Under Pressure

Last week, I got a new piece of equipment: a pressure cooker! A pressure cooker uses an ordinary stove and basically acts like a big pot with an airtight lid. Why use a pressure cooker over just an ordinary pot? Pressure cookers can cook things a lot faster than normal methods because of the pressure they build up. At full pressure, most cookers go to about 1 bar, or 15 psi (pounds per square inch). When the pressure in the pot is raised that high, it also increases the boiling point of water from 212 F to about 250F. The difference in temperature accounts for the shorter cooking time. Pressure cookers are also used for canning (which ironically usually uses jars, not cans) at home. I found a recipe for pressure caramelized onions that actually used jars so I decided to try it out as my first pressure cooked recipe.

First, I put onions and baking soda (I still don't know the purpose of the baking soda) into small mason jars with some butter on top of it all. NOTE the caps were screwed on all the way, and then backwards a quarter turn to ensure the jars did not explode. If the caps had been screwed on all the way, the pressure differential between inside and outside the jars could have caused them to rupture.

Ooooo...artsy photography.

Then, I put the jars in the colander-like basket for the pressure cooker, and put about an inch of water in the bottom. I then put them in the pressure cooker for 40 minutes at 1 bar/15psi.

Jars.

Six jars...One pressure cooker....One delicious meal

In this picture you see the jars after cooking. Now, for those who don't know, mason jars that are used at home usually have two parts to their lids, unlike what you would buy at the store. There is a flat piece that goes on top that has the little button that pops after it has been open, and then there is an outer ring that screws on to tighten the flat piece down. (See here). So as I went to unscrew the lids of the jars, all of them came off...except one. Usually pressurized containers have to cool to allow pressure to equalize to the outside in order to open them, but this one was much harder than the others. I let it cool for about half an hour so I could physically pick up the jar, but it still wouldn't budge. Then, I noticed bubbles coming from inside the jar. It was still boiling. Don't believe me? Aha! I have video proof:

Wait, what? The jar was still boiling after half an hour at room temperature...and it was cool enough that I could pick up the jar? After a brief discussion with my physics teacher, we determined that a vacuum must have formed inside the jar, which lowered the boiling point of the water inside the jar. The idea is that as the jar pressurized, the gases expand out of the jar because it is not completely sealed. When the pressure cooker is taken off heat, the gases slowly cool and condense. On this particular jar, it must have happened that the lid was on slightly too tightly so as the pressure increased outside the lid, it created a seal. The cooling of the gases inside the jar caused some of the gas to revert back to liquid or solid in the jar or on the onions (called adsorption [no, thats not a typo]). As these gases became liquid or solid, they were no longer contributing to the overall pressure in the jar. Therefore, an area of negative pressure was created and caused a suction that made the lid nearly impossible to get off. The principle that allows the pressure cooker to be effective also works in reverse. As the pressure inside the jar went down, so too did the boiling point of water. Hence, the remaining water was able to keep boiling at a much lower temperature. After about two hours of cooling, and trying to pry the lid off with a knife, the mixture continued to boil until I used a can opener to create a hole to break the vacuum seal. Pretty cool, huh? I'm not sure how much sense that made, but if you have any questions feel free to comment and let me know! Stay tuned for the next post!