1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B How can we melt ice in a microwave efficiently?

  1. Dec 2, 2017 #1

    I like Serena

    User Avatar
    Homework Helper

    I've noticed that when I put a prefab meal in the microwave, the parts that are frozen over just don't defrost any time soon, even though the rest of the meal gets really hot.
    The reason is that the ice structure doesn't allow the polar water molecules to vibrate.
    My question: what can we do so that the ice does defrost - and quickly?

    I am aware that a modern microwave has a defrost mode, which switches power on and off.
    Basically it just means that we give the ice time to absorb heat from liquid water.
    That works of course, but it's pretty slow, certainly if the frozen parts are big.

    So I was wondering, suppose we add a harmless extra substance to a prefab meal.
    One that also has polar molecules and that become embedded in the ice.
    As a candidate I've found ethanol, which has a much lower freezing point than water, so it can still be liquid inside the ice.
    Would that work to defrost the ice quickly?
    And since ethanol has a lower boiling point, isn't it even conceivable that it evaporates (so that we don't become 'drunk' while eating ;))?
    Are there other suitable polar molecule substances that might be used instead?
     
    Last edited: Dec 5, 2017
  2. jcsd
  3. Dec 2, 2017 #2

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    For speeding up the defrosting process without cooking / boiling small regions, there is a system by Pabasonic that's been called "Chaos". It uses random short bursts at full power instead of a continuous low power. I use it and it seems to work. Here is a rather ancient link about it. It's the random nature of the pulsing that does the biz, I believe.
    Mixing other substances with your food opens up a can of worms regarding health and safety. Ethanol gives good tax income and would put the price up.
    One possibility would be to use metal skewers inside the frozen item. It works well for cooking large 'jacket' potatoes and the skewers could be re-usable with home freezing.
     
  4. Dec 2, 2017 #3

    jim mcnamara

    User Avatar

    Staff: Mentor

    One partial answer: food processors (frozen dinner manufacturers) are very aware of the issue. They try to overcome it with packaging. There are products that have a (usually clear) plastic film that is a steam barrier. And they modify the heating instructions to partially to ask you to remove the film over (example) the dessert-like substance. Low-end products are deliberately deprived of this extra TLC, to save cost, and possibly to coerce you into buying the better product.
    Better = more expensive.

    Best possible answer: https://what-if.xkcd.com/131/
     
  5. Dec 2, 2017 #4

    DrGreg

    User Avatar
    Science Advisor
    Gold Member

    I wouldn't put anything metal in a microwave oven unless it's labelled as being safe for use in microwaves.
     
  6. Dec 2, 2017 #5

    Nidum

    User Avatar
    Science Advisor
    Gold Member

    Where the food is of a suitable type knead it for a few seconds before microwaving so as to break up the ice and generate lots of cracks and channels for hot liquid and steam to penetrate .
     
  7. Dec 2, 2017 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    It's not a magic process, you know. I think the only necessary thing is to ensure the skewer has a lowish resistance so that the waves are guided in rather than to cause local extreme heat or arcing. A silver wire (2mm diameter) worked very well for me and didn't produce any fireworks in potatoes.
    I remember having a Microwave oven (ITT, I think) that had a probe thermometer on a lead that you could insert into the food and the oven would turn off when the object was hot enough. Again, no arcing or excitement.
    The problem with conductors in microwave ovens is when they are physically thin and high resistance - like the metallic lines on some crockery. Local high currents along with high voltages is what does the damage.

    @Nidum: that's hard to do when the food is frozen solid.
     
  8. Dec 2, 2017 #7

    davenn

    User Avatar
    Science Advisor
    Gold Member


    Agreed ... as per the OP's first comments ......
    Think, frozen microwave dinners ..... plastic tray with a clear plastic top ... contents are frozen solid
    eg
    upload_2017-12-3_10-43-24.png

    Dave
     
  9. Dec 3, 2017 #8
    IIRC, you can get special mats for heating hard-frozen food that will absorb microwaves and get hot enough to thaw the pack from the base.

    Don't 'microwave from frozen' single-servings of fries (potato chips) use this trick via a metallised pad in the box ??

    Plan_B is to leave hard-frozen packs out for long enough for natural thawing to begin. Time to lay the table, set out plates etc, warn would-be diners, run to wash-room again...

    FWIW, when our microwave ovens are running on 'reduced power', they're providing full power at proportionately gapped intervals. Also, we put spacers under meal-packs to give all-around heating...

    And, yes, our oldest microwave oven has a plug-in thermometer, plus a 'zap until cooked' program...
     
  10. Dec 3, 2017 #9

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Apparently (So Panasonic and others claim) a random length and spacing of power pulses is more effective and provides more even defrosting. As I understanding, the absorption of water is different from the absorption of ice and that's the problem. It's important to keep the defrosting regions different sizes. Hence the Chaos idea.
     
  11. Dec 3, 2017 #10
    I've been saying this for years until one day one of our postgrads put one of those stackable aluminium pots (I forget what they are called) in the microwave. He ignored the howls of derision and heated his lunch. We all said it must be because it doesn't have a lid, he put on a lid and repeated the process. No problem. I'd like to hear an explanation for this. I know things like those twisty plastic things with wire in get extremely hot, red even, but large metal objects?

    Cheers
     
  12. Dec 3, 2017 #11

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Just having metal in the cavity is not a recipe for disaster. I have had two ovens with no turntable and they both allow the use of a metal shelf for two layers of food.
     
  13. Dec 4, 2017 #12

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    If microwave meals really aren't fast enough for your lifestyle best change your lifestyle :-)
     
  14. Dec 4, 2017 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    :smile:
     
  15. Dec 5, 2017 #14

    PeterO

    User Avatar
    Homework Helper

    We made an ice beaker (sides and base about 6 mm thick) , filled that ice beaker with tap water then placed it in a microwave for a few minutes. We boiled the water without melting the ice.
    That would indicate to me that there is no quick way to melt ice in a microwave.
    btw: I think a microwave causes the water molecules to spin, rather than vibrate - and in ice (a solid) the molecules can spin.
     
  16. Dec 5, 2017 #15

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Yes. The different absorptions account for the problem. I think the reason that the chaos works well is that the local regions where melting can happen are probably separated spatially by the mode stirring or the turntable. Having random pulses avoids the hot spots ever being in the same place during a rotation. The process is never as quick as heating water but it is made 'quicker' by this method. Your experiment was an extreme situation and the improvement, starting from deep frozen throughout, is probably more noticeable. (I liked your demo, though)
    I agree with CWatters about lifestyle, though! If you look at the cooking instructions forever cooking 'From chilled' and 'From Frozen', there is not a lot of difference in the time; barely a difference of one glass of wine (for the bon viveur) or an extra rant on PF.
     
  17. Dec 5, 2017 #16

    I like Serena

    User Avatar
    Homework Helper

    I can actually put many frozen meals in the microwave for 6 minutes and they are done.
    It's just that for a number of them that doesn't work due to the ice.
    Using the microwave to defrost already takes about 10-15 minutes and then it's still cold.
    Is that with the chaos theory solution @sophiecentaur? Or is my microwave just too old?
    My current solution is to heat it for 4 minutes, then break the ice as @Nidum suggested, and then finish it with another 2 minutes.

    And yes, I could change my lifestyle, but I'm still curious which solutions the manufacturers already apply.
    And I'm wondering why they haven't come up with e.g. an alcohol solution, which may be because of health and tax regulations as @sophiecentaur suggested.
    Or maybe it just doesn't work.

    So to test my alcohol theory, I've put 2 glasses in the freezer. One with whiskey (40%), and one with water.
    I intended to put both of them in the microwave to see the difference in melting.
    However, the whiskey never froze! Apparently the -15 degrees Celsius or whatnot is not sufficient to freeze it!
    It actually still tasted quite good. ;)
    So I'll retry after deluting it a bit.
     
    Last edited: Dec 5, 2017
  18. Dec 5, 2017 #17

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Our standard for frozen means (home made) is 5mins on 900W, decant onto plate/bowl, stir/spread out, 2mins on 900W.
     
  19. Dec 5, 2017 #18

    I like Serena

    User Avatar
    Homework Helper

    Now suppose we have a big turkey for Thanksgiving, which is frozen solid, and we didn't take it out of the freezer in a timely fashion.
    As I understand it, the recommendation is a couple of days to defrost depending on mass.
    How can we still get it prepared in a timely fashion?
    What if we had injected it beforehand with a couple of shots of whiskey or some such?
    Would we be able to defrost it significantly faster?
    It neatly circumvents any issues with food regulations and tax, and I wouldn't really mind if there's a slight whiskey taste to the turkey. It saves on the whiskey sauce after all. ;)
     
    Last edited: Dec 5, 2017
  20. Dec 5, 2017 #19

    I like Serena

    User Avatar
    Homework Helper

    Interesting. I wonder if that indeed means that my microwave is outdated.
    When I use its defrosting mode, I cannot actually set the power. I can only set the type of food and its mass.
    Oh wait! Does that mean that you do the same thing I already do? Just hit it with full power, break up the ice, and then hit it some more?
    I have 1000 W, which explains that I need 1 minute less.
     
  21. Dec 5, 2017 #20

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Chaos is a trade name, I think. It requires a more advanced power supply ("Inverter") than many ovens use. The pulses are quite short and it's much more complicated than the several seconds of ON-OFF on older ovens.
    Chaos comes into its own for large items, I think, in which you can get large regions of ice after a long cook time. That is a health risk sometimes. For small items (single person meals etc) the diffusion time for heating is quite short so it doesn't matter. I still think that local overheating could spoil some items. 1kW could cause a serious hot spot in meat which could toughen it up or dry it out.
    I seldom use chaos, preferring to take the frozen item out of the freezer and to let it defrost overnight. A large piece of meat is quite a challenge to defrost ASAP.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: How can we melt ice in a microwave efficiently?
  1. How does salt melt ice? (Replies: 14)

  2. How to melt ice? (Replies: 41)

Loading...