eli5: how do pressure cookers get food cooked more quickly in a way that simply using a higher heat does not?

47 views
0

obviously cooking on very high heat is faster but it wouldn’t mean you could have a huge hunk of meat nice and tender in a couple hours. but why. i don’t even understand enough to know if i chose the right flair.

In: 2

Think of when you boil water to make some pasta. The water heats up until it start boiling, but then it stays at that temperature. If you turn the heat up it will start boiling “faster” but the water doesn’t actually get any hotter. In as ELI5 of a way as I can explain: this is because it takes a lot more heat energy to turn liquid water into steam than it takes to heat water up to a boil from room temp. Essentially all the heat you’re pushing into that water leaves with the steam, and the water stays the same.

Now how does this relate to the pressure cooker? The pressure of the air around you is what determines the temperature the water will boil at. If you got rid of the air and exposed your pot of cold water to a vacuum, it would boil. Likewise, increasing the pressure artificially inside the pressure cooker allows the water to boil hotter.

Pressure cooking is a way of using higher heat than you can get in unpressurized pot.

On the stovetop, you are limited to the 212°F (the boiling point of water at average air pressure)

At the higher pressures, the boiling point of water is higher (you can reach 30 psi inside a pressure cooker, and that pressure, water boils at 250°F, giving you a hotter cooking cycle.

Science time!

Water doesn’t get hotter than 100 Celcius in a standard environment, no matter how hot the flame under the pot is. That’s just a science fact. If you want to cook something at higher temperatures and it’s in water, you have a problem. Adding salt does raise the boiling point a little bit but not that much.

If the air the water is boiling into were at higher pressures, then water will boil hotter than 100 C. This is what a pressure cooker is for.

It is literally impossible for water (at sea level pressure) to be above 100°C. If it ‘got hotter’, it would be steam instead. Given that almost every thing we can consume has a bunch of water to it, the highest temperature you can cook something at sort of comes out to that 100°C too (the exceptions involve steam and cooking methods that partially dry out the food and things that end up with less water than you started with). Adding more heat to the burner doesn’t change that, except to dry the food out faster (which is usually an unpleasant texture anyway, as you noted).

But what about *not* at sea level pressure?

With more pressure, water boils at a higher point. Most pressure cookers can safely and conveniently reach 120°C boiling points, which allows the food to cook significantly faster *without* removing the water first. And of course, higher temperatures usually mean faster cook times.

So water boils at 100 degrees C, everyone knows that. What you may not know is that it is impossible for liquid water to ever get *hotter* than 100 degrees C. At that point, applying more heat energy will just make it convert to steam more rapidly, but only in the vapor form can it get hotter than boiling point.

That is, at normal sea-level air pressure that’s all true. If you increase the pressure, you increase the temperature of the boiling point and thus increase what is essentially the maximum temperature you can get your food to, and that’s what makes it cook faster.

Of course, this affect mostly only works when cooking something immersed in water or a mostly-water liquid (which is pretty much anything you would cook with other than fat) like soups, stews, or brazes. There is no physical effect that keeps air or metal from getting hotter than 100C so baking or grilling under pressure wouldn’t really do anything.

All this is the same reason that frying food in oil is so different than boiling it in water. Frying is done at much higher temperatures, usually 200C or more, which is not physically possible with water (unless the air pressure were around 20 bars). If you “fry” something in 100C oil the results will be very different.