Under Pressure: Everyday Science of Pressure Cookers

pressure-cookerIt’s the season for soups and hot, hearty meals. But when you’re busy performing polymer testing for clients all day, those meals take too much time at the end of your very long day to cook. That’s where Instapots come in and save the day (or at least dinner prep).

Instapots have returned to mainstream cooking, and we couldn’t be happier about it. In thirty minutes, our low-and-slow meal is ready to eat. That’s got us thinking: How do these little countertop gadgets pack enough power to cook faster than their hefty counterparts while retaining all the flavor and moisture? Spoiler alert: It’s science!

Instapots are a brand name of multicookers, but are known more specifically for their electric pressure cooker feature. Pressure cookers do just as they say: They cook using pressure. Why does that make a difference? Because cooking under pressure increases how hot your food can get.

At a temperature lower than its boiling point in an open cooking system, the microscopic bubbles of steam in the water hit the surface and are crushed by the atmospheric pressure. When water hits 212° F, the pressure of the vaporizing water matches the air pressure, so the microscopic bubbles can escape the surface without being crushed, and thus convert into steam. The water rests at that boiling point until it’s all evaporated.

If you’re cooking anything containing water (which is most meats and vegetables) or cooking anything in water, that item cannot get any hotter than the water’s boiling temperature until the all of the water evaporates into steam.

So what a pressure cooker does is use the boiling and evaporation process to its advantage. When water converts into steam, the volume expands. If something increases in volume but its container does not, the pressure increases. And if you increase the pressure of the air, then the water would have to be hotter in order for the molecules to match the pressure and produce steam. Higher pressure means higher boiling temperature for water (this is also why so many recipes indicate different cooking temperatures depending on your elevation.)

Using an airtight lid–braced, sealed with silicone or rubber, and locked–an Instapot locks the steam from boiling water inside the pot, which in turn increases the pressure, which increases the boiling temp, and speeds up the cooking time. Pressure cooker lids are designed so that as pressure builds, the seal grows tighter, keeping the steam locked inside even when force is exerted.

This is why your Instapot recipes should always call for at least 2 cups of liquid (whether stock or water). That liquid not only helps keep the food moist and flavorful, but it is critical to producing the necessary steam in order to cook the food.

Of course, when you think of things “under pressure”, you think 1) of the hit song by Queen, and 2) of things bursting. High pressure, if unregulated, often produces explosive results. Back in the day, when pressure cookers were stovetop devices, they required frequent monitoring; even with a steam release valve, high-pressure heating systems have a tendency to burst if the pressure exceeds a critical range. Electric multicookers, however, have sensors and automatic shut-off functions to prevent your dinner from splattering all over the ceiling. Instapots, for example, have developed their steam release valve to trigger automatically if the pressure reaches above the “safe operating range.” So you can safely have a truly easy meal in a fraction of the normal cooking time.

Instapots and pressure cookers are a great example of how science impacts our daily lives, sometimes without us even realizing! All of the same science used to cook your delicious dinner is used in the complex independent testing that we perform here at SGS PSI.  For example, sealed digestion vessels–used to prepare samples for elemental analysis–generate higher pressure. The higher pressure allows the strong acid to react with the sample at a much higher temperature. Just like the higher pressure inside the Instapot cooks your food faster, the higher pressure inside our sealed digestion vessels digests our ICP-OES samples faster.  

We encourage you to stay curious and leave us your everyday questions in the comments below. Your question could be featured in our next blog post!