Scientists Conduct Rigorous Study of Why Grapes Shine in the Microwave

An article published Monday in a well-known scientific journal begins with the following phrase: "It is a universally recognized truth that a pair of grape hemispheres exposed to intense microwave radiation will produce a spark that will ignite a plasma." A truth universally recognized. But what is the cause of this wonder microwave?

If you are unfamiliar, placing the grapes in a microwave oven to produce sparks has become a popular tip on YouTube. This new research done by Canadian scientists shows that interesting advances can come from anywhere, even by studying nonsense.

"This is a diet that has not been studied significantly before," Gizmodo told one of the newspaper's authors, Pablo Bianucci of Concordia University in Montreal.

The trick usually shows two grape halves connected by a thin sheen of skin. After a few seconds in the microwave, they start to fire. Although various explanations exist online, the researchers wanted to study the phenomenon more rigorously.

The researcher imagined both cut grape slices and hydrogel beads – made from a material absorbing a lot of water – when triggered in the microwave. They quickly realized that grape skin was not needed to get the sparks, as evidenced by the spark in the hydrogel beads, held together only by their weight and the shape of the dish in which they were sitting, according to research published in the Proceedings of the National Academy of Sciences.

The specific geometry of two circular objects in contact with circular water in an electromagnetic field creates concentrated resonances at the point of intersection of spheres or half-spheres. This becomes a very small hotspot with a high energy density, sufficient to create a plasma from the ions in the region where the objects are touching.

Does the research deserve to be published in such a prestigious journal as PNAS? Catherine Murphy, professor of chemistry at the University of Illinois, is certainly the opinion. "The fact that they have been rigorous enough to go through the peer review process proves that they are doing a good job on a technical level," she told Gizmodo.

But paper is much more than a gadget, Murphy said. This type of directed energy research could find significant use in other directed energy systems, such as explosives or high intensity laser pulses. In addition, the article presents a way to image the electric fields in these types of physical configurations and could lead to advances in photonics more generally.

Many original things could yield useful scientific results if you apply rigorous thinking and conduct a thoughtful experiment, as this work shows. We also know that if you accidentally burn your house with microwaved grapes, it's not my fault.