Microwaved grapes make fireballs, and scientists now know why

Plasma forms because the fruit is the right size to trap the devices’ electromagnetic waves

exploding grape

GRAPE PLASMA  Microwaving grapes creates plasma, and scientists have determined that the effect comes down to a hotspot produced where the two grapes touch. The effect also works with a grape and a hydrogel bead (shown), or two hydrogel beads.

Slepkov Biophotonics Lab/Trent Univ.

Here’s a recipe for homemade plasma: Cut a grape in half, leaving the two sections connected at one end by the grape’s thin skin. Heat the fruit in a microwave for a few seconds. Then, boom: From the grape erupts a small plasma fireball — a hot mixture of electrons and electrically charged atoms, or ions.

This trick has been floating around the internet for decades, and previous explanations of the effect have focused on the importance of the connecting skin. But two whole grapes bumped up against one another do the same thing, as do similarly sized waterlogged beads called hydrogels, researchers report in the March 5 Proceedings of the National Academy of Sciences.

The team determined that the grapes act as resonators for the microwave radiation, much like a flute resonates with sound waves. A single grape is just the right size that the electromagnetic waves get trapped within the fruit, bouncing back and forth. Using thermal imaging, the researchers showed that a hot spot appears in the grape’s center, as a result of the trapped radiation. But if two grapes sit next to each other, that hot spot forms where the grapes touch, and salts within the grape skin are ionized and released, producing a plasma flare.

The effect makes for a spectacular kitchen fireworks display, but one you may not want to try at home — it could damage your microwave.

LIT UP Grapes produce a burst of plasma when microwaved. Scientists have now determined that this process occurs because the grapes trap radiation inside.

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

More Stories from Science News on Physics