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Why did my broccoli catch fire in the microwave? Why did the pyrex bowl get so hot?

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I often do experiments in my microwave. The latest one I did was to put 2 small pieces of broccoli stem (fresh) into a pyrex bowl and microwave the bowl for about 3 minutes.

At one point I saw a big flame originating from the broccoli, so I pulled it out of the microwave (the fire went out as soon as I opened the door). The bowl was very hot.

Can someone with a good scientific knowledge of these things help me understand these phenomena? It seems to me that after the microwave has eliminated the water from the broccoli, it should stop absorbing heat.

Tnx.

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  1. Microwaves will excite any asymmetrical molecules, not just water. If you like your experiments more than your oven, lay a couple of pieces of steel wire a millimeter or two apart and turn on the magnetron. You'll get a pretty spectacular light show as they arc back and forth.

    2 Replies
    1. re: alanbarnes

      Aah! Now you've got me thinking.

      My organic chem is a bit rusty, but I vaguely recall that lots of organic molecules have some repeating parts attached to a "fixed" part, e.g. sugar--a repetition of a bunch of (I'm guessing) COH parts, attached to some kind of non-repeating part. And I think I recall that at least some fats are similar. Which of course would make them asymmetric.

      (Is HOH symmetrical? My p-chem knowledge is totally nonexistent.)

      If I'm right, that would mean that there are plenty of asymmetric organic molecules; thus, I suppose, if you excited them enough, they'd all catch fire.

      1. re: Howard_2

        My orgo is still sparky and lemon-fresh! Yes, water is symmetrical -- but only down one line of symmetry.
        Broccoli is complex. Lots of recent studies have examined antioxidants in broccoli -- these are often big, complicated molecules with many rings and substituents.
        Vitamin K is pretty simple, and also asymmetrical: http://www.chemicalbook.com/Structure...
        The sugars in the broccoli probably account for the fire: they got hot and combusted. What temperature they needed to reach would depend on what sugar it was, but most of the sugar in broccoli is fructose (48.8% of total sugar), which burns at a lower temperature than sucrose, at least.

    2. I'm pretty sure that I've heard the Pyrex contains some metal in the glass, so that could have been the problem...

      1. Interesting you posted this today as I had a lunch conversation with a coworker about a show he watched on TV on things that catch fire in a microwave. Metal is easy to imagine, but a whole raw carrot? Or a grape sliced halfway through? I honestly never knew things besides metal were even capable of catching fire in a microwave!

        1. Broccoli has a high iron and magnesium content. These minerals act like tiny pieces of metal and create arcing effects in a microwave.

          2 Replies
          1. re: shelley1925

            Oddly enough, my husband microwaves frozen broccoli all the time and has no problem. I don't think the iron and magnesium are in the broccoli in little bits of pure metal.

            1. re: eleeper

              I microwave fresh broccoli all the time and have no fireworks.

              3 minutes is hella long for two small pieces of broccoli -- it's far more likely the OP cooked the bejeebers out of it and the dried-out edges burst into flame.

          2. Microwave ovens employ dielectric heating to excite (and thereby heat up) molecules with asymmetric charges. Prime among these is water and, since this is omnipresent in food, we have microwave ovens. Other molecules such as sugar also get heated in this way but not as much as water. When you stuck your two bits of broccoli in the oven for 3 minutes you supplied more than enough energy to convert all the water in the broccoli to steam. With the oven door closed, the steam would be largely trapped and would continue to heat up. Also, certain components in the broccoli would continue to heat. The fire is the result of a combination of the superheated steam and the continued heating of the broccoli. Opening the microwave door would have turned off the power and also released the superheated steam. There would also be some slight heating of the glass and this may have been exacerbated by a phenomenon known as thermal runaway.