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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4@Aaronut - it can also ignite if it's heated too long: en.wikipedia.org/wiki/Microwave_oven#Hazards– justktCommented Oct 1, 2010 at 17:36
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14I 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.– Shog9Commented Oct 1, 2010 at 17:41
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2I 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.– JoeCommented Oct 2, 2010 at 3:46
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1I 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 (bit.ly/fT6EA3) (first item on the page).– immutablCommented Feb 28, 2011 at 12:51
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1Related blog on the Washington Post website.– justktCommented May 18, 2011 at 16:21
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6 Answers
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:
answered Oct 1, 2010 at 18:02
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11The 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. Commented Oct 1, 2010 at 19:17
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56I 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.– AaronutCommented Oct 1, 2010 at 19:50
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1@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?– AaronutCommented Oct 20, 2010 at 14:40
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5...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."– AaronutCommented Oct 20, 2010 at 15:42
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6Hmm.. so when the metal detector goes off at the airport, I should just say that I have a banana in my pocket.– intuitedCommented Feb 15, 2011 at 22:25
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.
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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– SpecKKCommented Oct 1, 2010 at 22:22
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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. Commented Jan 8, 2016 at 10:32
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1Freshly arrived explanation on that effect: 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 ♦Commented Feb 25, 2019 at 12:14
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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.
It could have fermented and alcohol ignited it
Bananas is the only fruit that contains radiation. That is why it will catch fire.
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3All fruits (and non-fruits) "contain radiation". Else it wouldn't be possible to do carbon dating, for example.– rumtscho ♦Commented Oct 24, 2016 at 12:34
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The (slight) radioactivity from a banana's potassium has nothing to do with it catching fire in a microwave. Commented Oct 24, 2016 at 22:22
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5It would be great if your answer contained a source that you can cite that indicates this. Commented Oct 25, 2016 at 6:03
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.
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