Cooking Under Pressure

By Talon J. Cotterman

Have you ever wondered how pressure cooking gets your food cooked thoroughly at a much faster pace, or maybe why people up in the mountains have special cooking recipes for high altitude cooking? The answer to these both boil down to the same idea: the relationship between pressure and temperature.

Getting into the science first, you can see below what is called a phase diagram. This one is specifically for water. What you, me, and the cookbooks have in common is we all think of the boiling point of water as 100°C. However, you can see this is only true at 101kPa, which is equivalent to 1 atm. or put even more simply, the air pressure at sea level.

The most relevant information for our purposes is how the temperature changes as we change the pressure. We can see that increasing pressure makes the water stay liquid at higher temperatures.  This is how pressure cookers cook our food faster and more efficiently. We also see how at lower pressures, water starts to boil at temperatures below 100°C. Eating food cooked at lower pressures may be dangerous as it may not be cooked thoroughly or to a safe temperature.

That explains our high altitude cooking a bit, but how do pressure cookers actually work? It is simple, but genius. The water in the cooker boils off into steam and we simply trap that steam in with the existing air. That is why most cookers have a vent to let the extra steam out the top, so we maintain the pressure and don’t end up creating a high pressure bomb in our kitchens.

Some more fun facts about the phase diagram are the triple point which means you can have ice, water, and steam all at the same exact temperature as long as the pressure is right (See the 0.6kPa). You can also see borders where ice goes directly to steam in the lower temperatures and pressures. This is exactly how dry ice works. It is just carbon dioxide going from solid to gas in a temperature and pressure below its triple point.