Have you ever heard that if you place a wooden spoon across the top of a pot of boiling water and pasta the water won't boil over? I've used this method several times and it works! But why?Turns out the science is pretty basic. First, why does a pot boil over in the first place? The answer: too much heat mixed with pasta water (aka water made thicker from the pasta residue). This heat creates lots of bubbles and the heavier water mixture doesn't burst like plain water because it's heaver (surface tension is greater). Since it takes more heat to break pasta water bubbles, the additional bubbles that are created by the continual heat get backed up in the pot and then you've got a boil over.
Now for the wooden spoon. Think of pasta water in two situations:
1. When you allow pasta to boil in a pot without anything on top, the boiling water has heated up the pot (a good conductor of heat) and the air surrounding it. The water is free to boil at it leisure, allowing heat to escape through the bubbles. It gets hot inside those bubbles.
2. When you place a wooden spoon on top of the pot, there's a very low heat conductor sitting on top of your pot. When the boiling water gets closer to the spoon, it shys away from the cool object (transfers or looses heat to the cool wooden spoon). In addition, the spoon breaks up the bubbles once they get to the top (remember surface tension), thereby preventing a boil over.
I would think that if you let the water boil long enough, and that'd be a long time, the wooden spoon would heat up just as hot as the boiling water and the water would boil over. But I'm not sure. It seems like that would take a while (and no one boils their pasta that long...think pasta mush). I also think the water would be gone by the time the wooden spoon heated up. Maybe I should try it and report back.See this link for more info on what materials conduct heat best (and worst).Photo of boiling water via whatscookingamerica.net
9 comments:
i will have to try that, if someone told me that before i would have just thought it was a wives tale. but know i know... thanks bp science!
really? that works, eh? I've never heard of that before but it makes a lot of sense now that you've explained it. And I am a victim of the pasta water boil over so I'll definitely have to try this out.
I dig science Tuesdays.
Hmm, I'm going to try that. I quite like the science tip of the week. Especially when it is so useful. What does "bp's" mean?
bp stands for boiled pizza.
thanks for this post. it's nice to have the science spelled out. i use this technique often too.
ok, if it wasn't already official, it is now. You are awesome.
I'm going to try the wooden spoon method for my next pasta boil.
Currently I've been adding a teaspoon or so of oil to the water in my pot before it gets hot. That coats the inside of the pot and the water bubble stay down, too slippery I suppose? That is another way to avoid boil over. BUT, it's less healthy than your way!
Unfortunately your reasoning concerning the low thermal conductivity of wood is wrong, it will not be cool on the surface, it will be hotter than a similar object made from metal. (The heat transfer coefficient from vapor to the object is the same, less heat is conducted away for the wood -->higher temperature). So this is not a temperature-driven effect.
What happens is that the foam acts as an insulator for the boiling water. If the heat supplied by the stove and the heat leaving the pot through the vapor is not in equilibrium the pot will cook over.
If you now place the spoon on top of the pot, it will destroy bubbles and release vapor, thereby increasing the rate of heat transfer through the layer of foam.
The shying away of the foam from the spoon is probably an aerodynamic effect: the release vapor is convected around the spoon. The flow/pressure field of the convection then give the impression of the foam shying away...
Thank you Johannes for that clarification. Always good to get more information on any subject!
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