Why do Certain Vegetables Arc in the Microwave?

  1. Hello,

    This is my first post, so please forgive me if this has been covered before. In my searches I was unable to find any previous threads specifically about this question. I made a statement in a link sharing forum that was immediately disputed. I'm not interested in winning an argument so much as clarifying or revising my post. Any links shared here may be posted in my response.

    Here's the story:

    I chopped up and microwaved a green pepper, from my garden. An impressive display of sparks ensued. After much Googling, I found that most explanations (including the USDA Microwave Ovens and Food Safety website) point to the mineral content. The Canadian Food Inspection Agency website goes further in specifying that "these minerals include iron, magnesium and selenium."

    I also learned that grapes create plasma in the microwave due to circumstances having to do with the fact that they are round and juicy (which my pepper was not).

    My post in the aforementioned thread simply stated:
    I did not expect this to be such a controversial statement (especially considering the other things I say there), but apparently I am an idiot for accepting this explanation for the lighting storm in my microwave.

    No one who attacked my assertion has offered any alternative explanation, only that there is not enough metal in a pepper to cause this or that iron in food is not metallic.

    Can anyone help me understand what is really going on? Thanks!
     
  2. jcsd
  3. Chi Meson

    Chi Meson 1,772
    Science Advisor
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    Iron is not a requirement, but the fruit/vegitable must be juicy. And peppers ARE juicy. All juices are "electrolytic" and contain ions (sodium ions especially) which carry electric currents. Tap water contains enough mineral ions to carry electric currents, why not fruit juice? "Some" minerals is enough; you don't need "a lot." For the phenomenon to occur, the fruit/vegetable must be the right size. Roundness is not as important: I have experienced the effect with cubed carrots. What happens (simplest explanation) is the microwaves cause a resonant current inside the fruit. The juice heats up and vaporizes, and the cloud of vapor becomes the medium in which the microwaves resonate. Ionized vapor heats up and reaches the state of plasma (thousands of degrees F).

    Let me guess that your pepper was chopped into approx 1/2 inch pieces?
     
    Last edited: Mar 15, 2009
  4. Thanks. This makes sense to me.

    The pieces were probably around 1/4 inches in both the store bought pepper and the garden grown pepper.

    I suppose a fresh picked pepper from my garden might have a higher moisture content. Do you suppose it could be enough to be considered a variable? It also had much thinner walls than those big, thick store peppers.

    If I'm understanding this correctly, would it be safe to assume that the moisture content and/or physical size of a pepper is more likely responsible for arcing than the mineral content?
     
  5. LowlyPion

    LowlyPion 5,328
    Homework Helper

    Curious about the size issue with respect to dicing and resonance. Aren't microwave wavelengths about 4 or 5 inches for 2.5 ghz radiation? (Wikipedia says that's the radiation frequency anyway.) Why the shorter resonance distances?
     
  6. Chi Meson

    Chi Meson 1,772
    Science Advisor
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    I never understood that either. Resonance for sound waves in pipes occurs when pipe lengths are at 1/2 or 1/4 of the wavelengths. The 1/4 length resonance occurs when one end of the pipe has an "open end" (non-inverted) reflection while the other end has a "fixed end" (inverted) reflection. I do not know the specifics of this electric resonance, but it seems that food cut to 2 or 4 inch lengths does not lead to this phenomenon. Food cut to approx 1 cm cubes seems to work most often.

    When re-heating diced carrots for my 3 kids (back when each were infants) I witnessed this happen a total of 4 times. Yes, I was trying to.
     
  7. LowlyPion

    LowlyPion 5,328
    Homework Helper

    I never observed this as a problem, though I do have a turn table that rotates. Sometimes with mylar shreds left on the top of a container, and some labels that have metallic inks, I've seen that, but not in a bowl of hashed veggies. I heat butter explosively if not careful. I designed a vented container that fixes that (I figured it was a water content problem), but generally don't have much problem with other things except maybe the occasional kitten.
     
  8. I think it depends on the period/frequency rather than the wavelength (ignoring for a moment that they are related), and the drift velocities achieved by the ions/charge carriers.
     
  9. Chi Meson

    Chi Meson 1,772
    Science Advisor
    Homework Helper

    Oh yeah. The charges (ions) are not gonna be moving at the speed of light now, are they? The point is that the current within the fruit/veggie gets so large that it heats up phenomenally, and the liquid vaporizes, then plasmatizes (or whatever the word is). If the pieces are on the order of 1 cm, while the wavelength is on the order of 12 cm, then the ratio of speeds (photons vs ions ) would be 1:12. SO could the net speed of ions in electrolytic juice within fruit reach 25,000,000 m/s? Seems a bit much.
     
  10. With fruits in the microwave people usually use grapes.
     
  11. Moonbear

    Moonbear 12,265
    Staff Emeritus
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    Gold Member

  12. Hello,
    I feel it is dependent on not only size but also how it is stored once brought from the store. Another reason could be the method of cutting pepper or if it is for roasting dry or with oil
     
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