Steam

[newshound]

I can't believe the amount of nonsense in some of the follow-up posts.

What this link above says about blanketing, etc, is potentially correct in the context of some industrial heat exchangers.

But the reason a pressure cooker cooks quickly is because of the elevated temperature. The principle reason for removing the air quickly is that once this is done and the valve is floating on steam only, the water has reached the maximum safe temperature of the pressure vessel.

Getting rid of the air also reduces the oxidation of some of the organic matter in food.

[harry]

The main benefit of steam is the latent heat of vapouristaion, ie the heat needed to turn water into steam. Even at less than 100degC, steam would be quicker.

The principle reason for removing the air quickly

The temperature and pressure are firmly linked in saturated steam. The safety consideration is pressure only.

Yep there are a lot of thick idiots here. They clearly aren't capable of understanding the link I posted above.

Before retiring I ran steam boilers, steam heating and steam power equipment for forty odd years. And designed and installed it.

Pressure cookers are a compromise device. Ideally steam is not produced and "consumed" in the same vessel.

You could run a cooker on pressurised water but the energy available is less than with steam so it would be slower.

[John Rumm]

Indeed - I think harry must have some kind of commercial steam cooker with an external source of steam in mind! (either that, or physics works differently for harry)

[polygonum]

One of the things my mother used to pressure cook was the Christmas pudding. With great care taken to ensure that the wetness couldn't get in and spoil the pudding with wetness.

One of the things I used to pressure cook was my own version of a vegetarian haggis. Although I first did this to reduce cooking times, I realised that it was the elevated temperature that caused the taste to develop. I don't think the pressure actually had much of an impact - but because you can't fully separate them, I could be wrong. (Increasing cooking time alone did not have the desired effect.)

[John Rumm]

The pressure can play a useful part on some foods - especially those that need to hydrate when cooking - the pressure makes the water penetration much faster (even to the extent of making chicken bones go a bit floppy etc).

For "wet" things (casseroles, stews etc), the main gain is the higher temperature, but the pressure may soften things more.

[newshound]

You can only have steam at less than 100C in a system running below atmospheric pressure.

Yes an unrestricted supply of steam is very effective for heating something because of the latent heat deposited. But in a pressure cooker the energy flux comes from the gas heating the base and there won't be much difference whether the thing being cooked is in the water or the steam. In fact if it is in the steam, the heat loss from the cooker lid may make the steam cooler than the water.

Of course they are. But I was talking about a sealed vessel containing water. I could have worded it better. The point is, at (say) 1 bar over atmospheric pressure, the temperature in the gas phase is lower if air is still present.

The energy has to come from somewhere, and this determines the power.

[whisky-dave]

thought were were talking about this.

[harry]

The steam can't be cooler or hotter than any water present. When the steam gives up it's energy it becomes water (at the same temperature) And vice versa.

That is my whole point that some here can't grasp.

A balance is reached. When the safety valve lifts surplus energy is being vented as steam.

The Steam "used" goes to heat the food and heat losses from the vessel.