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Grape Balls Of Fire: How Grapes In The Microwave Create A Plasma

Mar 17, 2019

It’s not every day that a scientific study reads like great literature, but here’s how a recent paper in the Proceedings of the National Academy of Sciences begins:

“It is a truth universally acknowledged that a pair of grape hemispheres exposed to intense microwave radiation will spark, igniting a plasma.”

In other words: cut a grape in half, put the pieces next to each other in the microwave, and voila – sparks, and maybe even a glowing cloud of plasma.

People have been doing this in their kitchens for a while, but there’s never been a solid scientific explanation for the phenomenon. Until now.

“For decades I think it was assumed that when you cut the grape in half and you leave this little connecting bridge of skin, [it acts] as a connecting channel, almost like a piece of metal,” said Aaron Slepkov, the senior author on the new study explaining why microwaved grapes produce plasma.

“We were just curious about why that's the only geometry that anybody's ever found before,” said Slepkov,  an associate professor and Canada Research Chair in the Physics of Biomaterials at Trent University. “So, we went exploring this, and much to our surprise we found that you don't need this kind of bridge.”

The phenomenon also happened with two whole grapes, as long as they were very close together or touching, he said.

“We realized that water is a very special material at microwave frequencies,” Slepkov said, adding that water has a large index of refraction.

That means that the water in the grapes can pack microwave radiation really tightly, which traps light inside the grape.

Slepkov and his team came to understand that the plasma formation didn’t have to do with conductivity, after all.  

“So, the breakthrough for us was seeing that this is actually about photonics,” he said. “This is about optics of microwave light as it interacts with geometries of water, rather than about conductive channels in metallic-like objects.”

Slepkov said something strange happened as the two spheres got closer together in the microwave.

“The packed light inside of each sphere can sense the other sphere…and they move towards the point of contact. So, by the time that they're touching, almost all of the light is concentrated, almost as if these were two tiny lenses.”

If you want to try this at home, you have a lot of options. Slepkov has had success not just with grapes, but also blueberries, cherries, and olives.

“It seems to work because mostly, they're balls of water,” he said.