Why do pressure cookers cook so fast?

Pressure cookers use pressures of around 15 psi to cook food. According to Wikipedia, pressure cookers cook at 125C. Naively, I would expect it cook 6% faster since it’s 6% hotter in Kelvin or maybe even 25% faster because the freezing point of water is 0C so it makes some sort of sense to think in C.

Except pressure cookers cook things roughly 3 times faster. How does that work?

You’re probably thinking along the right lines but taking the wrong starting point - what you need to consider, I suppose, is the temperature at which things start happening to food, and figure in the extra 25 degrees in terms of that. There’s no real reason why your baseline should be zero Kelvins or Celsius.

Makes sense, otherwise my carrots would cook, albeit quite slowly, whilst still in the ground.

I’m not a physicist, but as a rough guide to speed of reaction I usually figure it’s going to double for each 10 degree C rise in temperature. At 15 psig the boiling temperature is going to be 121 deg C, which I would guess would be around 4 times faster than at 100 deg C.

Think of it this way. Cooking food is a way to bring the temperature of the food up from a starting point of between 0 and 20 degrees C (Unless it’s frozen or your kitchen is really hot) by putting it in 100 C water. The food then heats up through conduction of heat from the hot water to the cold food. The important factor to the speed of this process is the temperature difference. When the food heats up, the difference becomes smaller and smaller, and heat transfer goes slower and slower.

With 125 C water you’ll have a temperature difference of at least 25 C all the way to the end, even if your goal is to heat the food to 100 C, and even if the speed of the heat transfer wont be spectacularly higher at the start it’ll stay high for longer.

Well, some cooking involves keeping the food at a high temperature for a while to let various processes happen like Bill explains above. But this is the reason you get there faster with a pressure cooker. :smiley: