If you increase the pressure, the boiling temperature increases as well. In the other direction: if you decrease the pressure enough you could even make water boil at 18 dergees C.

However I met this strange thing in my wifes kitchen called "pressure cooker". I don't understand how you can cook things quicker with that. If you increase the pressure inside this tool, you are just increasing the cooking temperature. And thus nothing will even boil.

What did I misunderstand?

  • 3
    What refrained you to google it? – user36790 Jan 26 '16 at 13:31
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    @user36790 my interpretation didn't match the explanations I found online, I was hoping for someone to give an explanation where they'd eventually point out what I exactly misunderstood. – privetDruzia Jan 26 '16 at 13:38
  • Why does food cook? Because the water is hot or because it boils...? – Nephente Jan 26 '16 at 13:40
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    Things do not have to boil to get cooked.It is the energy that changes the molecular bonds that results in cooking and the higher the temperature the faster. PV=RT and the volume is constant so temperature goes up. – anna v Jan 26 '16 at 13:41
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    You aren't misunderstanding anything. The entire point of a pressure cooker is it allows you to cook something by boiling it at a higher temperature than if the vessel were open to the atmosphere. – Solomon Slow Jan 26 '16 at 14:06
up vote 3 down vote accepted

First consider a pot with a loose lid over it. As the temperature rises, the vapor pressure of the steam rises as well (even before boiling) and the steam will push its way out under the lid. You'll get cooking temperatures over 100C, but not by much, and a lot of the energy you expend on cooking just goes out into the air instead of into your food. And to add insult to injury, your lid will rattle around on top of the pot.

Now consider the pressure cooker. The lid is well sealed and tightly locked down. The vapor pressure rises with temperature as before, but now it has nowhere to go; the pressure can't relieve itself by pushing the lid up and escaping. The energy you put into heating the food and water stays in the pot, and consequently you can achieve higher temperatures inside the pot. In fact, the "pressure" part of a pressure cooker is only a side effect of trapping the hot gasses inside the pot; the real benefit is the higher cooking temperature.

Think of it like this:
Assume that you want to make pasta. For a piece of pasta to be "cooked" a certain amount of energy has to be given to it (depends on the type of pasta) (also as anna v mentioned It is the energy that changes the molecular bonds that results in cooking and the higher the temperature the faster). The way to transfer this energy to it is by putting it inside hot water. The choice of boiling is actually because the hotter the medium the quicker the energy transfer. I am pretty sure that you can make pasta even at lower temperature than 100 oC, but will need more time.
Now, the classic way of doing it is by using a traditional cooker, where water is boiling very close to 100 oC (as Asher mentioned, it might be a bit over with the lit on, or may be because of impurities (salts)). If you choose to cook your pasta in a pressure cooker then what actually happens is that by increasing the pressure (the way Asher described) you increase the boiling temperature of the medium and thus the rate over which the transfer will occur, which will result to much (I haven't tested it but I think it is much much much) faster cooking.
Attention, you do not violate thermodynamics, you just take advantage of what we know.
Warning, I wouldn't test it with pasta because it already needs a small amount of time to make it! I would try it on a food like stew maybe.

Also, though this might not be an actual use for this, you could make steak very rare with it.

In the case of a very rare steak, you need it to be cooked just enough to make it safe, kill bacteria, and make it easier to digest, but no more than that. If you put it high pressure, you could heat it up to a high temperature without cooking it. That way, you sterilize the steak and cook it just enough, but without cooking all the way through.

  • 1
    You can't heat a piece of meat to a "high temperature" without cooking it. If you want a very rare steak that's safe to eat, try sous vide cooking, not a pressure cooker. – Solomon Slow Jan 26 '16 at 15:38
  • If it's high pressure, then you will need to heat it to a high temp before it will cook. – DevilApple227 Jan 26 '16 at 15:40
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    That is surprising to me. Where did you hear it? I always thought the reason for using a pressure cooker, was to accelerate the process of cooking with hot water or with steam by enabling you to do it at a higher temperature. You seem to be saying that the pressure cooker forces you to cook at a higher temperature because the food is more resistant to cooking. Unless I'm missing something, that doesn't sound like much of an advantage. – Solomon Slow Jan 26 '16 at 16:15
  • @DevilApple227 , it is the HIGH TEMPERATURE that cooks the food, which may or may not involve boiling, and has NOTHING to do with pressure. – David White Jan 26 '16 at 23:29

protected by Qmechanic Jan 28 '16 at 8:30

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