The snack that shows how the world breaks

The science and history of wintergreen candy and its famous sparks

Hello, Snackers. It feels like magic to make little bursts of light while eating candy, but what’s actually going on there? It’s a mystery that has puzzled scientists for generations, so I’ve brought in one of the smartest people I know to help me understand it all.

Prologue: Here, have some candy

If you’ve never chewed Wint-O-Green Life Savers in a dark room, you must try it at once. If you have done it before, well, it’s time to try it again. The mints are readily available, most likely at your nearest convenience store/bodega/gas station/etc. and the only other thing you need is a mouth (pliers or a hammer also work). Science experiments are often fun, but they rarely offer such great thrills for such minimal cost and effort.

When you crush a Wint-O-Green Life Saver, it generates little bursts of light. “Little,” like really, really tiny. Even in a super-slow-mo video, it’s hard to take a screengrab that shows the glow. Look very, very closely here:

And yet, as fleeting and faint as it is, it’s definitely there, and its very existence is unnerving. Food isn’t supposed to light up when you bite it.

The scientific term for this short-lived glow is triboluminescence, which refers to light generated by rubbing, scratching, crushing, or breaking a substance.

You can find the same phenomenon in quartz and certain other minerals, as well as many other hard candies and even just plain old raw sugar (more on that shortly). Triboluminescence also appears to be the reason for the mysterious glows that sometimes accompany earthquakes, although scientists still don’t know for sure what’s going on (cue “X-Files” theme music).

Screengrab from Nova video on earthquake lights. This example occurred in Mexico in 2021.

Triboluminescence, at both the small and large scale, has been alarming people for centuries because, well, it’s freaking weird to have light without an obvious source.

But it turns out that the triboluminescence generated by wintergreen candies is especially fascinating. In fact, this little trick you can do at home—chewing Life Savers—holds actual scientific insights including, possibly, some extraordinary technological breakthroughs.

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1. What on earth is happening here?!

Before we get to the unnerving human history part of the story, which sent me deep into the archives, we need to discuss what, exactly, is creating those sparks.

There’s a fair amount of science to explain, which is not my area of expertise, so I've asked my friend Mike Sowden to help me—and you—understand it all.

Mike writes the excellent newsletter Everything Is Amazing, which has a curiosity-driven vibe similar to that of Snack Stack but gets way more into the science of things. (Here, for example, is Mike’s recent post on impossible colors and here’s one about an ancient megaflood. Go check out his work and subscribe to his newsletter.) Please also know that Mike is English and I will not be editing any of his British spellings and you should hear all of his text in the voice of, I don’t know, let’s say Sir David Attenborough.

Okay, take it away, Mike.

Hello! This is Mike. Jolly pleased to meet you all.

This a truly fascinating topic, because today, in the year 2023, or as sci-fi nerds like me call it, Blade-Runner-plus-four, modern science is still a touch hazy on exactly how this thing happens. That is amazing for such a common phenomenon, especially as it’s associated with a substance we’re so addicted to: sugar, that delicious, tasty, no-good ruination of our Western diets. (Mmmm…ruination.)

But here’s what triboluminescence is not. It’s not the same thing as a spark, the kind you get from banging two random rocks together, or using a flint or steel firestarter to get your campfire lit. That is a different physical process: a physical and chemical reaction that ejects glowing fragments that, while incredibly short-lived, should be hot enough to set your tinder ablaze. In contrast, there’s essentially no heat with triboluminescence. If you were thinking of lighting a fire by munching sugary treats above it with your mouth open, don’t do that. Or, you know, do that, whatever lights you up, but don’t expect anything meaningful to happen, except maybe some extra dentistry at some point.  

So what’s behind this light, if it isn’t a spark? 

It seems it’s down to stray electrons getting excited. I mean that in the scientific sense: as electrons gain or lose energy, they hop up and down between energy states, absorbing or releasing energy as they do so. When they release it, it’s in the form of photons. So when particular substances are crunched, splintered, crushed or scroncked (that’s not a real word, Doug), the electrons on their surface momentarily gain energy, then lose it again as that energy dissipates. This process emits photons of light, either traveling directly to your eyes or absorbed by other materials nearby, making them glow extremely briefly—a fraction of a split-second. (In the case of Wint-O-Green Life Savers, what emits the flash is the chemical methyl salicylate, better known as wintergreen oil.) 

But the energy at work here is basically nothing. It’s light with no heat, which makes this thing just perfect for kid-friendly party tricks, and maybe much more …

By the way—just for fellow Brits, if you’re wondering about the difference between Life Savers and Polo Mints (from Nestlé), the answer is nothing but the name. They are identical, as far as I can tell—and, yes, Polos will triboluminesce! Give it a go. 

One weird thing about this effect, though: it seems that in many cases the emissions of visible light are far outweighed by the ultraviolet light that’s released. UV light is only invisible to us because the lenses of our eyes naturally filter it out, but seen through an ultraviolet camera, triboluminescence should be quite the light show. Worth bearing in mind if you’re a food photography nerd.

Okay! Back to Doug for the Ye Olden Times stuff. 

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2. A brief history of people getting scared by this

Thanks, Mike! See, he’s good at this. Also, let me note that Mike’s comment about the Polo brand name made me picture a stuffy British aristocrat, in full polo regalia and fresh off a match, leading their horse into a darkened bathroom so they can chomp some wintergreen candies together. This would be an excellent plot point in a Downton Abbey-style period drama. (Free idea, Hollywood!)

Anyway, the oldest known documentation of triboluminescence in sweets comes from Francis Bacon—yes, that Francis Bacon—in his landmark work Novum Organum, published in the early 1600s:

It is well known that all sugar, whether candied or plain, if it be hard, sparkles when broken or scraped with a knife in the dark.

Fun stuff, although not as fun as the hijinks documented by one Father Giambattista Beccaria, who wrote in A Treatise Upon Artificial Electricity, in 1753, that you can totally mess with your friends using this trick:

You may, when in the dark, frighten simple people only by chewing lumps of sugar, and, in the meanwhile, keeping your mouth open, which will appear to them as if full of fire. … By breaking in the dark the outward crust of extremely refined sugar, which covers certain comfits, a light springs out of it, which is exactly like that of electric sparks.

All of this is to say that scholars around the world have known about this strange but kinda fun phenomenon for centuries (if you want the full investigation of people trying this over the years, you’ll want to read “Survey of the literature on mechanoluminescence from 1605 to 2013,” by P. Jha and B.P. Chandra, which was published in 2014 in the journal Luminescence).

But interest in this sparkly wonder didn’t really take off until the late 1800s. A light-studier-guy [Mike, is that the technical term??] named Eilhard Wiedemann coined the term “luminescence” in 1888 and one year later he offered up “triboluminescence.” In the 1890s, other scientists started to get serious about investigating the triboluminescence of sugar and sugary things like rock candy. Here’s a whole paper on the subject in Nature in 1899, if you’re interested. [I am!! — Mike]

One of the enduring subjects of inquiry regarding triboluminescence—the main question, really—was about the precise nature of those little flashes of light. Are they dangerous? As Mike explained earlier, they’re not. But even in the 1960s, a lot of people weren’t sure.

In 1968, the question made it into the pages of The New England Journal of Medicine, a discussion that was then picked up by The Wall Street Journal. The discussion began with a young woman at the University of Nebraska, who learned the candy-chewing trick at school and later showed it off to her father, a doctor, who was alarmed. He wrote to the NEJM:

We have repeated this experiment successfully many times. We have learned that wintergreen Certs work nicely, but our research has been limited mostly to the brand Life Savers.

It appears to us that wintergreen-mint chewing may be a hazard and until proved otherwise should be discouraged in explosive atmospheres, such as operating rooms, oxygen tents and spacecrafts. We would never want to think that a spacecraft exploded because an astronaut inadvertently chewed a wintergreen mint.

After the letter ran, several researchers contacted the NEJM to offer assurances that the “sparks” were totally safe. The respondents included two graduate students at the University of Pennsylvania, who had frozen wintergreen oil in liquid nitrogen and then whacked it with a hammer in the dark, which honestly sounds like a good time, something I would pay money to do. (Move over, escape rooms and axe-throwing! The hot new entertainment concept is Dip Stuff In Liquid Nitrogen And Then Shatter It With A Hammer … Rooms.)

The NEJM also reported that its editors, excited by this easy-to-try experiment, “retired to a darkened men’s room to watch each other munch Wint-O-Green Life Savers.” This also sounds like a fun day at work, although it makes me wonder if there were any women on the journal’s staff in 1968 and if they were invited on this excursion to the men’s room.

Still, none of this should have been surprising to the original letter-writers or the NEJM staff, if they’d been paying attention to the newspapers thirty years earlier. Because in 1939, the triboluminescence of Wint-O-Green Life Savers was national news.

Mike here again. May I just add that one of the great joys of writing about science is that you find it’s a rich source of reckless lunatics? 

I couldn’t find any examples of anyone having actually started a laboratory fire or set parts of their own body alight using triboluminescence (for the reasons I explained earlier), but I feel utterly confident that it wasn’t from lack of trying. As soon as the potential for creating explosions was a question mark hanging in the air, you can bet a lot of chemists, physicists, and amateur maniacs were busily working late nights, straining their nervous systems and their marriages to breaking point, in an effort to make something awful happen that they’d probably regret later. So it always goes. I raise a glass to them all.

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3. An audience with the glow worms guy

Edmund Newton Harvey made his name as a researcher who studied bioluminescence—plants and animals that could glow all by themselves. He wrote a book about them, called Living Light, which was published in 1940; he was also the subject of a profile in LIFE in 1945.

Glowing railroad worm, as photographed by Harvey. Actually, in a way, this is a worm selfie—Harvey simply placed a railroad worm on a piece of film in a darkroom, and that one worm created these imprints.

In the early 1920s, when Harvey was giving a public lecture, a member of the audience asked if he knew about another type of mysterious glow:

Someone told me that if you take a Nabisco wafer [Doug’s note: the audience member probably meant a Necco Wafer] with a wintergreen flavor and break it in half, there will be a flash of light at the moment the wafer is broken. I wished to try the experiment and on going to a grocery store to purchase my experimental material, the salesman informed me there was no such thing as a Nabisco wafer with a wintergreen flavor. You may draw your own conclusions but I regret that I cannot show you the experiment. However, if you have an opportunity, you might try it yourselves.

That paragraph appears in the middle of an article about animal luminescence, published in 1923. For more than a decade (I checked), that was all Harvey had to say on the matter.

But the encounter clearly stuck with him. Eventually, he bought some candy and started doing his own experiments. In 1939, he published a short article in Science titled “The Luminescence of Sugar Wafers,” in which he discussed the results. He started with Necco Wafers—per the audience member’s misstated suggestion—but also tried Life Savers.

Harvey reported that the chocolate, licorice, and cinnamon flavors of Necco Wafers did not produce luminescence when crushed, and neither did mint, lime, and cherry Life Savers. But wintergreen, lime, clove, and sassafras Necco Wafers, and wintergreen and clove Life Savers, did light up—and in both candy brands, the wintergreen versions created the brightest sparks. (By the way, Life Savers styled all of their flavors back then in the same way: Wint-o-green, Vi-o-let, Pep-o-mint, Lic-o-rice, Cl-o-ve, and so on. Not kidding.)

Harvey’s piece in Science quickly spread to the popular media, presumably because it offered a fun little experiment people could easily try at home with candy (imagine the frenzy on social media if this discovery happened now). Dozens of articles appeared in newspapers and magazines around the USA, from Boston to Tulsa to Minneapolis. Multiple writers covered it, although the largest reach appears to have come from a syndicated column called “Today’s Science,” written by Gobind Behari Lal.

(Sidebar: Lal’s own life story is worthy of a book or biopic. He was one of the first science writers with a regular column in the mainstream press, the first Indian to win a Pulitzer Prize, and a strong supporter of the Indian independence movement; he was also still writing columns and giving interviews until just days before his death in 1982, at the age of 92.)

It appears that Harvey—who, again, was mostly interested in the bioluminescence of animals and plants—only started looking into crushing candy after the comment at the lecture. That audience member’s simple statement, and Harvey’s great curiosity, ended up inspiring generations of researchers interested in the specific triboluminescence of wintergreen candy, a small but fascinating field that endures today.

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4. How the world breaks (and what it means)

Back over to Mike now:

So far, so pointless, right? At first glance, it’s hard to see a practical application here: you can’t set anything alight, you can’t make things glow for longer than a blink of an eye, there’s no heat. Is this like the Mentos-dropped-in-Coke thing: incredibly cool and great for awakening your excitement about the wonders of science, but utterly useless in every other way?

Maybe! But since this is an effect that only occurs when the surfaces of materials break, could that be a great early warning system for structurally compromised buildings? If buildings contained triboluminescent materials and sensors were set up to detect any flashes, that could potentially be a way to spot a building starting to collapse long before it’s visually apparent. (It’s the fractures at a microscopic level that begin the process of structural collapse, and until now, those have been incredibly hard to spot. Maybe this might help?)

There’s even a way to pump up the “volume” on this effect—i.e., to increase the intensity of the emitted light. Studies in 2007 found that sonication–the application of sound waves–increased the brightness of triboluminescence. The reason is cavitation, the formation and collapse of tiny bubbles of air that can release an enormous amount of energy into a small space and amplify any that’s already there.

(If you love your Cold War submarine movies, you might have heard the sonar technician yelling “CAVITATION DETECTED, SIR,” the telltale sign of an enemy sub noisily coming out of hiding. And if you like your science fiction, Dune features a weapon which is made many times more destructive using sound waves, maybe via some futuristic application of cavitation which…right…yes, Doug, I’ll wrap this up. Yes. Probably for the best.)

So there you have it! Every food tells a story, but rarely does it involve such intricate levels of scientific intrigue. Go find a roll of wintergreen Life Savers–or Polos, if that’s more readily available—and get to chewing. Even if you can’t see the sparks, know that they’re there and they’re pretty damn fascinating—and by the way, your breath is smelling great right now. 

Happy snacking. Be curious about the world. Have a lovely day. Ta!

— Doug & Mike

PS — Check out the archives for even more snack history.

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