MadSci Network: Physics
Query:

Re: Why do grapes spark in the microwave?

Date: Tue Dec 23 13:13:36 1997
Posted By: Adrian Popa, Staff Optical/Microwave Physics, Hughes Research Laboratories
Area of science: Physics
ID: 878101195.Ph
Message:

Greetings:

Your question is full of interesting physics and my answers are a bit complicated. Iíll try to make them understandable. During the early days of microwave ovens I had a research project to study the microwave properties of many different types of foods in microwave ovens; however, I never tried cooking grapes!

First let me define 3 terms:

  1. Dielectric - so called because it permits the passage of the lines of force of an electrostatic field but does not conduct current. Some times called an insulator (e.g. plastic, rubber, glass, teflon, distilled water).
  2. Dipole antenna - two linear conductors (commonly equal in length) separated at the center by a transmission line feed (e.g. TV rabbit ears antenna).
  3. Plasma - a high temperature ionized gas (e.g. neon sign, welding torch, gas laser, an electric arc)
Microwave ovens come in many different designs and sizes and your experience with grapes may not be able to be repeated in another design of oven. I found that I had to place the grapes that I tested in the center of my oven, raise them up on top of an inverted water glass. Also, my experiments did not work with a cup of water inside the oven along side of the grapes. The cup of water inside the oven is a safeguard load for the microwave magnetron power tube. A microwave oven should NEVER BE OPERATED with out some material to cook inside the oven and all manufactures tell you to put a cup of water in the oven if you want to test the oven. Heating just a few grapes with nothing else in the oven could possibly damage the microwave tube if the water in the grapes evaporates to quickly.

Microwave ovens generate between 500 watts and one thousand watts of microwave power at a frequency of 2400 megahertz (MHz or 2400 million cycles per second), corresponding to a wavelength of 12.5 cm (4.9 inches)in air. However, the oven wavelength in the dielectric distilled water is reduced to 1.4 cm (0.55 inches). As you shall see this is an important part of the answer to your question. Also, for an arc to occur in the air inside a microwave oven, a microwave field of at least 30,000 volts per centimeter must be generated! With the grapes separated by one millimeter (0.04 inches) we need to generate a 3000 volt potential between the grapes to generate arcing! This is a considerable amount of voltage.

MY EXPERIMENTAL RESULTS WITH GRAPES AND CRANBERRIES

First, I found that the experiment would not work with a cup of water in the oven to help protect the microwave tube. The grape experiment required all of the power my oven could produce (800 watts) and it could not share this power with a cup of water.

CAUTION:
Not being able to have a cup of water in the oven causes me to caution anyone about trying to repeat this experiment because they might damage their microwave oven.

For my experiments I purchased large 2.54 cm (1 inch) diameter red grapes. To protect the oven's microwave tube, I limited the oven timer to ten seconds for each test so that the grapes could not dry out during the experiment. I also used fresh grapes for each experiment so that I knew they were water filled and not dried out.

I found that single grapes would eject steam out of the stem hole forming little rocket engines which often propelled the grapes about the oven. If the stem was left in the grape, so that the steam could not escape, the grape skin would quickly rupture in a small explosion as it was heated. This reminded me that most microwave oven instruction books tell you not to cook eggs in the shell in an oven for the steam build up inside the hard shell will eventually cause the egg to explode causing a big mess in the oven.

When I put two grapes close together with the stem holes close to each other the arching and sparking you described occurs. The arc made a 120 Hertz buzz following the pulsing power of the microwave magnetron tube.

When I placed the stem holes tightly together the arcing would not occur.

When I heated single grapes and touching grape pairs together in the oven for 10 seconds, the touching pairs were at a significantly higher temperature after heating than the single grapes were! This suggests some form of enhancement of the microwave field in the grape pairs (perhaps a dipole antenna).

I also repeated these experiments with large cranberries. They have less water in them than grapes and are smaller in diameter. I was able to produce smaller arcs between cranberry pairs; however, I had to cut a small hole in the berry ends to let the steam escape to form an arc.

ANSWERS

There are two general classes of antennas, metallic conducting antennas and dielectric antennas that concentrate electromagnetic fields. The common antennas most people are familiar with are antennas made from conducting wires and rods such as the rabbit ears on indoor TV antennas or the multirod TV antennas on millions of roof tops. Dielectric antennas include various geometric solids including cylinders, spheres and plastic focusing lenses.

Non conducting dielectric materials are used for microwave cooking ware because they are relatively transparent to microwave energy. Also dielectric heating of food, particularly the water molecules in food, is the key principle used in microwave cooking.

Dielectric spheres one or more wavelengths in diameter form a special class of microwave antenna structure. When a dielectric sphere is immersed in a microwave field the spheres concentrate the electric field lines along an axis as shown in Figure A. If the sphere is slightly elongated, the field will usually align with the longest axis. This is exactly what water filled grapes (which are one or two wavelengths in diameter) will do in a microwave oven. The concentrated microwave field inside the grapes quickly heats the grapes to a high temperature after only 10 seconds of heating.

GRAPES IMMERSED IN AN ELECTROMAGNETIC FIELD. EQUAL POTENTIAL LINES ARE COMPRESSED INSIDE THE GRAPES AND IN THE GAP BETWEEN THE GRAPES INCREASING THE VOLTAGE GRADIENT IN THE STEAM FILLED REGION CAUSING AIR BREAKDOWN (ARCING)

Next I put two grapes with their stem holes tightly together and the pair of grapes form a larger more efficient dipole like antenna as the microwave energy field flows between the two coupled grapes. I believe this is why the coupled grapes are much hotter after 10 seconds of heating than single grapes are.

Finally, I slightly separated the stem holes of the grape pairs by about one millimeter. As the grapes are heated each grape emits a jet of steam toward the other grape and the concentrated microwave fields from the spheres reach more than 3000 volts exciting the steam into a plasma state as shown in Figure B. The plasma forms a short circuited conductor between the dipoles and we get the arching in the region of steam between the arrows shown in Figure B. When the grapes have expelled their steam pressure I found that the plasma extinguishes and the arc goes out.

This explanation is based on spheres filled with pure water and we know that the grape juice is acidic not just pure water. A more complex reaction is occurring within the grapes. However, the external resultant arcing would probably be about the same for both cases.

I said in the beginning that this is a complicated experiment with a complicated answer. It would take some expensive microwave equipment and time to study the arcing grapes in more detail in the laboratory. I hope this helps answer some of your questions. If you have more questions about the experiments please send me an e-mail note.

Best regards, your Mad Scientist

Adrian Popa


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