Scientists now know why microwaved grapes make plasma fireballs

To cook up homemade plasma, all someone needs is a grape and a microwave oven. The effect makes for a spectacular kitchen fireworks display. But don’t try this at home — it could damage your oven.

The recipe is simple: Cut a grape in half, leaving the two halves attached 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 fireball of electrons and electrically charged atoms called ions. The hot mix of electrons and ions is known as a plasma.

This trick has been floating around the internet for decades. Some people thought that the effect had to do with the skin linking the grape halves. But two whole grapes bumped up against each other do the same thing. So do waterlogged beads called hydrogels, tests show.

Researchers in Canada found that the grapes act as resonators for the microwave radiation. That means the grapes trap this energy. For a time, the microwaves will bounce back and forth inside the grape. Then the energy breaks out in a flash.

With heat imaging, the team showed that the trapped energy forms a hot spot at the grape’s center. But if two grapes sit next to each other, that hot spot forms where the grapes touch. Salts within the grape skin now become electrically charged, or ionized. Releasing the salt ions produces a plasma flare.

Hamza K. Khattak of Trent University in Peterborough and his colleagues reported their new findings in the March 5 Proceedings of the National Academy of Sciences.