we refer to.
6. CMR1 stands for Cold-Menthol Receptor type 1; TRPM8 stands for Transient Receptor Potential Melatasin type 8. I personally prefer CMR1, as the name actually makes sense. These receptors in the TRP family are actually channels that allow ions (charged molecules) to go through, thus creating an electrical signal. Specifically, menthol causes calcium ions (Ca2+) to flow through the channels. For detailed mechanism, references 5, 8, 9, 10, and 12 provide great explanation.
7. There is controversy as to whether menthol plays a role in cigarette/nicotine addiction. It is a concern that menthol may increase the delivery and absorption of nicotine from cigarettes. Some observations suggest that people who smoke menthol cigarettes smoke more, and for a longer time. Chemistry aside, the increased smoking may simply be due to a more pleasant smoking experience, thus reinforcing the habit. Again, there are advocates on both sides, and more studies are needed to confirm these claims.
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