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Wow, that sounds like a silly question, but really! My wife froze bananas in the freezer with the peel still on. You can't peel them frozen. I put it in the microwave for one minute at 50% power. After 30 seconds the thin end of the peel (where a bunch connect to each other) was on fire, with a visible flame (about the size of a lighter). What on earth happened?

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That is very odd indeed. A fire needs an ignition source, and the only thing I know of that can actually ignite from microwaves is metal. Was there a metal twist tie or something? – Aaronut♦ Oct 1 '10 at 17:30
@aaronut, Not that I noticed before putting the banana in. Once it came out, the end was charred. There was no metal visible, but it could have been hidden by the mess at the end of the banana. – yossarian♦ Oct 1 '10 at 17:34
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@Aaronut - it can also ignite if it's heated too long: en.wikipedia.org/wiki/Microwave_oven#Hazards – justkt Oct 1 '10 at 17:36
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I suspect the bananas were, like many of the fruits you find in grocery stores, coated with some sort of food-grade wax. The dry woody end of the banana would have allowed this wax to heat to its flash point and ignite, catching the stem itself. – Shog9♦ Oct 1 '10 at 17:41
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I thaw my bananas in a bowl of warm water ... the only problem is that dealing with half-frozen bananas will chill your hands rather quickly. If I need the bananas stil frozen, I use a sharp paring knife, and cut the peel off. – Joe Oct 2 '10 at 3:46
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3 Answers

up vote 63 down vote accepted

It's caused by the high amount of potassium in the banana. Microwaves react with metals, bouncing off and cause arcing. You can even create a cool light show by putting a raw peeled banana in the microwave. Don't worry, it won't explode, but it will make a mess, it's also harmless.

This can also happen in some frozen vegetables depending on the soil conditions they grew in. As noted in the linked article, other high concentrations of metals such as magnesium, iron, and zinc can be the responsible mineral.

Momentary brief sparking is harmless and won't harm your microwave, but the USDA recommends turning off your microwave if you see sparks nonetheless. If it does start a fire, unless it's a very small fire, do not open your microwave door. The safest thing is to turn off the microwave and let the fire burn itself out by consuming all the oxygen. Opening the door could create a dangerous backdraft induced fireball.

Edit: Found a banana in microwave video.

See Also:

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The high potassium content of bananas is very useful for people who maintain certain types of gamma ray detector systems...the K-40 isotope provides a nice calibration line without having to deal with regulated radioactive sources. I have put part of my lunch in a counter for calibration purposes... BTW--don't fret the radioactive dose: you get more from cosmic rays than from any reasonable consumption of high potassium foods. – dmckee Oct 1 '10 at 19:17
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I don't feel entirely convinced by this. Yes, bananas are high in potassium, but the potassium is ionized and should not be prone to arcing like a solid metal. If this didn't matter, you would be able to see sparking with salt as well (sodium is highly reactive!). I obviously can't argue with results, but I feel like there has to be some other/deeper explanation. – Aaronut♦ Oct 1 '10 at 19:50
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+1 educational, and--now with a video--entertaining! – keithjgrant Oct 1 '10 at 22:00
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...which makes sense because bananas are quite high in sugars as compared with vegetables, which never spark in my microwave. The same chowhound thread also suggests that it may have something to do with the exact configuration of the molecules, and apparently, chopping the vegetables after steaming them is also cited as a common cause. The whole issue just seems way more complex to me than simply "X is high in minerals." – Aaronut♦ Oct 20 '10 at 15:42
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Hmm.. so when the metal detector goes off at the airport, I should just say that I have a banana in my pocket. – intuited Feb 15 '11 at 22:25
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This may also be related to the dielectric antenna effects that cause grapes to spark in a microwave :

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.

...

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.

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More videos and an explanation of grape plasma produced by a microwave maartenrutgers.org/fun/microwave/microwave.html#disc – SpecKK Oct 1 '10 at 22:22
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I keep my bananas in the frig and warm them up (one at a time) in the microwave. It is the stem which burns/smolders/smokes, in as little as 20 seconds. I don't know why, but I think it may have to do with the lack of water.

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