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I am an active cook and make noodles all the time. I had this marvel just two times in my life. You have a pot of chicken broth (there's nothing in it, no salt even), you heat it on a regular electric stove with the lid on top, and then suddenly, in a blink of an eye, it literally explodes boiling. In an instant, one second tops. The moment you recover from shock, you realize 20-30% of your broth is on the floor. I swear I don't put salt or anything. It happens very rarely. But I'm still eager to know what may cause it and what I can do to avoid it. Did it happen to you?

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  • Was there anything other than the broth in the pot? If there’s no nucleation sites it could be related to superheating, but I’d more expect that in a microwave with a smooth glass vessel. So, was it a particularly smooth pot with nothing other than liquid/broth? Mar 29 at 16:59

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As commented by @fyrepenguin this sounds very much like a boiling liquid expanding vapour explosion (BLEVE) or a superheating nucleation event.

In these processes the key is that the liquid has heated above its regular atmospheric pressure boiling point without visible signs of boiling. This can happen in a few manners:

  1. In the case of superheating and nucleation, you have a very smooth vessel, typically glass or ceramic that does not contain any points of nucleation. The lack of nucleation points means that no bubbles are produced and the introduction of a nucleation point causes rapid boiling, often explosively. The nucleation event can be as simple as a bump of the vessel, introduction of nucleation point(s) from dust or a powder, or even a drop of liquid falling from a lid. There are lots of videos of this available on Youtube as it is quite easy to induce in a microwave, but I've never seen it on a stove-top.

  2. In the other scenario, you would need a vessel that seals tightly with the lid. If this happens, the pressure inside the vessel can be quite a bit above the surrounding atmospheric pressure. This results in an elevated boiling point - see graph here. This principle is how pressure cookers work - they raise the boiling point to cook food more rapidly by the higher temperatures reached. Sudden loss of containment of the pressure will result in rapid explosive boiling. The loss of containment could be due to the pressure in the pot overcoming the effects of gravity and friction on a lid causing it to rapidly open.

  3. Inspired by @L.Dutch's answer, and I think this is the most likely mechanism. Broth can have gelatin layers in it, and can even have the whole broth be a semi-solid mass if you have enough bones and connective tissue in the broth. In this case, the bottom of the solid gelled layer heats up and melts, but the heat transfer to the rest of the gelatin is slow, so the liquid reaches boiling below the gelatin, causing a pressurized zone below the solid layer as it adheres to the sides of the pot. Once enough pressure builds up, there is sudden release of the mass above, which blows up quickly, forcing anything above it out of the pot. This is essentially a BLEVE.

    I've seen this in action in labs when reheating solid agar for microbiology plates in a microwave. The agar melts at about 85 C, but is slow to transfer heat through, so you get boiling below the solid mass, which bubbles up, forcing any solids and liquid above the boiling layer up and out - makes a hell of a mess inside a microwave.

The first is easily preventable - put a small scratch in the bottom of your (presumably metal) pans or put something in the pan that will cause nucleation - a spoon would work, or a baking bean/pie weight.

The second is more tricky. For it to happen you would need a very tightly fitting lid. Many lids, especially glass ones, have a small hole in them which prevents pressure build up. You could simply leave the lid very slightly ajar/tilted. You should also check that the lids can't bind if you put them on slightly off centre or angled - a loose lid will prevent the rapid boiling as you won't get pressure build up.

The third should also be preventable. Mix the broth before storing. In addition, stir the broth before heating to break up any solid layer that might be present. The layer should be easily penetrable with a spoon.

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  • Put another way, if you can remove the lid from the pot, it’s probably not going to cause a BLEVE, since gas can force its way out easily. I wonder if you had a cold ill-fitted lid that you placed on a pot that just barely fit, then it expanded to seal it shut, you might be able to build up pressure. But the only likely scenario for those kinda of pressure would be a pressure cooker as you mention, where pressure buildup is the whole goal. Mar 29 at 20:58
  • Thank you! I have a small hole in my lid. Could the hole "not work" somehow? Mar 30 at 4:10
  • @SergeyZolotarev Very unlikely. There will be some pressure differential which causes the steam to come out in a jet from the hole, but this should be enough to prevent a BLEVE. Inspired by L.Dutch there is a 3rd mechanism, which I have seen in the lab with agar for making plates. I'll edit that in.
    – bob1
    Mar 30 at 8:03
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    Not all wooden spoons work as nucleation sites. I only tried that technique twice: The second time, I picked the wrong wooden spoon and got to experience the fright of boiling vapor being shot frighteningly close to my eyes. Maybe just buy a pot minder
    – Brian
    Apr 4 at 20:02
  • @Brian Interesting. I'm very surprised that a wooden spoon was smooth enough to not cause nucleation.
    – bob1
    Apr 4 at 20:12
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In a chicken broth you will have water and fats, produced by the chicken meat.

This mix will stratify, with fats, being less dense, going on the surface, while water will stay lower.

Now, when else you can have water under oil at high temperature? Exactly, when water falls into frying oil, and what happens is the same: water becomes vapor, forms a bubble which then burst under the oil.

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  • Thank you, but it doesn't explain why it happens so infrequently Mar 30 at 7:36
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    You've reminded me of the 3rd mechanism - you are almost right here, and I think this is the most likely mechanism. The gelatin from the broth forms a dense solid layer at the bottom of the pot. A small amount of liquid below it heats up to boiling and the gas result blows upwards, forcing the liquid above out of the pot. It's spectacular to see. I've only seen it in the lab with agar for microbiology media in a microwave, but the mechanism applies to a stove top too.
    – bob1
    Mar 30 at 8:06
  • @bob1 I am not sure that gelatin would exist in a hot broth.
    – L.Dutch
    Mar 30 at 8:38
  • @L.Dutch The 3rd would happen only during initial heating and the broth above would be cool or cold. I've had layers that were a couple of cm (~1 inch) thick in the past, which might have been enough to do a BLEVE, but I tend to transfer the broth to another pot for making soup, so never seen it happen in my kitchen.
    – bob1
    Mar 30 at 8:41

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