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Picture this.

  1. I make 1/4 cup of popcorn kernels in the microwave. No paper bag - just a glass bowl with a glass lid, where the lid has holes in it to allow steam to escape. After about 5 minutes, about 3/4 of the kernels have popped. If I go longer, the popped kernels start to burn, and even some of the unpopped kernels burn too. So I stop at 5 minutes, with the bowl about 80% full of popped popcorn.

  2. I then empty the glass bowl of all the popcorn into another bowl, for eating. But before eating, I make another 1/4 cup of popcorn kernels in the microwave again. This time, at about 4 1/2 minutes, there are so many popped kernels that it lifts the glass lid off the top of the bowl, perfectly cooked. I empty it into my eating bowl...

I didn't use oil or anything, but I got perfect air-popped popcorn from just a microwave. Only thing is, it took two tries. Also, this happens every time. Part of the reason I make a second bowl at all is because the popcorns are just so much better than the first bowl's.

How can I get the first batch to cook just as well as the second one? I have tried "pre-heating" just the bowl in the microwave for about 30 seconds. It does help. But apparently this is actually very bad and dangerous for the components of the microwave.

I don't know what's happening. But if I had to guess, there are tiny water molecules in the microwave that are messing things up. Or, something about the bowl being warm helps the kernels pop.

Do you have a suggestion on how to make a perfect bowl of air-popped popcorn in one go?

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  • Is this a common way of making popcorn? Apr 17, 2023 at 1:53
  • 5
    Perhaps an increase in water contents in the air inside the microwave? Dry corn is, well, fairly dry, but not entirely (or it wouldn't pop). Add a drop of water to the first bowl and see whether the moist air aids the popping (for example because the air itself heats up as well). Apr 17, 2023 at 7:25
  • Does the second batch include the un-popped kernels from the first batch, or are they completely new kernels?
    – TripeHound
    Apr 17, 2023 at 9:03
  • 3
    When I do this (frequently) the bowl gets very hot by the end of the (first) batch. (I once shattered a Pyrex bowl used for this with a few drops of water, it was so hot.) I would be shocked if the second batch did not come out much better, starting in a hot bowl. These days, I use a (Nordic Ware) plastic/silicon bowl designed for microwaving popcorn. I believe everyone has discounted the likelihood that each kernel pops because it's releasing a bit of steam. and all that (aggregate) steam is probably what heats the bowl.
    – Jeffiekins
    Apr 18, 2023 at 21:54
  • 2
    In case it wasn't obvious: We are all excited to hear back from the experiments! I know you are busy in the kitchen right now but the best lab is a waste without good reports ;-). Apr 19, 2023 at 6:36

4 Answers 4

21

You might need to experiment a bit, but my guess is your bowl [or lid] isn't perfectly transparent to microwaves & is itself heating. In effect that's 'wasting' the microwaves. Second time the bowl is hotter, giving the popcorn a better chance.

You could test with just half an inch of water in the bottom of the bowl. If the rim gets hotter than the water, or even hot at all, reject the bowl for microwave use.
Personally, I won't use any bowl proven to absorb microwaves itself. It makes the process unpredictable… apart from the risk of trying to pick a bowl out that you expect to be cold at the top, to find it's skin-flayingly hot.

Try something plastic & heat-resistant instead, or just a paper bag, like supermarket microwave popcorn.

Comments below seem to be intent on inventing more & more weird & wacky ways to try avoid this simple two-minute elimination test, or alternatively propose ridiculous science-free theories.
I'm determined not to respond to any more of these ;)

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  • 2
    It only takes about 2 minutes...
    – Tetsujin
    Apr 16, 2023 at 17:43
  • 3
    A better test is an empty bowl next to a container with a little water - the bowl shouldn't get warm at all. And if it's glass, it won't - some ceramics absorb a bit but glass really doesn't
    – Chris H
    Apr 17, 2023 at 8:37
  • 4
    Note OP wrote: "tried "pre-heating" just the bowl in the microwave for about 30 seconds. It does help." - so maybe the bowl is transparent enough, and only by being filled with hot popping corn for a few minutes is heated up enough to make a difference.
    – Martin
    Apr 17, 2023 at 8:45
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    OP doesn't state whether or not this did actually warm the bowl. If it did, then that's exactly what we're trying to establish. Sack the bowl & use something else.
    – Tetsujin
    Apr 17, 2023 at 8:49
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    I would argue that the bowl stealing heat from the popcorn is a more likely cause than the bowl stealing microwaves. If it were the latter, the bowl would be blocking microwaves during the second batch just as much as the first, and there would be a minor if any difference between the two.
    – Abion47
    Apr 19, 2023 at 17:41
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I am as surprised as you are, and as curious, frankly. By proceeding systematically we should be able to find an answer.

Something that is involved in the process has been changed by preparing the first batch. This is one of the following things:

  • The microwave (electronics, temperature);
  • the air in the microwave (moisture, temperature);
  • or the bowl (temperature).

The easiest to exclude is the bowl: I assume that the bowl is at least a bit warmer to the touch when you take it out. Just use a second, identical bowl for the second batch. If the difference persists, it was not the bowl.

The oven vs. the air are harder to discern. You could try to vent the oven thoroughly but quickly, e.g. by pointing a blow drier on high fan but "non-heat" at the opening for a few seconds. If that has an effect, it is the air.

If not, it must be the oven. The electronics surely warm up and may run better (hit the frequency better, for example). Yes, I just learned from your post that you shouldn't run the microwave empty. Therefore, simply run it with a dummy. Ideally that dummy would not change the moisture contents of the air in the microwave. For example, you could could use the usual batch of popcorn, but inside a large, sealed Ziploc or other plastic bag which is deflated as much as possible when you put it in to allow for air expansion. If you then vent the microwave as described above, and the difference persists, it is likely that the "warming up" of the microwave itself is the reason. In that case you can always simply microwave a glass of water first as the "throwaway batch".

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  • I would guess the warmer bowl is the key - it warms by conducting heat from the kernels of the first batch, which we don't want as rapid heating is why they pop. By the 2nd batch it's taking less heat. To test, prewarm it with hot water, than dry it before the first batch.
    – Chris H
    Apr 17, 2023 at 8:39
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    4. The actual corn... 4a. The corn for the first bowl was probably sitting on top in storage. 4b. The corn for the second bowl may be sitting out, exposed, for a few minutes while the first bowl microwaves. These seem like small variables, but we're also talking about a small amount of stuff. I propose a test as such: take out enough popcorn for two bowls, mix it all together, and leave it sitting spread out on something like a cookie sheet for an hour or two so that 5 minutes extra sitting out shouldn't make a difference. Apr 17, 2023 at 13:13
  • 1
    Please don't microwave Ziploc bags. They melt.
    – MJ713
    Apr 17, 2023 at 21:26
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    @MJ713 Wait -- you have never heard of Ziploc® brand Zip’n Steam® Microwave Cooking Bags!? ;-) More seriously: If you avoid contact with overly hot items (and the glass bowl should prevent that in our use case) they are just fine. If in doubt, throw out the test kernels. Apr 17, 2023 at 21:34
  • 2
    Modern consumer magnetrons should drop in power output as they get hotter. As the large permanent magnet gets warm and approaches its Curie temperature, it experiences reversible demagnetization and its field strength drops.
    – user71659
    Apr 17, 2023 at 22:30
2

That's an interesting effect.

I have a similar bowl I use, but its a special-made "microwave popcorn" bowl, almost identical to this one (if not this exact brand). Its ceramic, with a plastic lid that is designed to (mostly) stay on but not air-seal. Costs in the neighborhood of $15(US). I've been using these to make microwave popcorn for around 30 years. I haven't noticed your exact effect, so either my special-made bowl avoids it, or its so rare I make that much that I never noticed before.

enter image description here

So my first suggestion would be to try it with a bowl that's designed for this purpose. I'm not a chemist, but ceramic is way different than glass, and that's likely to make a difference. The glass bowl should be dissipating much more heat into the air inside the microwave, so a lot of the heating done for your first batch may be wasted heating the air outside the bowl.

I have learned a few other things over the years that you might find helpful. #1 Is particularly applicable to minimizing unpopped kernels without burning, which is what I think you are asking for.

  1. Don't do it on high power, and don't overfill the bowl with kernels. There's always a bit of a narrow edge in making popcorn between getting as much of it to pop as you can and burning the popcorn. The higher the power you use, the tighter that edge is. I've found I like to use 7 on my microwave's 1-10 scale. There's a pattern of popping where it starts slow, gets intense for a while, then slows again. You want to stop it after the intense bit when you start to hear about 1 second between pops. If you start to smell a burned smell, hit that stop or open button like its the buzzer on Jeapordy. Otherwise you'll find you're now doing #3 below instead.

  2. My kids don't have the patience for this (and don't like dancing on the edge) so what they do is put in like half a cup of kernels (1/3 is I beleive the reccomended capacity), run it on high power, and stop it while its still popping. That wastes a ton of popcorn kernels. The remainder can be repopped, but it isn't as good. This offends the engineer in me, but it is one solution I feel compelled to bring up. If I'm understanding you correctly though, this is the exact thing you are (sensibly) trying to avoid doing.

  3. Let it burn! As long as I'm listing inferior options, I'll list the opposite of the kids' approach: you can just let it burn, then use a big metal spoon to dig the smoking ball of carbon (a diamond in the making) out of the middle of the bowl safely and deposit it in the sink (or better yet on the concrete outside). It stinks up the house, and of course you have to work around any burnt pieces you don't dig out, but the remaining popcorn is just fine, and you'll find there are few if any unpopped kernels. (This isn't something one typically does on purpose, but if you get distracted and don't stop it soon enough, this is what you'll find yourself doing)

Stuff that doesn't impact how many of the kernels get popped before burning, but is important for other reasons:

  1. Put your toppings in the bowl ahead of time. If you don't, the popcorn comes out as dry as the Sahara. For butter, I like to pre-melt some (about 1tbs) in the bowl and dissolve some extra salt in before putting the popcorn in. (Don't use margarine with calcium added for this! The calcium likes to separate out and create hard deposits on the bottom of the bowl, and that makes the popcorn taste weird too.) Trying to pour hot butter over popped corn after its popped will shrivel up the kernels it gets poured directly on, and its very tough to distribute evenly. You can (and probably will) still salt to taste after popping though.

  2. If you want to go low fat, you can dissolve your salt in the bottom of the bowl in a puddle of water before popping. Trying to put salt on dry unbuttered microwave popcorn after popping is an exercise in frustration. The salt just rolls right off of it.


Experiment

Since I have the equipment, I went and tried this out experimentally last night. I ran two batches right after each other using my usual method, which is:

  1. 1tbs of melted (via 15s in the microwave in the bowl) butter, mixed with salt.
  2. 1/3 cup of unpopped kernels
  3. Run in microwave at power level 7 (out of 10). Timer is set at 3:50, but that's a failsafe.
  4. Stop the microwave when popping slows down to about 1 pop a second.

Results

The bowl was indeed much warmer for the second batch. In fact, the butter was bubbling when I put it in the bowl, so I probably didn't even need to run that 15s melting cycle (but did so anyway).

The first batch completed in 3:40 (280 seconds). The second batch completed in 3:05 (245 seconds). That's about 12% quicker. Both batches finished with about 70 unpopped kernels.

There was no noticeable difference in taste between the popped kernels of the two batches.

Conclusions

  • If I had instead held the time constant, I think its fair to say that all or nearly all of the kernels in the second batch would have popped. I would imagine it would have also burnt some of the popped corn, but this may be worth testing.
  • Since both the air inside the microwave and the bowl were considerably warmer when the second batch was started, this doesn't really show which is responsible for the time difference. Buying a second bowl would allow us to isolate out that variable (or a second microwave, but the former option is considerably cheaper, and I do kind of need a new bowl).
  • Warming the bowl up (eg: in the oven) before use would be another way to test this. It may also end up being a good way to achieve the quicker pop time, but if my current theory that this only achieves a 12% reduction in pop time and no other effect is correct, it doesn't seem likely it would be worth the trouble of doing routinely.

More study seems called for. I'll be applying for grants for this important work shortly.


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  • Interesting experiment.... I have to ask if the lid was still wet when you put it back on for the second batch? I also think that running the microwave at 7/10 would extremely helpful in spreading out the heat more evenly. I have been running mine on "HIGH" only which may be why I got different results. Both the water i added and 7/10 in yours would distribute heat more evenly Apr 21, 2023 at 19:58
  • @nickcarraway - "Wet"? No. but when you pop with butter, it lightly coats all interior surfaces of the popper, so yeah it probably had some of that.
    – T.E.D.
    Apr 21, 2023 at 20:00
  • @nickcarraway - Can't stress strongly enough how important I've found it is to knock the power down to prevent burning. That's why I put that bit first.
    – T.E.D.
    Apr 21, 2023 at 20:32
2

I am the original question-asker reporting back...

I was able to make perfect popcorn by adding about 1 teaspoon of water to the bottom of the bowl. But please don't try this unless you have a glass lid with holes in it (see below).

There were concerns about the bowl, the microwave, and the lid. Ultimately, I believe it was the humidity under the lid from the previous batch that was creating the perfect environment for the kernels in the second try. I never mentioned that the lid with holes in it frequently had some condensation on it before I put it back on for a second batch, because I didn't think it would be relevant.

To test the humidity theory, I added about 2 teaspoons of water to the bottom of the bowl, then mixed up the popcorn kernels in it. After about 9-10 minutes (about twice as long as before), I had a bowl of perfectly cooked popcorn. It was incredible that just about every single kernel had the exact same color (brownish-orange) inside the center of the popcorn, especially after that length of time. The bowl was 101% filled with popped kernels, although I stopped it before it could lift the lid off. It's possible that by using less water, you could achieve the same result in less time.

Chemically, this was a bit like buttering popcorn to help it cook more evenly, except with water and a microwave instead of oil and a stove. It's a very unusual way to think of using water because in any other context, water has a very high specific heat - it is objectively a bad heat conductor. Never would you think of stationary water as a "heat conductor", except inside a microwave, as microwaves work mainly by heating the water molecules in food.

The reason why I think the lid must have holes in it for this to be effective is because you have to give a way for steam to escape. The kernels won't pop (and release steam) if there is already a saturation of steam inside the bowl. Also, without holes in the lid, the steam inside the bowl will become so incredibly hot, that you'll likely just wind up superheating the water or glass or something, potentially melting and breaking the glass especially if it's the more common soda-lime glass. The final popcorn will probably be wet, too, and it will take a lot longer to pop all the kernels. This popcorn was surprisingly dry!

I could not determine anything from microwaving the bowl with water in it. The bowl says Pyrex on it. The lid is borosilicate glass. The bowl has some paintings on it. It's still possible that this was a factor.

So in conclusion, I added a really small spoonful of water to my popcorn kernels, mixed it up, cooked it twice as long in glass with holes in it, and got perfectly popped popcorn, dry and evenly cooked.

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  • 1
    actually, if you don't have holes in the lid, the pressure inside the bowl will make some holes appear between the lid and the bowl. This is probably not desirable, but you can also just put the lid on a bit "off" and let steam escape that way.
    – Esther
    Apr 26, 2023 at 19:09
  • Believe it or not, the reason why I got the lid with holes, is because I once used such a heavy lid with a lip on it that the steam couldn't do that effectively enough. I watched the center of the lid started glow red and eventually cracked one time making popcorn this way. The lid was pretty heavy and old Apr 27, 2023 at 22:24

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