Pressure Cookers
Pressure cookers have been around for a long time.
Originally used mostly for canning, the pressure cooker has more recently gained in popularity as a dinnertime emergency responder – helping to cook dinner a lot faster than in an oven or on the stove.
Yet pressure cookers are just cooking pots with a lid placed tightly on top.
So, how does merely closing a pot with a lid allow for food to cook so fast?
Let's find out.
Escape plan
Look at a glass of water.
No, look really closely – on the scale of molecules.
This is not a haven for the lazy or idle.
Rather, it is a knock-down battle as molecules knock each other around.
This is a world of push and shove, and molecules are always being pushed out of the water and into the surrounding air.
Ah, but getting out is not so easy.
The surrounding air will oppose this, trying to keep the water in its place.
Still, there will always be some molecules moving fast enough to break through and evaporate. This is true for bodies of water like lakes …
… or even solid ice.
Long story short, …. evaporation happens. (I know, it sounds like a very nerdy bumper sticker.)
An offer you can't refuse
So, in every cup of water, a battle is going on.
On the one side, water molecules are fighting to get out.
On the other, the outside air is working hard to keep water in its place. Under normal household conditions, the air has the upper hand.
But things can change.
The heat is on
Heat a pot of water on a stove and what happens? If it is heated up enough it will boil – obviously.
But how?
The reason is that heating the water gives the molecules more energy so they can move around faster. At the right temperature – the boiling point – the water molecules have enough speed to break out and boil away as a gas.
Lowering standards
There is another escape plan. The water can escape if the air lowers its guard. To be more accurate, if the surrounding air had lower pressure the water could escape more easily. The boiling point would be lower. In fact, without any air, water boils at room temperature.
In fact, when cooking at high altitudes, with its lower pressure and lower boiling point, adjustments to how food is cooked needs to be made. Food takes longer to cook.
The empire strikes back
Of course, all this goes two-ways.
Lowering the temperature means the water molecules have less energy to escape – evaporation goes down.
But also increasing the air pressure makes it hard for water to break out, the boiling point temperature gets higher.
**Note: This is an important point for the plot!
The outer limit
It turns out that the commandment for liquids is thou shalt not be hotter than thy boiling point.
Once the boiling point is reached, (212º F under normal conditions) a liquid cannot get any hotter.
It is forbidden.
Turn up the heat on a boiling liquid and what do you get? More boiling liquid.
Note: This too is important!
Cooking in the fast lane
So, now let's talk about pressure cookers.
Here, the food is placed in the cooker, a lid is secured to the top and it starts cooking. What happens?
As the food heats up, steam escapes the cooking food and fills the sealed container.
The hot, trapped steam increases the pressure inside. The increased pressure keeps more steam from boiling. The boiling point temperature increases.
The boiling point temperature is the hottest the water in the food can get. As it goes up in the cooker, the water is allowed to heat up hotter than it could otherwise. The food cooks hotter and faster.
The bottom line is this: The pressure inside the cooker eases restrictions on how hot the food can be. This is useful for not only canning, where high temperatures are important for sterilization, but also to cook food quickly.
What a difference a little lid can make!
On the web
An animation recapping how pressure cookers work.
Boiling water in vacuum chamber
Video demonstration of how water, in the absence of air pressure, can boil even at room temperature.
Don't miss out on future posts! Sign up for our email list and like us on Facebook!
Check out more hot topics, go back to Home Page
