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Why does food get colder on the sides quicker than in the middle?

  1. Sep 8, 2012 #1
    i noticed that when food is spread in a plate dish and kept to cool,it cools down in the sides first and then in the middle.why is it so?is it because of the contact with edges of the dish?or is it due to the contact with air?please exaplain(it works with semi-solid food and with rice too.basically almost anything that can be spread)
     
  2. jcsd
  3. Sep 8, 2012 #2

    Ryan_m_b

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    Staff: Mentor

    It's all to do with the surface area. Food at the edge is open to the air more than food in the middle which mostly borders other hot food.
     
  4. Sep 8, 2012 #3
    Odd, I notice just the opposite. When I spread ice cream in a dish I notice the edges getting warmer first.
     
  5. Sep 8, 2012 #4
    There is a higher temperature gradient between the air on the outside and the food at the edge than there is between the food at the edge and the food at the center.

    This is true for hot solid foods as well as for ice cream.

    Therefore, the edge will have more heat transport than the center (whether in or out).
     
  6. Sep 9, 2012 #5
    thank you for answering!
    it seems that the food getting cooler and and ice-cream getting hotter are the same phenomena
     
  7. Sep 9, 2012 #6
    thank you for answering!what confuses me though is why is there a higher gradient between air and food on the edge when compared to middle
     
  8. Sep 9, 2012 #7
    is it possible that the rising hot air from the whole dish creates a column of hot air preventing cold air from coming,while the cold air enters from sides colling down the sides.
     
  9. Sep 9, 2012 #8
    similarly,in the case of ice-cream,a cold "pocket" of air should be formed(due to colder air being denser and settling down),while the sides aren't protected as much.this could be a possible explanation,but i already see holes in it.
     
  10. Sep 9, 2012 #9

    mfb

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    You have hot food and cold air - the gradient is large everywhere on the surface. However, your edge has more surface than the middle.

    That depends on the temperature and your room - convection tends to be a bit more effective at the sides (where cold air comes from), if your food is hot enough.


    In terms of ice, it is the same, just with "hot" and "cold" exchanged.
     
  11. Sep 9, 2012 #10
    i just tested my theory.i spread some rice across the dish and left a little circle
    (or hole for that matter) in the middle.the hot air would still show it's effect,which happened.the rice did not get colder in the middle
     
  12. Sep 9, 2012 #11
    The reason for the higher heat gradient between the air and the food surface is because the only way for heat to be transported from food to air is through radiation and convection.

    However, between the center of the food and the outside of the food, there's an additional mechanism for heat transport: conduction.

    The air is also a heat reservoir, which means that its overall size is so huge compared to the food, that any heat added to it results in a negligible change in its temperature, so it can always be "cold". However, the food is small, and is very hot, so it must cool. The difference between the center of the food and the edge of the food is realistically, not going to be big (lets say its boiling soup. then its gonna be around 100 degrees. Air's around 20 degrees).
     
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