2019-03-12 Why Microwaved Grapes Shoot Sparks

We love food science because it helps us make better barbecue. And because it's cool. Take exploding grapes, for example. What? You never fired up a grape? Cut a grape in half, leave the halves attached, and stick the grape in the microwave oven. Press start and soon the grape will shoot out sparks or flames. Cool trick, huh? But what's going here?

Scientists recently studied the phenomenon to find out. Experiments show that the sparks are not caused by the conduction of electricity. They are caused by microwaves trapped inside each grape. Here's how. A single microwave wavelength measures about 5 inches long. But water inside the grape compresses the microwave wavelength to about 1/2 inch, about the thickness of a grape. Now, with a whole grape, the compressed microwaves bounce around inside the grape and concentrate in the center. With a single grape cut in half with the halves still touching (or with two grapes touching each other), the microwaves concentrate at the spot where the two meet. At that spot, a strong electromagnetic field creates a plasma, which is a superheated gas in which electrons have been stripped from the molecules. Plasma is the same thing that emits light in a plasma TV. In the grape example, the electromagnetic field sparks a plasma by stripping an electron from a sodium or potassium molecule in the grape. That spark triggers reactions in other atoms in the air, which lose electrons as they absorb the microwaves inside the oven. Poof! You get a spark.  

Interestingly, in the hotspot where the grapes touch, scientists determined that microwaves were 80 times smaller than the microwaves are in air. That means it's the water inside the grape that's causing the microwaves to concentrate. What if substances other than water could concentrate microwaves in a similar way, or concentrate them even further? That's the interesting part, and it has many scientific possibilities. Another interesting thing: most food consists primarily of water. What else can we fire up in the microwave? Check out an explosive grape video here.

grapes emitting sparks in microwave oven

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