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Cooking in a perfect thermos

  1. Apr 25, 2013 #1
    I just got one of those new fancy stainless steel thermoses recently to keep my morning coffee piping hot for hours, and I started thinking...

    The reason many coffee stops put their coffee into thermoses is that keeping it on the burner burns out the taste after a while (like 7-11:frown:). My guess is that taking it off the burner and putting it into the thermos prevents the coffee from over-cooking. However, arguably, if you brewed a pot of coffee at 170 degrees and poured it into a "perfect thermos" where there was no heat loss, then wouldn't the coffee continue to "cook" at that 170 degrees? Why is that any different than keeping the pot of coffee on the burner and maintaining its temperature at 170 degrees?

    The salient difference is that, on the burner heat has to be continually added to the system to compensate for the dissipative effects of heat loss from the top of the coffee pot due to the lower ambient temperature in the room, whereas this is not the case in the perfect thermos. However, as far as molecular interactions, the average kinetic energy between the coffee molecules would appear to be similar between the two, and isn't this what "cooking" is? So why does one continue to cook and the other not? Or are they both cooking in a different way, other than just the anisotropy of the burner case heating unevenly from the bottom?. Or is it just the fact that, say, water burns off the top of the 7-11 coffee and leaves the system that accounts for the different effects?

    As another example, say you are cooking 2 roasts and get the internal temperature on both up to 350 degrees to the point where they are both nice and juicy. Now, you leave one in the oven at 350 degrees and you take the other out and put it into a perfect thermos. You leave both roasts in this state for the next 12 hours. What do you have now? We can bet that the roast that stayed in the oven is going to be burned to a crisp. But what about the roast in the perfect thermos? Ostensibly, its internal temperature has stayed at 350 degrees the entire time just as the roast in the oven. So hasn't it "cooked" just as much as the roast in the oven? What is the difference and why?
     
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  3. Apr 25, 2013 #2

    sophiecentaur

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    It's not just a matter of 'keeping it hot' by insulation. Cooking a roast involves bringing the temperature of a what is the equivalent of several kg of water from around 20C to around 80C average (and that's a medium roast). This takes a while because meat is an insulator. Your thermos would need to hold many kg of water at 100C in order to hold enough energy for this (see 'method of mixtures'; beloved of A level students in the 1960s). Any less water would just cool down before the internal temperature was high enough.
    Then there is the problem that cooking is an endothermic reaction and actually 'consumes' energy.

    There is a similar situation with Microwave Ovens. They are dead quick at 'heating through' a small thermal load but a 2kg+ joint of beef will still take a longish time because the heat needs to get inside with only a moderate temperature difference (the waves don't penetrate very far, in fact).
     
  4. Apr 25, 2013 #3
    Cooking involves chemical reactions that consume heat reducing the temperature in the perfect thermos.
    So the temperature will drop.
    BTW a steak keeps cooking after being taken off the grill. A midium rare steak delivered too late to the table might end up as midium. which is why a smart cook will always under cook the steaks.

    Disclaimer: It is safer to eat meat well cooked.
     
    Last edited: Apr 25, 2013
  5. Apr 25, 2013 #4

    Danger

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    Yeah, but then you miss out on that whole "dead animal" ambience that makes dining a pleasure.
     
  6. Apr 26, 2013 #5
    That's exactly right, it keeps cooking, that's my point. And then it starts to stop cooking once it cools down to ambient temperature. This cooling down would not happen in a perfect thermos/insulator.

    Will it? In the example I gave above, the roasts were heated until they were a steady 350 degrees throughout. At this point, haven't we saturated any chemical properties of the meat/water that may absorb any ambient heat within its thermos container? I mean, once we've saturated its specific heat capacity, doesn't the mean kinetetic energy between the molecules remain constant? In a hypothetical perfect insulator, of course.
     
  7. Apr 26, 2013 #6
    Maybe the steak keeps cooking because there is still a temperature difference between the red inside and the cooked outside? By the time it gets to your plate, the outside cools off a bit (but remains cooked), having given up some of its heat to cooking the inside (and the air). I'm also assuming here that the color of the meat, or it's "cooked" value is directly proportional to how hot it is....
     
  8. Apr 26, 2013 #7

    sophiecentaur

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    Part of the reason for the cooling is not heat loss. It's an endothermic reaction - requiring heat. That will cool the meat, despite its being insulated.
     
  9. Apr 26, 2013 #8
    The thermos vs the hot plate also has the benefit of limiting water loss through vaporization and thereby reducing the coffee mixture's water content which will effect taste. If one could depressurize and remove water vapor in an isothermal process, from the heated coffee mixture, this might be a benefit to coffee flavor strength.
     
  10. Apr 26, 2013 #9

    sophiecentaur

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    Absolutely. Who needs Quorn, anyway?
     
  11. Apr 26, 2013 #10

    Borek

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    At least some of the compounds responsible for smell and taste are volatile. They won't run away from the sealed thermos, but they will get easily lost from the boiling pot (which you can't seal to not risk explosion).
     
  12. Apr 26, 2013 #11

    sophiecentaur

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    This is getting to be like an episode of Masterchef. Time to turn over.

    There must be some low temperature chemical / enzymal changes taking place. well after the cooking period. When I put the remaining half of a beef joint on the side. after carving (rare, usually, with bloody fluids oozing out), I notice that, by the end of the meal, the beef has cooled down considerably and the red/pink has changed to grey/brown - all the way through, so it can't just be surface oxidation.
     
  13. Apr 26, 2013 #12

    Danger

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    You continue to drive my Google-button into stress failure. Having looked it up, I can safely say that I probably wouldn't like it. (Although I do use artificially-bacon-flavoured soy bean bits for Caesar salads rather than the real thing because I'm lazy.)
     
  14. May 1, 2015 #13
    If you get the internal temp to 350F, at anywhere near earth's surface pressure (app. 15psi), the roast will be dry as a bone, pardon the expression. Liquid H2O can't exist above 212F, remember? Cook your roast at 350F until it is "nice and juicy", and the internal temp is around 165F. Now, if you increase the pressure, ala a pressure cooker, you can raise the H2O temp above 212F.
     
  15. May 2, 2015 #14

    sophiecentaur

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    The dryness or otherwise of meat will depend on the cooking time. How quickly the internal water gets eliminated will depend on the self insulation and the rate at which heat actually gets transferred in there - also how fast the vapour could escape. The experiment has not been described in great enough detail for a good prediction of the result. If the thermos has no heat source in it, apart from the hot (externally) meat then I can't see how it could ever end up over cooked and dry.
     
  16. May 2, 2015 #15

    russ_watters

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    Um....no. Meat with an internal temperature of 350 F is called "leather". Meat only gets cooked until the internal temperture is 140-170 F, depending on taste. It doesn't sit in the oven at a hot temperature. Again, cooking takes a long time because the meat is insulated: as soon as the internal temperature reaches the desired level, it is finished cooking.

    So because in cooking the meat never reaches an equilibrium, there is no way for a thermos to help.

    Now....then, there's crock pot cooking: crock pots are at least a little insulated.
     
  17. May 3, 2015 #16
    If you bring the internal temp of your roast to 350F, it's already burnt to a crisp, before it even gets to the perfect thermos. It contains no water, save perhaps some bound up in chemical reactions. The temp can only be raised that high because all the water is gone; boiling point 212F. "Nice and juicy" is a distant memory. You can test this, but you won't be able to use a meat thermometer. None will register that high.
     
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