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It seems that almost everywhere, I see the advice to keep the top of a double boiler away from the water in the bottom--with the explanation that the top will get too hot if it touches or is semi-immersed in the water. But steam is hotter than water, as I understand it. Therefore, I should think that the problem with touching the water would be under-cooking rather than over-cooking as well as uneven cooking if (say) half of the food is subjected to the water temperature but the other half is subjected to the temperature of the steam.

This may not be very important at times, but if you are cooking some things (e.g., with eggs), it seems that it would be worth fully understand what is going on.

Your thoughts? Thanks.

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The temperature is less of an issue. The water is at the point where it turns into steam (if you are working with boiling water, some techniques go for barely simmering or just very warm water), so that the water and the air/vapor mix is pretty much at the same temperature, give or take a few Kelvin.

But there’s a huge difference in specific heat capacity. Water can store a lot of thermal energy, gas can’t. That corresponds to heat conductivity:

When your upper bowl doesn’t touch the water, it can absorb only a relatively small amount of energy from the vapor per time, then it cools down the vapor which has to be heated up again. So the contents of the bowl will warm up slower, giving you time to stir and ensure evenly rising temperature. (Remember that the top of the bowl is touching room temperature air.) Whereas when the upper bowl touches the water, there’s a huge thermal mass that can give you a much larger flow of energy to the bowl, which can overheat the food or create local hot spots (at the bottom), simply because it gets heated too fast, so that e.g. egg dishes get fully cooked or curdle.

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You seem to be mixing up a double boiler and a pressure cooker. The articles you read are correct - the temperature in the double boiler is lower when the inner vessel does not touch the water.

First, in a double boiler, the steam-air-mixture will not get above 100 C. Instead, the steam will expand. This is why you see a cloud of vapor rising out of the double boiler. And new, cold air comes in, so you don't even have a 100 C steam, but a below-100 mixture of air and steam.

Second, the food does not touch the steam. It touches the inner vessel. On the outside, the steam touches the inner vessel. But it is not simple conduction of heat that happens on the boundary; the steam also condenses on the surface of the inner vessel, which uses up a lot of energy, and reduces the temperature of the inner vessel. (This is also why steaming is a slower cooking method than boiling).

Third, I am not entirely sure what the temperature equilibrium of a double boiler would be - whether it is very close to 100 C, or noticeably below it - but you usually don't cook in a double boiler at equilibrium. It heats up slowly (see the above two points on heat transfer) and that time allows you to heat your food very gradually. If you were to hang the inner vessel directly into the water, it would soon come up to 100 C (especially the thin steel vessels that are frequently used for double boilers) and then the bottom of your food would be prone to overcooking. In such a case, you can just about save eggs, if you stir or whisk them with sufficient intensity, but any chocolate will distemper badly.

So, in summary, if you want your food to get heated slowly, don't let your double boiler's inner vessel touch the water.

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  • No I know nothing about pressure cookers. But thank you both, Stephie and Rumtscho, for your detailed answers. It makes sense as I read both, but I'd never have gotten there on my own. (Obviously.) Cheers! Apr 11, 2022 at 1:31
  • You say “And new, cold air comes in, so you don't even have a 100 C steam, but a below-100 mixture of air and steam.” but I’m not totally convinced on that point - if the expanding steam is forcing its way out of the vessel, how/why would cold air make its way in? Jan 19 at 20:32
  • @fyrepenguin You're mixing two gases (steam and air) which make contact over a large area. What you get in such a case are usually weirdly-shaped currents flowing in all kind of directions. Having a gas that only expands without letting other gases in - that's an explosion. At least that's my understanding of physics, which is admittedly not my area. If you have sources that explain it better, I'm happy to read them!
    – rumtscho
    Jan 21 at 13:09
  • @rumtscho the key in this case is that you’re constantly producing more steam from the water in the boiler. Steam is about 1600x the volume of the same quantity of water, so it’s not the same steam that’s constantly expanding, but new steam being generated and forcing its way out of the container. At least, that’s what I would intuitively expect. I wouldn’t mind taking this to chat if you like, but also willing to drop it. Jan 21 at 22:26
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What people are missing here is the latent heat of vaporization. Just because steam is at 100°C (and it can easily be hotter that than), most of the transferred energy does not come from the temperature of the steam, it comes from the latent heat of condensation, which is 540cal/g. The contents of the double boiler can easily go above 100°C, but for most practical situations it does not because of other factors in the system, including convective and evaporative cooling and emissivity from the food and the vessel.

In the end, the system works very well if the top pot (or bowl) is touching only the steam (not the water), and especially if you are using a gas range if you turn off the gas after the water comes to a boil and before putting the top pot or bowl on since the heat from the gas can directly heat the upper vessel and cause it to be much hotter than you want, burning the ingredients in it (whether chocolate or egg or whatever).

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