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Liquids in a pressure cooker can reach higher temperatures because the boiling point increase as the internal pressure rises.

But do those liquids continue that "rolling boil" once the cooker is up to full pressure? Or does the increase in pressure keep the liquids from continuing to actively boil?

Background: When I cook meats in a pressure cooker, I place them on a cooking rack to keep them from being submerged. I was wondering if the liquid is boiling enough internally to effectively baste the meats in that froth.

I am using an Instant Pot electric pressure cooker which reaches about 12 psi.

  • It would be very difficult to reach a rolling boil. See my answer for details. (As you note, increased pressure raises the boiling point, and so it becomes self limiting. If well insulated, the extra energy input would be stored as pressure, until the safety valve opens). – DrM Jun 12 at 16:00
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For as long as it is plugged in and the power setting is appropriate, it should keep boiling as if it was in a normal pot. Imagine if you were to take a normal pot deep underground where you get the same atmospheric pressure and watch it boil, it would look the same as boiling as sea level.

It is the water vapor that sustains the higher-than-outside pressure inside the pot. The lid has a valve that impedes the release of vapor. So, without continuous boiling, pressure would decline gradually (like once you turn off the pot).

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    As more things become dissolved in the water, boiling point inside the cooker would change somewhat. Even then, for as long as there is enough energy supplied to the pot, it would continue to boil regardless. – user110084 May 8 '17 at 7:41
  • I'm sorry, that's not correct. The closed lid makes a big difference. The extra energy goes into building pressure. Boiling in a sense, is self limited. See my answer for details. – DrM Jun 12 at 15:57
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An electric pressure cooker actively senses pressure (generally through the deformation of the pot), energising the heating element only when the pressure drops. So while it definitely boils while being heated, the majority of the time it's not actively boiling, just at boiling temperature.

As for whether that will "baste" an object inside the pressure cooker: No, but that doesn't matter. The relative humidity in the pressure cooker is 100%; pouring liquid over the object won't have any effect on how moist that object is (not that basting has much effect on the moistness of meat in any case).

  • pouring liquid over the object won't have any effect — It does if it's washing away dry rubs or sauces, or other effects of physically immersing food in water. – Robert Cartaino Jun 12 at 13:34
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    That will definitely happen, and would even if there was no rolling boil, because of droplets of water condensing on the lid and falling on the food. – Sneftel Jun 12 at 13:47
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At 12 psi, water boils at 243.7F. That said, it would be difficult to maintain a rolling boil.

The rolling boil is easiest to establish in an open or well vented pot where water vapor can escape as fast as it is produced. In a fully closed pot, adding heat raises the temperature, which increases the pressure, which raises the boiling point and in effect inhibits boiling. The pressure cooler, with a safety valve, is intermediate to these two extremes. Even so, the heat required for a rolling boil would still have to be in excess of what is needed to maintain the elevated temperature and pressure plus lift the valve plus replace the heat carried away by the super heated water vapor. We can see that maintaining a rolling boil in the pressure cooker is going to be challenging compared to an open pot.

There are a few concepts that help us understand what is happening in a little more detail.

Vapor pressure - is the pressure exerted by a vapor in equilibrium with its condensed phase. Vapor pressure generally increases with temperature.

Partial pressure - the pressure above the liquid in the closed pot, has a contribution called the partial pressure, from each component in the gas phase. The partial pressure due to the liquid in the pressure cooker, is identically its vapor pressure.

Boiling occurs when the vapor pressure is equal to (or exceeds) the pressure in the environment, the ambient pressure.

Equilibrium - occurs when opposing processes are in balance. Examples can include evaporation and condensation to establish a vapor pressure, or heating and cooling to establish a temperature.

With that preamble, lets consider what happens in an open pot compared to what is happening in a pressure cooking.

In an open pot, pressure above the liquid remains close to that of the surrounding air. Once it reaches the "boiling point", adding more heat results in more steam being produced which then carries away more heat and so the temperature stays close to the boiling point. Adding enough heat, can bring us to a fast rolling boil.

In a pressure cooker, we have a closed pot (up to some pressure where the vale opens), and the steam cannot escape to carry away the heat. Even before we reach a boil, adding heat increases the temperature and thus increases pressure inside the vessel and thus increases the temperature needed to reach boiling. Over this range, boiling in effect is self attenuating and energy is stored in the increased pressure.

If pressure becomes sufficient to open the valve, boiling might pick up from that point, but holding the valve open becomes part of the energy budget along with the extra heat carried away in the super heated water vapor and again, it is energetically expensive to maintain a rolling boil.

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    If heat is being added to the system (by the burner), but not being removed (by boiling), where do you think it's going? – Sneftel Jun 12 at 8:24
  • @Sneftel The heat is stored as pressure, and some is lost through the walls, unless the safety valve opens! I edited the answer to make it more complete and hopefully more clear. It is pretty obvious that it would be very hard to reach a rolling boil. The subject is taught in the regular coursework for chemistry and chemical engineering, four times - general chemistry, p-chem, thermodynamics and statistical mechanics in grad school.. – DrM Jun 12 at 15:52
  • And when the pot reaches full pressure and the release valve start venting steam? The boiling is depressed to what can vent but it is still boiling. The pressure can not build indefinitely to the point of equilibrium- that would just be a bomb. In an instant pot the temperature is controlled to allow an equilibrium at a target temperature (and thus pressure) without venting but this is not a normal pressure cooker. – Sobachatina Jun 12 at 17:43
  • So, I suppose your answer is correct that it will reach equilibrium without boiling only in an instant pot- not because the pressure will keep building- but because the pot will control the temperature and apply less energy when it reaches equilibrium. – Sobachatina Jun 12 at 17:48
  • @sobachatina Your first comment is true in so far as venting will let it boil more (or a little). But (a) the OP asked about a "rolling boil", and (b) you are trying to make a quantitative argument without having the numbers on which to base it. We don't how big is the pot, how much it vents, or the several numbers that go into losses through the walls. Don't forget also that at equilibrium there is no net increase in temperature or pressure. – DrM Jun 12 at 17:55

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