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I've been experimenting some with do it yourself microwave popcorn, and some trouble getting the 'burn time' right has led me to a question that I can't find a good, definitive answer to.

What is the scientific explanation for the mechanism for how popcorn burns? I'm not looking for "Because it gets too hot", but what gets too hot, how does it actually get that way? Microwaves don't just heat everything up (like a stove does), so it's not as straightforward as that. You can put many foods inside of a microwave and massively overcook them without burning. So what's special about popcorn that causes it to burn? I would prefer either a detailed chemistry/physics answer, or a reliable, science-based source. From looking around online, there are lots of semi-reliable opinions on this, none of which agree with each other.

For example, this very old Chicago Tribune article on Popcorn seems to say that it's a problem of "finding" the kernels. Really? Others claim it is the kernel overheating, or the bag overheating, or the oil overheating (yet no-oil still burns, though that may contain some oil from the corn itself I suppose), or gnomes lighting them on fire with matches (Well, no, but it's about as reliable as anything else I've found).

And related to this, what control do I have over this other than time. On a stove, I can reduce the heat; for example, if I'm cooking on the stove with butter, if I cook at '3' I can cook almost all day without burning my butter, while at '4' it browns pretty quickly; so I can choose whichever setting based on that. What in a microwave would be similar (in allowing me to cook the kernels more thoroughly - not just take longer - without risking burning). What ingredients or microwave settings can I control?

  • To the last question: if you can't control the watt output, there is something weird about your microwave. Maybe there are models without such a control, I don't know - if this is the case, you can't do anything. – rumtscho Jan 26 '15 at 18:20
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    @rumtscho Most microwaves don't truly control the watt output; they just modulate on/off (so 80% power = .8s on .2s off or whatever). Panasonic developed the 'inverter' which allows their microwaves to actually cook at lower wattages, but most do not have that technology. It's possible the modulated on/off would help - I have done some tests and find it unclear if it just delays the popping/burning or not. – Joe M Jan 26 '15 at 18:26
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    Just a thought here regarding the last point -- if you're looking for more control, I'd personally suggest just cooking it on the stovetop in a heavy pot. From my perspective, microwaving popcorn is for when you just want to throw a pre-made bag of junk popcorn in. If I want to make "good" homemade popcorn, I always cook it in a pot (usually with a glass lid, to see what's going on better), and I've never burned it that way. I can also mix in a greater variety of things without making a mess and have a lot more control/subtlety in the way it cooks. – Athanasius Jan 26 '15 at 19:17
  • @Athanasius At home I agree (though I manage to burn it there too!). Unfortunately at work I don't think I'd be allowed to cook on a burner... – Joe M Jan 26 '15 at 19:34
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    When designing microwave popcorn, your variables are bag size, quantity of popping corn, microwave timing. The popcorn cornels will heat up and pop from the steam produced inside. Then burn if having to wait to long for the remaining cornels to pop. If the bag volume is small, it will retain steam long enough for additional cornels to pop. If to large the popped cornels dry out and burn before the remaining cornels complete their popping. I used a short length of 1.25" diameter pipe to measure to a higher precision popcorn cornels, used a small number 10 bag, then calculated timing. Good Luck. – Optionparty Jan 26 '15 at 22:10
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I'm going to try to take a crack at this answer, from my perspective as a materials scientist, which is kind of a combination of solid state physics and solid state chemistry.

How popcorn pops is from superheating the water in the kernels until there's enough pressure to break through the outer hull. Then, the starch inside the kernel is able to rapidly expand, cool quickly, and the starch sets into a foam. (FYI: This means that the water actually gets much hotter than it's boiling point inside the kernel)

Microwaves are able to excite (heat) water molecules really well, and water absorbs most of the microwave energy that's entering the food, preventing other molecules present from overheating & burning.

So, once the popcorn has "popped," the water content in the kernel is very low and it's mostly just starch, arranged in a molecular structure that will not melt. (Since it won't melt, the next phase transition that it undergoes is burning - where the hydrocarbons (carbon & hydrogen) in the starch reacts with oxygen to produce carbon dioxide/monoxide and "char" which is essentially just solid carbon.)

Now, the popped popcorn is in your microwave, absorbing energy, but it doesn't have a lot of water anymore. So the energy goes into the starch instead and heats it pretty quickly, burning the starch. Other foods that you can cook for a long time in the microwave probably have very high water content.

As to the oil, to me it seems like a way to more evenly dissipate energy among the kernels. Oil has a higher thermal conductivity than air, which means that it will more easily take high heat in one area in the microwave and transfer it to an area of low heat. So, if one kernel is getting much hotter than another, the oil will be able to take some of the heat from the hot kernel and "give" it to the cooler one. This makes it more likely that all the kernels will pop at the same time; reducing the likelihood that some kernels will pop and burn before some other have even popped. This also helps because microwaves are very directional. You may notice that there are "hot spots" in the microwave, and turn tables exist to try to minimize the influence these have on heating your food.

Let me know if anything doesn't make sense, I don't have detailed knowledge about popcorn but it's a little bit like the plastics I study in grad school!

  • Hmm. So similar to Optionparty in the comments to the question you posit that it's the popped kernels that burn. Seems testable! When I tested oil vs. no-oil popping, the oil popping both popped faster (presumably because the oil heated up more efficiently than the tiny amounts of water in the kernels of popcorn) and burned faster (but approximately in proportion to the difference in time). If you're right that the oil is distributing the heat more evenly - which makes sense to me - then that means that more oil is better, and that a high smokepoint oil may be a helpful solution. – Joe M Jan 26 '15 at 22:47
  • Does this also mean that, if there were more oil in the bag, that the oil would help delay the burning of the starch (because it would 'soak up' the microwaves - I know it's technically different but the term seems to be used that way)? – Joe M Jan 26 '15 at 22:50
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    @JoeM I'm not actually sure if a high smokepoint oil will that much more helpful. Obviously, microwaving butter is probably a bad idea, but the difference between something like olive oil and vegetable oil probably won't matter, since once the kernel has popped there's only a very thin layer on the outside of the foam. And microwaves heat up really quickly, so maybe 10C difference in smoke point might be the equivalent of a few seconds in the microwave. – Alex Bruce Jan 26 '15 at 22:51
  • @JoeM Yeah, I think using more oil would help more, although then you run the chance of the popcorn absorbing a lot of it and being super greasy. It's a trade off to me... – Alex Bruce Jan 26 '15 at 22:52
  • Sadly, I learned the hard way about using butter... though I did expect it to burn heading into it. – Joe M Jan 26 '15 at 22:55
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I think most of your confusion comes from the paradigm of water. Water (under kitchen conditions) will not get any hotter than its boiling point.

Oil has no such limitation. You microwave will heat oil well past water's boiling point and all the way up past the smoke point to the flash point of the oil. At the flash point, the oil will actually catch fire.

If you put a bag of popcorn in a microwave on high for an extended period of time you'll see this as actually burned corn.

Also, if a significant mass resting against the paper bag gets above paper's flash point (451 degrees F) it will also catch fire.

My little sister has proven this true time after time.

  • Where is this oil coming from? I've run the test without any added oil, and it still burns (ie, just kernels and a brown paper bag); further, it typically burns in one location (assuming you stop it within a reasonable time, I imagine 10 minutes later it might not be the case.) – Joe M Jan 26 '15 at 18:59
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    Corn contains oil. You know... corn oil. – Mr. Mascaro Jan 26 '15 at 19:01
  • Also there is no reason a microwave can't heat the carbohydrates and protein in the kernels past the ignition point too. – draksia Jan 26 '15 at 19:25
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    @draksia That's part of why I'm asking the question, though. I'm not a physicist, and I don't know exactly to what extent the microwave actually can heat carbohydrates and protein. Which of these - the oil or the carbs/proteins/whatever - is (first) responsible for the burning? I would think one would happen first consistently (assuming you pull it out when you smell burnt popcorn, so within ~30 seconds of the initial burning). – Joe M Jan 26 '15 at 19:49
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    @AlexBruce, it will never get hotter than its boiling point in that situation either... it's simply that it's boiling point is elevated while under pressure. – Mr. Mascaro Jan 27 '15 at 18:31

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