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I need to put a plastic bag in a pot of boiling water. The bag is rated to handle up to 200C (400F). I will be using a stainless steel (or aluminum) pot on a gas stove.

I am worried that the temperature of the metal touching the plastic bag will go higher or near the limit of what the plastic can handle, and it may affect the plastic bag integrity or the safety of the food.

I suppose, water inside the pot should be bringing down the temperature to near boiling temperature.

I do not have any temperature measuring equipment.

Anyone before has measured the temperature of the metal inside the pot, preferably near the middle (at the hottest possible point) ?

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At a full boil, the water and steam will all be 100C, even close to the bottom, so you'll be totally fine if the bag's just in the water. (With a big bag obstructing the flow, it may be a bit less than 100C at the surface, but I don't think that's a concern for you.)

If a significant area of the bag touches the bottom, all bets are pretty much off. At that point, there's no water there to hold the temperature down. Food burns when it sticks to the bottom of pots, even with boiling liquid around elsewhere, so I wouldn't be surprised if it can reach 400F.

So I'd do like Wayfaring Stranger suggested in comments: just keep the bag off the bottom. You basically want a canning rack, but all that really matters is that it's something that's mostly open space, so water can flow around (i.e. not a plate) but solid enough to hold things up.

Some makeshift canning rack substitutes would work too:

  • a bunch of jar rings (for canning racks, people usually tie them together, but for a one-off you might be able to do without)
  • something made out of crumpled/rolled up aluminum foil
  • any kind of oven rack that fits in your pot
  • a bunch of rocks or anything else heat-safe that sinks in water
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    I think there's an interesting unanswered nuance to the question: how hot is the water at the bottom of a boiling pot of water? Does the hottest water rise to the top before it boils off? Is the water at the bottom of the pan >100C but because it is trapped, it stays put? – Caleb Feb 10 '17 at 2:38
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    @Caleb If it's at a full rolling boil, it's basically all 100C, even at the bottom, and if it's a light enough boil, some of it will be a bit under 100C. If the pot were under pressure it could go higher, but since it's not, if it would rise above 100C it instead turns to steam, and immediately rises, without a chance to heat further. I've stuck a thermometer in enough boiling pots of water to be reasonably sure about all this. But since the temperatures are so much higher when there isn't water there, it doesn't really matter for the OP's purposes how hot the water is. – Cascabel Feb 10 '17 at 3:35
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    @Caleb - The point of concern is the surface of the pot not the water. A water drop skitters around on a frying pan because of the Leidenfrost effect. Basically the water evaporates so fast that the drop of liquid water is supported by a cushion of steam. en.wikipedia.org/wiki/Leidenfrost_effect – MaxW Feb 11 '17 at 4:48

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