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Yesterday on reddit a user posted this picture of a potato:

http://i.imgur.com/3nHAqaX.jpg

He said that he followed an online recipe. He put it in the microwave for 5 minutes, turned the potato, and put it back in for 5 more minutes. When he came back to check on it, the potato was on fire.

But the picture looks so surreal, it is even possible for a potato to start to glow red? Especially after only 10 minutes in the microwave? That would mean that this video recipe is actually quite unsafe. Or could it be a microwave malfunction?

Microwaved potatoes sound like a good idea, but now I don't think I would dare try.

Additional pictures:

Second potato angle

The glowing insides of the potato

The potato still in the microwave, just after he put out the fire

Burnt microwave plate

The brand of potato used

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    I wonder what Skeptics would think of this question. – Catija Jan 12 '16 at 22:06
  • Yes, yes it is. It's happened to me. cooking.stackexchange.com/a/5358/67 – Joe Jan 13 '16 at 0:50
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    Is it possible there were traces of foil on your potato??? – Lisa Jan 13 '16 at 2:31
  • I can tell you that when it happened to me, there was absolutely no foil. – Joe Jan 13 '16 at 2:58
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    How many watt Microwave? I've an 800 watt job, and it'll cook a medium potato in 5 minutes. After you boil all the water out in a microwave, things will catch fire. I've overheated damp books, only to see them burn, even smelted aluminum: instructables.com/id/microwave-smelter – Wayfaring Stranger Sep 9 '18 at 23:03
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"Should be fine" is like saying 'well, there's really low odds of something going wrong'. I have no idea what the odds are, but yes, it's possible.

I had microwaved potatoes many times, and then had a rather strange incident that I described in another question about microwaving potatoes, and included links to other incidents of people mentioned it happened to them.

I've since had a few years to think about what happened, and there's a few possibilities besides what I originally mentioned:

  • I don't know how old the potatoes were. They might've been older and a little bit dry.
  • I was half asleep, and it's possible that I didn't poke them a few times. I don't know if that could've led to the skin separating from the meat of the potato and that drying out & catching fire.
  • And I should mention that people didn't seem to believe me when I posted that other answer, as it got a couple of down votes. – Joe Jan 13 '16 at 1:03
  • That's very interesting, you even mentioned in the answer you linked that one of the potatoes was glowing ! – Sarumanatee Jan 13 '16 at 13:49
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    @Sarumanatee : mine was glowing too ... like it had burned away from the inside ... which makes me wonder if there might've been something inside the potato (like how you occasionally find a black spot in there) that absorbed a higher percentage of the microwaves and superheated 'til it caught fire. – Joe Jan 13 '16 at 13:56
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Cooked a potato, medium sized one in my Samsung microwave last night on the "Potato" smart setting .. thing caught fire , was on fire and burnt the inside roof of microwave, melted it .. Doesnt work at all now

  • it's for the best. If it hadn't broken it, you'd have a microwave that smelled like insects trapped in a halogen lamp every time you opened it for the next 6 months – Joe Sep 9 '18 at 17:18
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I felt this needed a physics answer.

tl;dr: It's unlikely but not impossible - we have to make some unrealistic assumptions to make it work for a sensible size of potato

Consider a homogeneous spherical potato in a vacuum*:

According to Wikipedia potatoes are 79% water, 17% carbohydrate (mostly starch), 2% protein and 2% fibre. Figures I've seen elsewhere vary by a few percent, so we can simplify to 80% water and 20% starch (the thermal properties of protein-and fibre-rich foods are close to those of pure starch so we can treat them as the same).

Water has a specific heat capacity of 4.3 kJ/kg/°C, and starch 1.75 kJ/kg/°C. That means that our model potato has a specific heat capacity of 3.7 kJ/kg/°C (a real measured value is a little lower at 3.43 kJ/kg/°C, but as we will see, water dominates the results).

First we consider the heat energy required to get the potato up to the boiling point of water (100°C), assuming a starting temperature of 20°C (room temperature), and a potato weighing 0.2 kg (just under half a pound, so a small jacket potato). 0.2×80×3.7 = 59.2 kJ. 1  kJ is 1 kW for 1 s, so a 1 kW microwave, 100% efficient, would heat this potato to boiling point in about 1 minute. This is considerably quicker than realistic for several reasons, and this is something we're familiar with:

  • The microwaves don't penetrate to the centre so much of the potato is heated only by conduction.
  • While this is going on the hot surface is losing heat to its surroundings by convection, conduction, and radiation.
  • Microwave ovens aren't 100% efficient at delivering their stated power into food, though it's surprisingly hard to get figures on losses.

Once the temperature of the potato is 100°C, it won't keep on rising; instead the heat will go into evaporating the water in it. The latent heat of vapourisation of water is 2300 kJ/kg or 460 kJ for our potato. This is near enough another 8 minutes (total 9 so far) at 1 kW, before we can start to get above 100°C by boiling off all the water (I assume the potato is isothermal, which of course isn't true, but thew timescales are long enough for significant heat flow).

Once we've boiled off all the water we can start heating the remaining dried potato (starch). We only have 20% of the mass left (i.e. 0.04 kg of starch, the specific heat capacity of which is 1.75 kJ/kg/°C). Red heat is at least around 700°C (for steel, but the potato would char to carbon at lower temperatures and carbon should be close enough to steel). The extra 600°C rise would take another 600×0.04×1.75 = 42 kJ of energy, or 42 seconds.

So it's just about possible to get a 200 g potato red hot in a microwave in 10 minutes, but only if we:

  • neglect all heat losses (except those due to driving off water), which would be significant. I won't go into detail but the potato would have a surface area of 154 cm2 and would radiate only 12 W net at 100°C but 168 W at 400°C (just below the autoignition temperature, and a significant fraction of the input power). The plate/turntable takes away some heat by conduction, and we can;t really neglect convection (but it's hard, hence the assumption of a vacuum).
  • assume 100% microwave efficiency.

On the other hand we assume no energy is released by combustion, and if we can get the dry starch to its autoignition temperature of 410°C (Wikipedia) it will burn, releasing quite a lot of heat (the heat of combustion of starch is 17.5 mJ/kg, but much of this is lost to the potato).

Another point that makes burning more likely is that we don't actually need to get to zero water in the whole potato before some of it can start burning. This would mean that more of the heat of burning some potato is useful in heating the rest, and would be of particular interest if the turntable of the microwave was jammed or there was a hotspot that received considerably more power than the rest. A similar effect would be caused by conductive (e.g. metal) particles on the potato, as they'd cause a small region to get very hot. These don't really fit with the even burning shown in the photo.

Those who are interested in the physics may wish to look at The Thermal Properties of Potato, T. Yamada 1970. It's in Japanese with an English abstract but the figures give clear results for the specific heat capacity and thermal conductivity. A 1-dimensional thermal model would be an interesting undergrad physics problem.


* not strictly required but for once a sensible simplification - and we physicists so rarely get the chance to follow this stereotype.

  • 1
    I am very split on whether to upvote the answer. I am pretty sure that food can catch fire before it has 0% water left - firewood is considered "dry" if it has less than 18% water, but it can burn (with some effort and smoke) at more than that. So the idea of a spherical homogenous potato in vacuum getting to be a lump of starch is entertaining (and probably doesn't even need too much heat, since you can dehydrate food in vacuum at low temperatures just fine), but not all that well related to the actual burnability of potatoes. At the same time, I just enjoyed reading! – rumtscho Jan 24 at 15:03
  • @rumtscho the firewood comparison is a good one, but driving off ~80% of the water doesn't change the conclusion given the error margins. I ignore how the water binds, because that's chemistry not physics, but that may have an effect. Given that I've listed most of my other assumptions that one should be in there too; there may be more that I've missed. – Chris H Jan 24 at 15:24
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    ... on second thoughts I suspect you get the firewood locally dry, and the combustion provides the heat to dry neighbouring regions. That means burning can contribute more to the heating than I assumed as it can start before the whole thing is up to temperature. The rather well-distributed red heat in the photo suggests we're not simply creating a very hot spot. I wonder about the effect of a stuck turntable. – Chris H Jan 24 at 15:29
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    OK, including a link to a source titled "The thermal properties of potato" did it for me, here is your +1 :) – rumtscho Jan 24 at 15:37
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    @rumstcho something localised (conductive dirt perhaps?) - could certainly have an effect (another point to add to my penultimate para). I'd forgotten that Q but the accepted answer is nonsense , directly equivalent to saying salt would ignite because it's made from sodium (and plenty of foods have more sodium per unit mass than a banana has potassium). – Chris H Jan 24 at 15:58
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i just warmed up mashed potatoes, after 12 seconds there was a flash, I opened door and the burned smell was horrific

  • Hopefully you got it out quickly ... my microwave smelled horrible for months – Joe Jan 24 at 1:15
  • Is there anything that would have caused the "flash"? Is your microwave a new or older model? What's your microwave wattage? (wondering if this is something high watt microwaves have vs lower wattage models) – elbrant Jan 24 at 3:03
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    Are you saying the microwave burned the mashed potatoes in 12 seconds? – UnhandledExcepSean Jan 24 at 15:45
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10 minutes on high should be just fine for a medium to large potato. I suppose it's possible it could burn if it's a very small potato, or if the potato is severely dehydrated. I would expect it to produce a lot of smelly smoke before it actually catches on fire, so you'll likely notice that it's gone wrong before you need a fire extinguisher.

  • 1
    Some microwaves are well sealed -- I had a glowing husk similar to what was pictured, and barely smelled anything until I opened it ... and then it took me a month to get the stench out of my microwave ... it smelled like when I had one of those floor-standing halogen lamps, and a bug got into it. (so much worse than a bug zapper, as you more roast it than burn it to a crisp quickly). It was rather similar to burning hair. – Joe Jan 13 '16 at 1:01
  • @Joe, that sounds like a terrible stink. What were you cooking? – mrog Jan 13 '16 at 17:00
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    the potatoes that caught fire. – Joe Jan 13 '16 at 18:06

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