The physics of the Instant Pot – Cheap food, energy efficient, healthy, easy and YUMMY!
With the holiday season upon us, I get a lot of questions about electric pressure cookers such as the Instant Pot. These cooking appliances are energy efficient rock stars and I would encourage adding these in any energy efficient-focused kitchen. Now I’ll be honest, the Instant Pot was a “thing” for a long time before I actually tried it out. Maybe it was the cheesy name that made me think this was a passing phase or maybe it was that I was already a huge stove-top pressure cooker fan and didn’t want to add another gadget and give in to the latest consumerism craze. Who knows why I resisted? Eventually, due to the ridiculous amount of questions from people at my outreach events about my thoughts regarding this countertop marvel, I decided to give it a try.
Today, rarely a day goes by that I don’t use this gem… From a huge batch of black beans (to reduce my purchase of canned beans {post with recipe coming soon}) to super quick stew meat this is a true kitchen energy efficient work horse.
Pressure cooking is nothing new – people have been using large pressure cookers to can their harvest for long term shelf stable storage for generations. Today’s electric pressure cookers bring that well-proven cooking strategy down to a size that’s perfect for family meals.
AND, the electric plug-in pressure cookers remove the stress and need to monitor and adjust the burner constantly to ensure things stay at a safe temperature and pressure. This is all done for you by the controls - just set it and walk away. The temperature adjustments are automatic. Also - since there is an embedded thermometer, this devise comes with additional safety checks to make sure things don't get dangerous.
Now, I would disappoint you if I didn’t explain the PHYSICS of the pressure cooker (because physics is oh, so cool), so here we go….
Phases of water
There are three phases of matter: Solid (ice), liquid (water) and gas (steam). (for my fellow nerd followers – Yes … there are more than three phases, but these are all we need to talk about for now).
SOLID:
LIQUID:
GAS:
And, we can change phases of a substance by adding or removing heat. Take liquid water, add heat and you get steam (gas). Take liquid water and remove heat and you get ice (solid).
For now - let’s just focus on the liquid to gas conversion. Put water in a pot, place it on the stove, turn on the burner and (obviously) the liquid water will increase in temperature until it reaches this magical temperature called the BOILING POINT. At sea level, this temperature is 212° F (or 100° C).
You can not get an open pot of liquid water at normal atmospheric conditions above 212° F – period. “Sure you can”, you think…. “Just put it in a 400° F oven, right?” Nope – the water will still not go above 212° F – it will all boil away first.
We are STUCK at a maximum temperature 212° F with liquid water …. Or are we?
"Shifting" the boiling point
If you live in Denver, Colorado, your liquid water in an open pot never even gets to 212° F, it gets “stuck” at 203° F due to Denver’s higher elevation and therefore lower atmospheric pressure.
So, wait a minute… if lower pressure in Denver leads to a lower boiling point temperature, could HIGHER pressures lead to a higher boiling temperature? Why, I’m glad you asked…. YES!
Here is a graph of pressure versus temperature for water. This graph is used in science to identify which phase water will be with a given pressure and temperature. The “T” is called the "triple point" which is a unique point of pressure and temperature where a substance can be all three phases – Cool but way beyond the scope of this blog post.
Let’s just zoom in a bit to the dashed line in the above graph. The brown line shows the transition between the liquid and gas phases. Here it is a bit closer:
So – looking at the blue line (atmospheric pressure), we draw a horizontal line on this graph from sea level atmospheric pressure from the vertical (Y) axis and once we hit the curve (the transition between liquid and gas), draw a vertical line down to the horizonal (X) axis to determine the “boiling” temperature – voila…. 212° F (or 100° C). The boiling point temperature we all learned in high school.
Next, go to Denver, CO and do this same exercise – follow the green horizontal line from Denver’s lower atmospheric pressure of 0.8 atmospheres, hit the curve and drop down to the boiling temperature of 203° F which is the point that water actually boils in Denver. Cool! (or cooler, literally J ).
Now – let’s looks at the other side of sea level atmospheric pressure. What if we INCREASED the pressure at the surface of the pot to two times atmospheric pressure (30 psia). Look at the red line now – same strategy… draw a horizontal line across to the liquid / gas phase change curve and drop down. We can now see that water will not boil (or transition from liquid to gas) until 250° F. That means the liquid water temperature is able to reach a higher temperature before becoming a vapor molecule.
In summary – higher pressure means higher water boiling temperature.
Higher water temperature means faster cooking. Temperature is simply a measure of the average kinetic energy or movement of the molecules and so faster moving water molecules means less cooking time!
Now let’s think about the energy needed to cook with Instant Pots…
In order to get the liquid water to this higher temperature, we will need to add more energy. This is true.
BUT – once there, the cooking time is reduced which is an energy saver.
AND – the real advantage of these electric pressure cookers is that unlike our stove tops, the heating element and cooking pot are enclosed in a well-insulated shell. The outside of the Instant Pot does not get hot, which tells us that the electricity that goes in, goes directly to the food, not to the kitchen. This is a huge energy saver!
LASTLY – Since we are starting at a higher temperature, any passive cooking strategies (see previous post) are even more effective since the starting point is higher.
So – there you have it. There are now many electric pressure cookers on the market. A few years ago, America’s Test Kitchen tested them and you can see those results here (https://www.splendidtable.org/story/is-instant-pot-the-best-americas-test-kitchen-reviews-multicookers).
Keep an eye out (or subscribe) as I will be posting more blog entries about my favorite pressure cooker recipes in the months ahead. Some examples…
a weekly batch of yogurt
a rocking curry chickpea stew
super cheap (and single use plastic - saving) huge batch of hummus.
Happy cooking!
