Steam

[harry]

For those who want information as opposed to idle conjecture.

[John Rumm]

Hmm, no mention of pressure cookers...

Still wiggling?

[polygonum]

If you had a sealed vessel (i.e. a pressure cooker which did not have any safety features) and brought up the temperature in exactly the same way as in a real pressure cooker, what would be the effect on the cooking? That is, you have the original air content, and you ignore the actual pressure, simply have a strong enough vessel to take whatever it gets to.

(Yes - I know that getting the food out might be a bit more difficult...)

[harry]

Dalton's law of partial pressures covers this:-

In practice this means that air in the steam results in a lower temperature (at any particular pressure) than would be the case if there was only steam. There are other problems too relating to the insulating effects of air interfering withe the steam heating.

The PVT relationship in steam are non-linear as steam is not a gas, it's a vapour. So for any particular pressure/temperature of steam you need steam tables.

[harry]

The principles are the same.

[polygonum]

As I expressly said that it would be run at the same temperature as a real pressure cooker, it might indeed result in a different pressure profile.

As the heat is being applied at the base, and the food will be at the bottom of the pan, what insulating effects?

What I want to know was what effect it would have on the cooking.

[harry]

It is the pressure that is controlled in a pressure cooker, not temperature. The assumption is made that all or most of theair has been eliminated. If it hasn't then the cooking process will take longer than expected. It's necessary to control pressure because the vessel/cooker has pressure limitations.

Air "bubbles" can lodge on the food so insulating it from the steam. Air and water are the enemies of the heat transfer process in any form of steam heating.

The other effect is that if you try to cook with a mixture of steam and air, it will be at a lower temperature at a given pressure ergo cooking will take longer.

This is vital in the tinned food industry and pharmacology, all tinned food is cooked in steam autoclaves and many pharmacological products are sterilised similarly . Not achieving correct temperature could kill someone using the products.

Less critical for other uses, just takes longer. It possible to heat water to boiling point in seconds, even virtually instantaneously in some cases using steam. Steam is far faster and more efficient to use than gas or electricity for heating liquids and gases/air. A steam powered kitchen is incredibly quick preparing meat and vegs. It's just generating steam is the problem.

[Chris Hogg]

Presumably as cooking progresses in a pressure cooker, the temperature will rise slightly as the air/steam mix escapes through the pressure relief valve and the partial pressure of the steam in the cooker increases.

Compared to industrial processes, the pressure cooker is a fairly crude but simple method of producing elevated cooking temperatures for domestic use. See

and in particular, with reference to your comments on sterilisation

[polygonum]

My question was *** IF THE TEMPERATURE PROFILE WERE THE SAME ... ***.

I am well aware that unless considerable care and attention were put into performing the exercise, it would not be the case.

[harry]

The cooking would be slowed down as the heat transfer rate would be reduced. The heating effect of steam is mostly by transfer of the LATENT heat.(Or "heat of vapourisation) This is the "wet" or "saturated" steam effect (as opposed to superheated steam).

The steam is at the same temperature as the water it came from, the only difference being the latent heat. So when wet steam encounters a cooler object, condensation immediately forms on the surface of that object (which must be removed as soon as possible). The cooler object is thus warmed. But if it was air and steam mixed, less latent heat would be available so the heating effect would be reduced.

[John Rumm]

Also worth noting that a domestic pressure cooker is not necessarily used as a steam cooker - you use quite a high water content, and may well have the thing being cooker fully immersed - so the steam element serves only to allow for an elevated boiling temperature.

(some of the taller ones have baskets that stand off from the base, so you can prepare something like stew immersed in liquid in the bottom of the pan, and then steam veg etc in the basket)

[harry]

The steam and water are the same temperature. There is more energy in the steam. So heating with the steam will always be quicker than heating with water the same temperature. So there is no point in covering the food with water.

It will take longer for the reason above and there is a greater volume of water to raise to boiling point before anything at all can happen.

[polygonum]

Thank you - I agree.

[John Rumm]

Hmmmm

yes and no....

per kg, indeed there is lots of energy, since the latent heat of vaporisation for water is high. However its about 1600 times less dense - so the amount you have (in mass terms) is tiny in comparison.

Not if the thing you are heating is immersed in the water.

Some food is by intention "wet", stew, stock, soup etc

Longer than what?

Longer than if there were less water? well no shit Sherlock!

Longer than the same meal prepared in a normal pan? Not so much - and that is where the pressure cooker will win overall.

[harry]

I can see your knowledge of steam is near zero. The reason steam heating issomuch quicker then water is as below.

[John Rumm]

Ironic since it seems to you without clue as usual.

Do you really try hard to appear so obtuse, or does it come naturally?

You will only reclaim the latent heat of vaporisation by condensing the steam back into liquid.

If the food is *NOT* in contact with the steam, and is immersed in the liquid in the bottom of the pan, then how will the steam have the opportunity to condense on it?

If the food is elevated such that its in contact with the steam rather than liquid, the steam will only condense while the surface temperature of the food remains below the dew point. Once the surface temperature has reached that of the steam, you will again get no condensation. So you may get some reduction in cooking time (compared to boiling), only if you reduce the water content, and thence the heat up time (given the rate of energy input is dictated by the hob and not the pan).

The bottom line is that the *major* contributing factor to the reduction in cooking time in a domestic pressure cooker comes from the elevated boiling temperature.

[harry]

The fact that steam can put so much energy into an object so quickly means it reaches the disired cooking temperature much more quickly. The exterior reaces cooking time very quickly. The cooking time is taken up by the time it takes the heat to penetrate to the centre of the object. Steam is much quicker then the water because the only limitaion to the energy transfer is how quickly the condensate can be cleared from the cold surfaces. Wereas with water it is dependent on relatively slow convection currents (so most of the object is surrounded by relatively cool water). And less energy is available.

[Clive George]

In a rolling boil?

[John Rumm]

Bzzzzzrt, sorry wrong answer try again...

Where is the energy coming from harry?

How can the steam put the energy into the food faster than its coming out of the hob?

Could you translate that into English?

Where is the energy coming from harry?

Go stick your head in a pan of boiler water and tell us how cool it is...

[polygonum]

Using a hub, the heat goes through the bottom of the pan, into the liquid there before it can do anything else!

It might be different if the steam were heated from above, or fed through a high-pressure steam line. Very, very different to a domestic pressure cooker.