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Why is food cooked differently by microwaves than by an infra-red grill?

  1. Jun 30, 2008 #1
    And why can't food be cooked with X-rays or radio waves?
     
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  3. Jun 30, 2008 #2

    Mentz114

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    The microwave oven is cooking food with radio waves. The frequency is chosen to resonate with something in all food ( water ?) so the food heats up throughout as energy is absorbed and becomes heat.

    The infra-red grill creates a lot of infra-red radiation which heats the food from the outside.

    I'm no chef but that's my take on it.
     
  4. Jun 30, 2008 #3

    G01

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    Yes, the frequency of the microwaves emitted by a microwave oven are chosen to resonate with water molecules. Thus, in effect, microwave ovens only heat up the water in the food and the water transfers the heat to the rest of the particles in the food by conduction.

    The resonant frequency of water is chosen because water is such a large component of anything we eat. So, it makes sense to use a frequency that heats up water. This is why we choose this specific microwave frequency to cook food.
     
  5. Jun 30, 2008 #4

    symbolipoint

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    Microwave into foods means the water brings up the temperature to 100 C, maximum (maybe some steam, but not much higher than 100 C).

    Infra-Red can be much higher than the boiling point of water; maybe reaching 300 to 500 F (not C). This can penetrate food from the outside, and depening on time and temperature, can give anything from caramelization, to burning.
     
  6. Jun 30, 2008 #5
    Microwaves are radio waves. In the case of microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz (2.5 gigahertz). Radio waves in this frequency range have an interesting property: they are absorbed by water, fats and sugars. When they are absorbed they are converted directly into atomic motion -- heat. Microwaves in this frequency range have another interesting property: they are not absorbed by most plastics, glass or ceramics. Metal reflects microwaves, which is why metal pans do not work well in a microwave oven.


    From: howstuffworks.com
    http://home.howstuffworks.com/microwave1.htm
     
  7. Jun 30, 2008 #6

    Integral

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    So it has been pretty well established in this thread that microwaves are RF energy which excites a vibrational mode in the water molecule. Further it has a "skin depth", that is it penetrates some distance into the material being heated. How far it penetrates depends upon the water content and density of the food.

    One major difference for cooking is that the outer surface of microwaved food is not heated, in fact it remains in contact with nearly room temperature air so is a heat loss surface instead of a heat gain surface. The layers of food beneath the surface are hotter then the surface, this is why microwaved raw meat is nearly inedible, meat is best with outer layer cooked first traping the jucies inside.
     
  8. Jul 1, 2008 #7
    Also, if you put some food on a plate and put it in the microwave, after the food is heated (often too hot to touch), the plate is usually still cold and you can grab it right out. This is because the microwaves don't affect the ceramic of the plate, but they do affect the water in your food.
    If you try to do this in an infra-red grill you will most definitely get burned quite badly!
     
  9. Jul 1, 2008 #8

    DaveC426913

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    I don't understand why I can pop a dish of frostily frozen fruit (strawberries and blueberries) into the microwave for 45s and have it come out still covered in ice crystals.

    I know mw ovens don't heat completely evenly- even if, like mine, they have a rotating tray - but how can ice crystals completely escape melting?
     
  10. Jul 1, 2008 #9

    mgb_phys

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    Ice is almost transparent to microwaves, the bond resonance in ice is very differnet to the bond in water. The defrost setting on your microwave actually turns the power on and off for 5-10secs, allowing the microwave generated heat in the melted part to transmit to the still frozen part.
     
  11. Jul 2, 2008 #10

    DaveC426913

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    This I knew - though I'd assumed the idea was to try to not actually cook the food before it had had a chance to thaw.

    But this I did not know!

    The relevant bonds are the ones between H and O in the molecule aren't they? So, in ice these bonds are constrained by the lattice and that changes their resonance?
     
  12. Jul 2, 2008 #11

    Defennder

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    Does resonance here mean the same thing as "delocalization"?
     
  13. Jul 2, 2008 #12
    Isn't the ice thing simply because the water molecules in a liquid are allowed to rotate, while the water molecules in (solid) ice cannot rotate? I thought the rotation was what generated the heat, or is this just a simplification of what really happens?
     
  14. Jul 2, 2008 #13

    dst

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    Resonance means exactly that, the molecules vibrate/resonate because they have the energy necessary to do so.

    Delocalization is something very very different.
     
    Last edited: Jul 2, 2008
  15. Jul 2, 2008 #14

    mgb_phys

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    Resonance is the natural frequency at which a bond vibrates, if you apply a field at or near this frequency the energy will be most efficently absorbed.
    Delocalisation refers to electrons which are not strongly attached to a particular atom, such as those in a metal that form the sea of electrons that allow metals to conduct electricity.

    It's not the OH bond inside the water molecule that is excited at microwave frequencies (that is much shorter/stronger and absorbs in the visible/IR) it's the longer weaker hydrogen bond between adjacent water molecules.
    This is why the response is so different in liquid water, where this bond is free to continually conect and disconnect to other water molecules, and ice where there are always 4(?) rigid bonds to the nearest neighbours in the crystal.
     
  16. Jul 2, 2008 #15
    The way microwaves work is by hitting a resonance frequency of water, I forget which one it is but I don't think it's the first. It would be possible to hit another resonance with x-rays but it is easier to hit lower resonances.

    My adviser was doing an experiment to test if ice would melt in a microwave. It was quite a challenge to get a freezer large enough and cold enough for the microwave to fit in and for the ice to stay solid. When he finally got what he needed, the ice didn't melt. Fifteen minutes in the microwave and not a drop of water, pretty amazing.
     
  17. Jul 2, 2008 #16

    DaveC426913

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    If he was putting the ice in the microwave, why did he have to put the microwave in the freezer?
     
  18. Jul 2, 2008 #17
    Because ice melts in room temperature... If you want to show that a microwave doesn't melt ice then you can't just put a block of ice in, see it melt and then say "well that was because the room is hot enough for the ice to melt, but the microwave didn't have anything to do with it, honest!"...
     
  19. Jul 2, 2008 #18

    DaveC426913

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    So he had to cool the air that was inside the microwave, and, I suppose, the inner walls of the oven itself. OK.
     
  20. Jul 2, 2008 #19
    A microwave is like a radar device. It has a magnetron tube and the inside is a faraday cage. It was discovered when they were doing research on radar and found that it melted a chocolate bar. It also has the health hazards of radar, especially if the seal gets damaged.
    http://www.gallawa.com/microtech/how_work.html

    There is plenty of misinformation about microwave ovens around, such that it cooks from inside out etc. Some of which has been repeated on this forum:

    The frequency is chosen to resonate with something in all food ( water ?)
    It is not at the natural frequency of water. “ Microwave heating is sometimes explained as a rotational resonance of water molecules, but this is incorrect: such resonance only occurs in water vapor at much higher frequencies, at about 20 gigahertz. Moreover, large industrial/commercial microwave ovens operating at the common large industrial-oven microwave heating frequency of 915 MHz (0.915 GHz), also heat water and food perfectly well.”
    http://en.wikipedia.org/wiki/Microwave_oven

    The frequency is the one given them by ISM as explained here
    http://en.wikipedia.org/wiki/ISM_band

    Microwave into foods means the water brings up the temperature to 100 C, maximum (maybe some steam, but not much higher than 100 C).
    Wrong! Under the right conditions a microwave can heat water above 100 degrees C, called superheating. It puts people into the emerg every year with bad burns. Also a microwave can superheat cooking oil and other liquids if given the chance.
    http://www.phys.unsw.edu.au/~jw/superheating.html
    Meat does not need to be heated above 100˚C to cook. Our meat thermometer for a convection oven shows a maximum temperature of 83˚C for lamb and poultry.

    A microwave heats other than water based products. EG Vinegar, cooking oil and even non-liquids.
    Other uses:
    If all you can find is a ballpoint pen that won’t write, you can put the ink part into the oven for a few seconds and bring it back to life.
    If you have some tape or envelopes etc that the glue has dried up, you can revive them in the microwave.
     
  21. Jul 2, 2008 #20

    mgb_phys

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    Sorry I didn't make that clear - microwaves don't use the water resonance.
    the frequency of domestic microwaves is around 1.8-2.9GHz this is a combination of a an efficent magnetron, a convenient size for the cavity and screening and because at the time it wasn't near any important radio bands. Unfortunately the 2.4Ghz for low power bluetooth and Wifi is now right in the middle.

    Good description of the microwave properties of water http://www.lsbu.ac.uk/water/microwave.html

    A microwave heats liquids by pretty much the same mechanism as water, although most things you would put in a domestic microwave contain a lot of water anyway, anything with a dipole moment will react in the same way.

    There are some interesting applications in metalurgy to sinter metals and ceramics by grinding the particles so that they are so much smaller than the uwave wavelength that the particle behaves more a like a dipole than a bulk conductor. Because the power is efficently absorbed through the sample it melts instanatly - you can make some allows this way that would be impossible with bulk melting.
     
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