Scientists now know why microwaved grapes make plasma fireballs

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.

Explainer: Understanding light and electromagnetic radiation

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.

Explainer: How heat moves

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 .

Microwaving grapes create plasma fireballs. The reason? Grapes trap the microwave’s energy inside themselves, research now shows.Science News/YouTube

Power Words

atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

electron A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

fireball A lump of rock or metal from space that hits the atmosphere of Earth. Fireballs are meteors that are exceptionally bright and large.

fruit A seed-containing reproductive organ in a plant.

hydrogel A “smart” material that can change its structure in response to its environment, such as the local temperature, pH, salt or water concentration. The material is made from a polymer — a chain made from links of identical units — that have free, water-attracting ends sticking out. So in the presence of water, it may hold (bond) those water molecules for quite a while. Some hydrogels are used in baby diapers to hold urine, in potting soils to hold water near to plants until they need it and in wound dressings to keep a sore from drying out.

ion (adj. ionized) An atom or molecule with an electric charge due to the loss or gain of one or more electrons. An ionized gas, or plasma, is where all of the electrons have been separated from their parent atoms.

plasma (in chemistry and physics) A gaseous state of matter in which electrons separate from the atom. A plasma includes both positively and negatively charged particles. (in medicine) The colorless fluid part of blood.

Proceedings of the National Academy of Sciences A prestigious journal publishing original scientific research, begun in 1914. The journal’s content spans the biological, physical, and social sciences. Each of the more than 3,000 papers it publishes each year, now, are not only peer reviewed but also approved by a member of the U.S. National Academy of Sciences.

radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

resonate To reverberate, like a ringing bell, producing a clear tone or frequency of radiating energy.

resonator (in physics) Something that traps energy at certain frequencies, causing it to reverberate. See resonate.

salt A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called “sea salt.” Common table salt is a made of sodium and chlorine.

wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.