Sunday, April 4, 2010

Physics and Food - Two of my favorite things!

After an extremely long break from writing, I am finally back. I've been busy with work, and when I'm not working, I'm "busy" recharging from my days on duty. Christine mentioned recently in her blog that she doesn't know how people with "real lives" have time to blog. I guess I'd qualify as one of these people, and she is absolutely right that blogging is hard to fit in to "real life" sometimes!

One day recently when I wasn't out on a job, I had some of the other engineers over for dinner. I made my family's favorite pasta sauce with angel hair pasta, and I made creme brulee for dessert. While I was waiting for the water to boil for the pasta, my friend Will and I were discussing the physics of boiling water.

For water to boil, the liquid on the bottom of the pot must get hot enough to change into gas and form a bubble, and this bubble must stay hot enough to float to the surface of the water without popping under the pressure on its way up and without cooling down enough to turn back into liquid. This means that the pressure inside the little air bubble has to balance the pressure of the surrounding water, which depends upon the pressure being put on the surface of the water. And the temperature of all of the water has to be high enough to keep the bubble from cooling down, (not just the water on the bottom).

I hope some of you will remember the ideal gas law from high school chemistry or physics:

PV = nRT

where P is the pressure of the gas, V is the volume, n is the number of moles (related to the number of grams of the substance), R is a known constant, and T is the temperature. Technically this equation only applies to gases that are under perfect conditions, but we can approximate our boiling water situation to one that can be explained by it for simplicity.

The question that Will and I were discussing is whether or not putting the lid on the pot helps the water to boil faster. You probably realize that putting the lid on will keep more heat in that would otherwise escape into the air, right? But does putting the lid on also increase the pressure on the top of the water? If this were the case, looking at the ideal gas law, the increase in pressure and temperature would balance out, meaning that the water would not boil any faster. However, putting the lid on does not increase the pressure because the lid does not seal tightly enough; air can still escape fairly easily. So the answer: putting the lid on the pot does help the water to boil faster because it traps the heat but doesn't increase the pressure.

As I was writing this, I wondered to myself: if the temperature of the water and the water vapor is increasing, why does this not in turn increase the pressure anyway? I think the answer to this is that the lid will still allow the volume to increase instead of the pressure. The water vapor will transfer its heat back to the water, increasing the temperature even further, but it is still allowed to escape from under the lid.

Once water comes to a boil, the temperature of the water will not increase any further. All of the heat coming from the stove will go into converting the liquid into gas. This means that whatever you're cooking in the water can't get any hotter than 212 degrees Fahrenheit, the boiling point of water. Now a pressure cooker is a different story. The lid on a pressure cooker seals tightly and keeps the vapor from escaping. This increases the pressure on the surface of the water, so it has to get even hotter before those bubbles on the bottom can form and rise. This means that the boiling point of the water is higher in a pressure cooker, so whatever you're cooking can be cooked at a higher temperature.

What about a double boiler, what's the purpose of that? A double boiler consists of two pots; the first is filled part way with water and sits on the stove, and the second sits on top of the first. The water should not be high enough to touch the top pot. The water is brought to a boil. Whatever you're cooking in the top pot is heated indirectly by the steam from the boiling water. This is a way of cooking the food more slowly and evenly. It's used to do things like melt chocolate; doing this in a pot sitting directly on the burner can heat the chocolate too fast, causing it to burn. It's also used to make custards and sauces for the same reason. I actually used mine recently to make the custard for a banana pudding.

It's interesting to write about the connections between two of my favorite things. Maybe I'll write some other physics of cooking posts soon. As always, let me know if there's anything specific you'd like me to write about, food related or not!

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