Chemist turns on to glowing candy

November 04, 1991|By Frank D. Roylance | Frank D. Roylance,Evening Sun Staff


Hey, kids. Next time Mom catches you in the closet or under the covers gobbling up your leftover Halloween candy, tell her you're doing a chemistry experiment.

That's not what Towson State University chemistry professor Linda M. Sweeting is recommending. But it just might work.

Sweeting, as part of the Baltimore region's observation of National Chemistry Week, put on a demonstration yesterday at the Maryland Science Center explaining what makes wintergreen candies glow in the dark when you chew them up.

It was just the start of a week of demonstrations, talks and teacher workshops scheduled at area colleges and the Science Center throughout the week.

"I think chemists in general, and the American Chemical Society in particular, are concerned that people react to the word 'chemistry' and 'chemical' with fear and trembling," she said. "They are really unaware that they themselves are made of chemicals and . . . that chemicals give them the energy to walk down the street."

This week's events are designed to "make the public more aware that chemists are involved in a wide variety of activities, some of which contribute greatly to our material comfort and high standard of living."

Sweeting, 49, is coordinator of organic chemistry at TSU. She's also an expert in magnetic nuclear resonance, which is used in medical diagnostics.

But she also is something of an world expert on the subject of candy and other substances that glow, being one of only three or four scientists on the planet who have made the phenomenon, called "tribolumenescence," a focus of research.

She got hooked on tribolumenescence in graduate school at UCLA, while working in the dark with a substance she was told was prone to decompose if exposed to light.

"I was transferring the solid into the bottle, and it was creating blue flashes," she recalled. "When I saw this, I was half expecting it to explode. It really scared me."

In researching the phenomenon later, she found it had been noted and described as early as the 1600s by Sir Francis Bacon.

But "no one had really figured out what was going on until early in this century." In the 1920s, a French scientist named Longchambon analyzed a spectrograph of the light from glowing sugar crystals and found it was identical to lightning.

So, what's going on?

Sweeting said the light originates in the candy's sugar molecules. The sugar crystallizes in an asymmetrical structure that leaves certain electrons in somewhat unstable positions. When the crystals are broken up by chewing, the electrons are freed to "fall" to a more stable orbit around atomic nuclei in the molecules. And as they fall, they emit electromagnetic energy.

But, if it's the sugar that produces the light, why is it only visible in wintergreen candies, such as Wint-O-Green Lifesavers?

Truth is, she said, table sugar also glows in the dark when sugar cubes are struck together, like striking a match. So do ordinary quartz rocks and adhesive tapes when they're unrolled in the dark.

They all share the same characteristic asymmetric molecules -- in the sugar or rock crystals, and the tape's adhesive polymers. When they are broken up, electrons fall to more stable orbits and emit light along the way. But it's a comparatively weak glow.

The difference with wintergreen candies, Sweeting said, lies in the flavoring. It's an oil extracted from the wintergreen plant, Gaultheria procumbens, which grows in North American woods. Wintergreen oil is naturally fluorescent.

"What that means is that you can excite its electrons by shining light on it, specifically ultraviolet light," she said. "And when you do, the electrons jump back to their normal home and emit visible light. Good examples or other fluorescent materials are black light posters and laundry brighteners."

"Wintergreen is the brightest of all," she said. "Spearmint is not bad; but none of the transparent candies work at all." That's because they -- like regular Life Savers -- are made from "glassy sugars," which don't have the same properties.

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