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Last week I microwaved some peanut butter M&Ms for 40 seconds, which is something I've done hundreds of times before to melt the chocolate on the inside. However, when I did it this time, there was some popping and then a big spark inside. There was no metal in the microwave, the bowl is microwave safe, but it did have a chip in it. I also left the M&Ms uncovered, while I usually cover them with a paper towel. Any idea what caused this? Someone elsewhere suggested this paper might be an explanation but I would think it would have happened before if that was the case.

  • Was the quantity (approximately) the same as before? – Chris H Feb 25 '19 at 17:40
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    @ChrisH Yes, roughly a handful. – HS-nebula Feb 25 '19 at 17:44
  • Grape is the usual object to demonstrate this effect: thenakedscientists.com/get-naked/experiments/grape-plasmas CD's and lit candles covered by glass bowls are fun as well. – Wayfaring Stranger Feb 26 '19 at 0:53
  • @WayfaringStranger, true, and I'd think the index of refraction would be significantly different in a combination of chocolate and peanut butter than in a grape, which may not cause mostly internal reflection. Maybe the shell is made of ingredients that would cause the sparks? – HS-nebula Feb 26 '19 at 5:47
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    @HS-nebula I think the shell is some form of hard sugar, which would be a lot different than the chocolatey innards. I Could be wrong about that, but its early right now, and I can't do a search until coffee hits. -I've never looked up the coating recipe. – Wayfaring Stranger Feb 26 '19 at 16:28
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Off the top of my head I would say the sparking and popping is unexpected: usually a significant of water content would need to be present for this kind of behavior, as water conducts electricity and everything else not so much. There is water in peanut butter, but it is around 1-2% and is emulsified with fat, and that should reduce the conductivity substantially. But, the voltages inside a microwave oven are quite high and could force a conductive path through the emulsion to cause sparking.

My best guess is that whoever makes M&Ms (can't remember now) changed their peanut butter formulation and used slightly more water. It is batch specific?

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