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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?

Rodrigo de Azevedo
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yossarian
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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 01 '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 01 '10 at 17:34
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    @Aaronut - it can also ignite if it's heated too long: http://en.wikipedia.org/wiki/Microwave_oven#Hazards – justkt Oct 01 '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 01 '10 at 17:41
  • @justkt: That's true, but to reach the autoignition temperature of even a highly-combustible substance would require far more than 30 seconds at half power. – Aaronut Oct 01 '10 at 19:46
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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 02 '10 at 03:46
  • I've thawed frozen bananas that way many times and never had that happen... now I know to watch out! – JustRightMenus Oct 02 '10 at 04:40
  • This has happened to me as well! But instead I was cooking an apple cut into pieces... and it has happened with a pear as well. (sorry, not much help...) –  Jan 26 '11 at 18:42
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    I did this when attempting to 'bake' a potato in my new 950W microwave. It would have burnt the whole potato had I not intervened.

    Quite something to watch. A scientist friend suggested it was related to this phenomenon (http://bit.ly/fT6EA3) (first item on the page).

     – immutabl Feb 28 '11 at 12:51
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    Related blog on the Washington Post website. – justkt May 18 '11 at 16:21
  • justkt - the article can't be read as the WP requires a subscription for me to access it. – Jude Jun 13 '17 at 04:21

6 Answers6

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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.

See Also:

Boltgolt
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hobodave
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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 --- ex-moderator kitten Oct 01 '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 01 '10 at 19:50
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    @dmckee: As our resident physicist, I'm hoping you can comment. How is it possible that stable potassium compounds react to the microwaves like pure metals? Doesn't there have to be some reaction breaking them down, or am I looking at this entirely the wrong way? – Aaronut Oct 20 '10 at 14:40
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    @Aaronut: I don't know. I don't even know if that is the mechanism at work here. I'm not a solid state physicist, and hesitate to speculate too much. Has anyone tried some other high potassium vegetables (broccoli, spinich,...)? The mechanism discussed in SpecKK's link looks viable, but requires two smallish dipoles and a low resistance path between them. I don't think the banana provides that---unless there is something going on in it's internal structure. – dmckee --- ex-moderator kitten Oct 20 '10 at 15:30
  • @dmckee: Interesting, broccoli and spinach don't seem to be frequently complained about. Both the GM and USDA links refer to carrots, apparently due to the soil conditions under which they were grown. I found a link about broccoli which also makes reference to sweet potatoes and cauliflower; all three are frequently microwaved and rarely have issues. Interestingly, I also found this chowhound thread about broccoli suggesting that fires (not necessarily arcing) could be due to sugars combusting... – Aaronut Oct 20 '10 at 15:40
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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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    @intuited You had better hope they grope your banana instead of igniting it with the backscatter. (Yes, I know it's X-rays and not microwaves :P) – Matthew Read Nov 14 '11 at 22:35
  • @Matthew Read: It's okay, my banana is not pierced. – intuited Nov 16 '11 at 01:29
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    Try microwaving a bowl of water with some potassium chloride and soluble salts of all those metals. I don't think it'll ignite. – Simon Kuang Aug 19 '14 at 21:05
  • Interesting, I got similar, but not same results with Potassium Nitrate + water in my mycrowave - instead of catching fire it started arcing like mad. – Art Nov 09 '15 at 01:17
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    If a banana contained metallic potassium it wouldn't take a microwave to set it on fire :) And high amounts of saltpetre would also.... impart an off taste. – rackandboneman Jan 08 '16 at 10:28
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    Oranges, grapes and of course, raisins (dried grapes) are also high in potassium. I've hydrated raisins with a very small amount of water in the microwave and also heated cold rice pudding (with lots of raisins). Both took several minutes and I've never noticed this. I very much doubt the potassium in food behaves like metallic potassium. Remember the internet has as much pseudo-science posted as real stuff - AND it's repeated more often. – Jude Jun 13 '17 at 04:29
  • The video link is broken. – Clément Jul 05 '17 at 01:27
  • To clarify: Metallic potassium will ignite all by itself when contacting water or wet matter. – rackandboneman Apr 24 '18 at 08:44
  • Fun fact, you can also create a "cool light show" using grapes, but for a completely different reason. – Aaron Franke Mar 20 '19 at 06:10
  • Oh, the IDEAS..! "I don't know the lab came to burn down", said Bond, smiling faintly. "All I did was put a frozen banana and some broccoli in the microwave and set it to 'Defrost'". "Nonetheless, excellent work, Commander Bond!", said M. "Errrm...but...". "But, M?" replied Bond, quizzically. "Well, you see...cutbacks in funding...end of the cold war...all that rot...", said M, sadly. "You mean..?", asked Bond. "Yes. Redundant. Sorry, old man". With that M drew a silenced pistol from his jacket and fired, killing Bond instantly. "Ms. Moneypenny?", called M. "Could you clean this up, please?" – Bob Jarvis - Слава Україні Apr 04 '19 at 19:38
  • I've had this occur w/ green beans and green peppers. It all depends on the arrangement of the pieces. If all pieces are well separated w/ space all around them, there won't be any sparking. If you have some pieces are close enough together or overlapping, you can get arching between them. – NSGod Dec 17 '19 at 22:32
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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.

SpecKK
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  • More videos and an explanation of grape plasma produced by a microwave http://maartenrutgers.org/fun/microwave/microwave.html#disc – SpecKK Oct 01 '10 at 22:22
  • I would expect a small, 5-6cm banana to be the most problematic then, it is would be a tuned dipole for the most commonly used frequency in microwave ovens :) Now even smaller ones, around 3cm... you could really call that a short banana. – rackandboneman Jan 08 '16 at 10:32
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    Freshly arrived explanation on that effect: https://www.pnas.org/content/early/2019/02/13/1818350116. The relationship may exist, but it is not exactly the same, since the grape effect relies on the shape. – rumtscho Feb 25 '19 at 12:14
  • Update: How Microwaving Grapes Makes Plasma. – Peter Mortensen Apr 05 '19 at 07:33
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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.

Max
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It could have fermented and alcohol ignited it

ganja
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Bananas is the only fruit that contains radiation. That is why it will catch fire.

Hazel
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I belive the potassium in the banana would be a good conductor, but I have a few things to add.

The microwave runs on, well microwaves a form of radiation that penitrates what's in there and heats from with in. A banana is also slightly radioactive, not harmful to humans though, and I belive the potassium is a conductor of both the microwaves and the radioactivity of the banana reacting to one another and the potassium is then chemically changed and then reacts to the microwaves emitted from the device That's my hypothesis anyway let me know what you think.

lance
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