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B Lower microwave absorption efficiency to increase penetration (cooking)

  1. Mar 16, 2017 #1
    A problem I encounter when microwaving food is that the centre is no penetrated by the microwaves so remains cool and I have to wait for the heat to be conducted through which takes a long time and undermines one of the advantages of microwaving which is the speed of the process. Would it make sense to have a microwave whose emission spectrum could be nudged a hair away from optimal absorption, allowing the microwaves to penetrate just a little deeper into the food? Ideally it could be controlled to match the nature of the food being heated. Obviously it shouldn't be possible to adjust it to the extent that it's just passing through the food.
     
  2. jcsd
  3. Mar 16, 2017 #2

    berkeman

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    Staff: Mentor

    Does your microwave oven have a built-in rotating platter? That is a big help in getting better uniformity in microwave cooking.

    http://microwaveturntableplate.com/wp-content/uploads/2014/01/microwave-oven.png
    microwave-oven.png
     
  4. Mar 16, 2017 #3
    Mine at home does but that doesn't penetrate a big, deep circular pot of nachos.

    The microwaves at work are of the catering type and don't have rotating platters; I'm not sure why. Presumably they have some other way of eliminating hotspots but the same problem applies; hot exterior and cold interior!
     
  5. Aug 16, 2017 #4
    For some products, cooking at a 70% to 80% power can assist.
     
  6. Aug 16, 2017 #5
    I already do limit power to permit more time for conduction of heat but the thing is, I'm not really interested in finding solutions that fudge existing designs into sort of working. I already know about these and wouldn't have come to a physics forum to ask! I want to know whether it's not feasible for a microwave to adjust the frequency ever so slightly that the microwaves penetrate deeper into the food (but not so much that they just pass right through, obviously). I don't know how the frequency of the waves generated is controlled and whether it's practical to vary them but I guess there aren't many people around who would really know the answer to how a microwave functions in such detail.
     
  7. Aug 16, 2017 #6

    berkeman

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    Of course there are plenty of folks here who understand how microwave generators are used in commercial microwave ovens. That's not really the issue here. And BTW making them chirp a bit in frequency would hopefully be fairly inexpensive, but would definitely add cost to the microwave generator.

    The harder question is whether some modulation of the wavelength of the microwave cooking radiation would improve the uniformity of the heating, allowing you to increase the power and decrease the cooking time, right? That's less of a physics/EE question, and more of a specialized material science question, it would seem....
     
  8. Aug 16, 2017 #7
    My vague understanding of the microwave is oven is that it targets one of the natural (resonant?) frequencies of the water molecule to enable efficient transfer of energy to those molecules. My understanding natura;/resonant frequencies is that they are the most efficient frequency to transfer energy to a system (the system the frequency applies to, so a water molecule). If the frequency is a little off then you still get energy transfer but less efficiently, right? If it's less efficient then where is the loss? I thought deeper into the food.

    I don't know how reliable this random page of information from 1965 is but it seems to suggest (along with an equation; two thirds of the way down) that penetration is determined by the frequency so I'd expect fancy microwaves for rich people to have such a function. Companies like to offer different models of machine to consumers so they can get the most money in exchange for similar goods and this seems like an obvious feature to include in a high-end model.
     
  9. Aug 17, 2017 #8

    DrClaude

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    That's actually not the case. The frequency used is nowhere near any resonance in water. (In addition, there would be no unique resonance frequency for all water molecules in different food stuffs.) The transfer of energy takes place through dielectric heating. You can have a look at Wikipedia for more details: https://en.wikipedia.org/wiki/Microwave_oven
     
  10. Aug 17, 2017 #9
    Find and read any basic information on microwave for food home or industrial use, and you will find that the frequencies are regulated.
     
  11. Aug 19, 2017 #10

    tech99

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    Gold Member

    The frequencies are regulated to prevent interference with radio services, but basically any frequency in the microwave region would be suitable for cooking - it does not depend on water resonances.
     
  12. Aug 19, 2017 #11
    I'm pretty sure I was taught that microwaves operate at resonant frequency of water at school but that shouldn't be surprising since I had a biology teacher who didn't know whether yeast was alive and a couple of chemistry teachers with way worse understanding than mine (I learned from the textbooks and not from them though; the same for my other subjects (except maths for which teaching was pretty solid) but some misinformation could still slip through).

    Thus, I don't suppose there's anything to be done about improving penetration by tweaking the frequency. I read that only two of the permitted frequencies are economical to produce but I wonder whether one penetrates deeper than the other.

    What about high-energy beta irradiation??
     
  13. Aug 19, 2017 #12

    davenn

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    yeah I was taught that bad science as well ..... it isn't true
    water molecule resonance is in the Terahertz, not in the low Gigahertz

    no, as you cannot "tweak" the freq anywhere near enough to make any significant difference

    Dave
     
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