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I was just cooking some noodles and staring at the pot waiting for them to cook made me wonder... Will my food cook quicker when the water is boiling and bubbling, or when it is at a temperature where it is about to start boiling but there are no bubbles in the water yet?

My instinct would be that food will cook quicker just before the water starts boiling because steam conducts heat slower than water and so when the food is in contact with the steam bubbles it receives less thermal energy. I'm basing this assumption off the fact that a steam bath/Turkish bath which is AFAIK at 70 degrees C and close to 100% relative humidity feels a lot colder than sticking my finger in a cup of water at 70 C. Although this begs another question, does the food even come in contact with the steam or does the desire of the bubble to stay round (minimum energy configuration) actually prevent it from even coming in contact with the actual food?

And what is it that intrinsically makes one substance a better thermal conductor than another? I mean what is heat anyways--I was always taught that it was the "kinetic energy of the particles", so are we talking $0.5mv^2 + 0.5I\omega^2$ where $m$ is the mass of the individual atom? If so, what is the relationship that equates this to degrees Kelvin? And even then, that makes sense for a gas, but what about a liquid or a solid where particles are not free to move around like they can in a gas, is it potential elastic energy of the particles displacing from the optimum minimum energy lattice configuration?

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The problem of heating noodles is irrelevant because they reach the temperature of the water within a few seconds. And when the temperature is the same, there is no more energy transfer. Noodles are just swelling until they become soft enough to it and I doubt that you can speed up that by vigorous boiling. You will just waste energy for evaporating excessive amount of water. –  gigacyan Jan 26 '11 at 10:57

1 Answer 1

My instinct would be that food will cook quicker just before the water starts boiling because steam conducts heat slower than water and so when the food is in contact with the steam bubbles it receives less thermal energy.

This is absolutely the opposite of reality! A steam bubble in contact with a body with a temperature less than 100°C will condense on this body, transferring about 540 cal/g!. In fact heating a wall by condensing steam is one of the most efficient heat transfer methods.

But now Your primary question: Those noodeles "cook" faster where they experience the higher temperature. In a pot on a fire, the surface of the water will be as low as maybe 80 or 90 degrees, when bubbles raise at the bottom, but do not reach the surface yet. (Again here: heat transfer by bubble evaporation is extremely fast, same reasons as for condensation) 10 or 20 °C less are a lot when kooking noodles (wheat starch) this may lower the reaction velocity to half the value at 100 °C, or even lower (rule of thumb for such reaction)

Thermal conductivity: most amorph solids/liquids have similar, low heat conduction. Crystalline solids are medium, metals are much higher, the thermal conductivity being related to the electrical conductivity. Google for: "Wiedemann Franz Lorenz" Thermal conductivity of gases can be calculated by kinetic gas theory, this was "triumph" for Clausius Maxwell and Bolzmann.

Georg

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A very good answer. It should be noted that the steam in a bubble, is at atmospheric pressure (or above because of surface tension), but the steam in your Turkish bath is dighly diluted with air. Also of note, in a pressure cooker things cook faster not just because of the higher temperature, but also because as long as the food interior is cooler than the water plus steam, pressure will drive steam into the food. –  Omega Centauri Jan 25 '11 at 22:28
    
Few seconds after you put noodles in boiling water they have the same temperature as water. A bubble of 100°C steam cannot transfer any of its energy to a 100°C noodle. –  gigacyan Jan 26 '11 at 10:51
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@gigacyan, I'd be more cautious with such assumptions. Noodles are made from flour (starch) and water, after drying the noodle has some porosity. In the very moment when surface of the noodle is immersed in boiling water, the starch there will swell (verkleistern) and from that moment on no convection within the noodle is possible. Heat and water has to diffuse in, both processes are rather slow. I think there is some reason, that all noodles have at least one dimension not more than say, 2 mm. –  Georg Jan 26 '11 at 17:11
    
@Georg: Well, it is made thin so that it doesn't take too long for water to diffuse through the noodle. If thermal diffusion would be as slow as you think, you could put your hand in the boiling water for a minute. In reality, couple of seconds would be enough to get a serious burn. –  gigacyan Jan 26 '11 at 19:19
    
Hello gigacyan, think where the tips of the temperature sensing nerves are located, and from which temperature on the nerves signal pain. –  Georg Jan 26 '11 at 21:54

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