Steam and Steamers

I have a very simple question that came up after the recent purchase of a bamboo steamer (made of bamboo, not for steaming bamboo itself!). My brother seems to think that food should cook quicker because steam is hotter, I maintain that the boiling water is the hottest part and that the steam is cooling air vapour. Now my brothr is by no means stupid (although he did believe that Welsh sheep have longer legs on one side of their bodies to cope with hills!) and I have a tendency to doubt my gut instincts so I am asking you, The Dope, to settle it. If I happen to b right can you throw in some science so I can make myself aoind more learned. Thanks.

TRUE steam is hotter than the boiling water. Look into a pressure cooker for that. What you are doing is cooking with hot water vapor. Put a thermometer in the “steam” to find the real temp.

Bollocks! Damnit, at least I asked you guys before arguing with him. If it is hotter how come it takes longer to cook in a steamer?

My guess is becase the food/steam contact area is smaller then food/water contact area…?

Steam is indeed hotter than boiling water. Boiling water exists at 100oC approximately. Above that temperature it vaporises and becomes steam. Once steam falls below 100oC it starts to condense back to liquid water so steam exists at higher temperatures than boiling water.

I think the problem comes because there is a common English usage of ‘steam’ that refers to the white clouds of visible water droplets that form over boiling water. That stuff isn’t true steam, it’s a fine mist of liquid water droplets. Steam itself is an invisible gas.

However steaming won’t cook food faster than boiling because there is very little steam produced relative to the amount of water and so it has limited capacity to transfer heat to the food.

If you are using a pot with a tight fitting glass lid, the steam condenses on the lid and other surfaces inside the pot. This leads me to believe that the temperature inside the pot could well be above 212F. Due possibly to the increased pressure caused by the weight of the lid?

This discussion won’t be complete without bringing in (drumroll, please) The Latent Heat of Vaporization.

“Ok, ok,” you say, “what the hell is that?”

The Latent Heat of Vaporization is basically this: The energy required to raise a substance from liquid to gas.

So, to raise the temperature of water (or anything, for that matter (my god! the pun!), but let’s deal with water), you have to put energy into it. Input X calories and you raise the water’s temperature Y degrees. UNTIL, the water gets to it’s boiling point, 100 degrees C. At that point, you have to overcome the molecular attraction of the water molecules to make it any hotter and therfore change from liquid to gas phase. Not only do you have to put in X calories to go from100 to 101 degrees, you also have to put in some additional energy. What this means is that the steam actually has MORE energy in it than the boiling water, and is therefore hotter.

Here’s a great page full of large type, pretty pictures, and a lot more scientific-speak than I’m capable of.

And re: pressure cookers and keeping the lid on. Raising the pressure raises the boiling point of water, and there fore it gets even hotter before boiling. I don’t know if it affects the LHoV.

Your brother is wrong. There are many things to consider here.

It is true that the temperature of the water is fixed at about 100 deg C, however hard you heat it. Put in more heat energy and you just produce steam faster. Steam itself can in theory be as hot as you like.

However, you have to bear in mind that the steam is coming out of the boiling water which is at 100 deg C. The phase-change from water to steam occurs at a constant temperature, so the steam should in fact be at the same temperature of the water, not hotter.

There is a caveat to this - bubble nucleation. Ther pressure in a bubble is higher than atmospheric due to its interface tension, so the steam in a bubble generated in the boiling water will be hotter than 100 deg C. When the bubble bursts at the surface, it releases a gout of this extra-hot steam at the surface. The steam directly above the surface of the water might be a tad hotter than 100 deg C because of this. You may be able to measure the difference with a thermometer held close, provided that the thermometer stays dry.

Hold a thermometer a little further away from the water though and it will measure 100 deg C at most. Why? Because it very quickly acquires a film of condensed surface water. Like the water in the pan, you cannot heat this water hotter than 100 deg C - it will just boil back to steam. The same goes for the stuff in your broiler - the very fact that the steam is condensing onto it demonstrates that it is cooler than 100 degrees. If the stuff were hotter, the water on it would be boiling, or it would have a dry surface.

Vegetables immersed in boiling water are going to reach the same temperature as it fairly quickly. Vegetables in a broiler are being heated by steam contact and by latent heat released as the steam condenses onto them, but they are also being cooled by air contact and convection (and also radiation to a trivial degree). my guess is that the stuff in the broiler will overall be cooler.

Steamer, not broiler. If anyone finds any marbles, they are probably mine.

One point I left out is that the air inside the pot will be pretty clear, that is, free of visible water vapor. The steam won’t condense until it contacts the relatively cool surfaces of the pot and the lid.
BTW; Boiling temp. varies a lot with pressure on the water. Thay’s why foods take longer to cook in boiling water at high elevation.

It seems to me that it will only read 100C because there is still water to be converted to steam. Once all water has been boiled away, then the temperature can rise.

It doesn’t need to be cooler than 100C. Water will stay at 100C until it gets enough energy to turn to steam. If the steam condenses, it is giving up this energy, not necessarily becoming cooler than 100C. Indeed, if the water is boiling, and the thermometer is already reading 100C, I cannot think of a way that there would be any significant portion of water that is not 100C.

I’m fairly sure you understand this, but I’m just clarifying. There is a lot of energy to transfer between “heating steam” and “boiling water.” The entire container can stay at 100C pretty much anywhere you measure inside, with steam becoming water, and water becoming steam. Apart from isolated pockets having a slightly higher temperature, as mentioned. The steam might absorb some energy in order to raise its temperature, but it would quickly transfer this energy to the water to make more steam, evening everything out.

Please correct me if I’m wrong.

Not quite. The rate at which thermal ennergy is transferred depends primarily on four factors: surface area, temperature difference, specific heat of the item being heated, and the specific heat of the surrounding medium. In our cooking example, all the factors are (approximately) equal, except the specific heat of the surrounding material, in this case, liquid water vs. water vapor. This is why you can reach into a 400-degree oven without your hand bursting into flames, while plunging that same hand into a 400-degree deep fryer is going to cause nearly immediate third-degree burns.

What about moving hot air, Q.E.D, such as in a convection oven or a hair dryer?

Moving air serves to speed up things somewhat. In a conventional oven, the air doesn’t move (much), and as the hot air around the food heats the food up, the cooler food tries to cool the air in contact with it. In a convection oven, moving the air around keeps the air around the food at more uniform, and higher, temperature.

The specific heat of water is 1 calorie/gram °C
The specific heat of steam is 0.5 calorie/gram °C (under "reasonable kitchen conditions)

Also important here is the density of the liquid vs the vapor.
A volume of water contains roughly 1250 times as many molecules of H[sub]2[/sub] as does the same volume of steam.

That should read H[sub]2[/sub]O rather than H[sub]2[/sub].

Don’t wanna be playing with H[sub]2[/sub] in the ol’ kitchen, do we Squink. :wink:

Lets start with the pan of barely boiling (Simmering) water at 100-C with surroundings at atmospheric pressure.

The water at the bottom of the pan is very slightly above 100-C for nucleation or incipient boiling to occur. As a tiny bubble of steam rises to the surface it gives up some of its heat to raise the temperature of the surrounding water.

When bubles reach the surface and burst they release steam vapor at 100-C and atmospheric pressure. Since this is a continuing process more bubbles surface and burst producing more steam vapor, and pushing the previously released vapor upward through the bamboo steamer. The vapor continues to give up its heat to the surroundings, the steamer and its contents and emerges from the top at several degrees less that 100-C.

NOTE: It is impossible for steam vapor coming off an open vessel of boiling water to be hotter than that boiling water

The only way that steam can exist at a temperature higher that 100-C is for it to be confined so that the pressure is higher than atmospheric. Consult “Steam Tables” for ‘saturated steam’ where you will find the steam temperature at satuaration or boiling related to pressure.

And atmospheric pressure equals 14.7 psi at sea level. Right?

Approximately, it varies with the barometeric pressure.

14.7 psig (g is for gage) is normally use for generic discussion purposes, and 212 F or 100-Cfor nominal b.p. of water.

If you want to be precise consult the “Steam Tables,” use a barometer, and a precision thermometer.