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B How can we melt ice in a microwave efficiently?

  1. Dec 5, 2017 #21

    I like Serena

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    The several seconds of ON-OFF is indeed what my defrost mode does, and which I don't consider to be particularly effective, so I never use it.

    And yes, I can understand that in theory a chaotically pulsed mode could work, applying power in extreme bursts to break up the ice, such that (close to) full power is still applied.
    That is, if microwave manufacturers can get such a system to work while still applying full power.
    So now I wonder...
     
  2. Dec 5, 2017 #22

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    As I understand it, one metal is different from another.
    It's all about whether the metal responds to an electric field or not.
    Copper will definitely respond since it's a conductor, and it will short circuit the microwave.
    That's why we have aluminium plates that are suitable for a microwave to heat from the bottom up because aluminium has a low conductance. Low enough not to fry the microwave and high enough to collect heat.
    I think aluminium foil will also cause a short circuit, but I'm not exactly sure why. I'd be happy to see more opinions on that.
     
    Last edited: Dec 5, 2017
  3. Dec 5, 2017 #23

    sophiecentaur

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    Full power would cause local frying of the food after a very short time. A burst of full power (the optimum time would have been found experimentally) can do some melting of ice locally (this will happen in the antinodes of the standing wave pattern). Moving the pattern or the food will allow more average power to be supplied. Moving and pulsing can allow the mean power to be increased without damaging the food. Random is just a refinement, probably. I haven't read of anything more magical about the pulsing system but I suspect that short fast pulses would improve on what the old (relay based?) switching of the magnetron power supply.
    Alternatively, the food could be physically whizzed around and about to spread the effect of local heating in antinodes. The cavity would need bearings like a washing machine then.
     
  4. Dec 6, 2017 #24

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    I've completed my experiment.
    I've diluted my 40% whiskey to about 3% and froze it (stir and shake because otherwise it doesn't freeze at all).
    At about 3% it does freeze, and I've hit it with maximum power (1 kW).
    Turns out that after 1 minute there is still some ice, but after 1:30 all ice is gone.
    My preliminary conclusion: injecting a turkey with whiskey _will_ defrost it in a timely fashion, although some confirmation would still be good.
     
  5. Dec 6, 2017 #25

    sophiecentaur

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    This surprises me. I have tried keeping Vodka in a freezer and it just separates out, leaving a block of ice and a remain volume of liquid. Perhaps your experiment was not at as low a temperature as mine.
    Also, have you ever actually tried to inject a frozen turkey with anything? It's much the same as a block of granite as far as a needle is concerned.
     
  6. Dec 6, 2017 #26
    I think they mean before you freeze it...
     
  7. Dec 6, 2017 #27

    sophiecentaur

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    Yep, I know but we already discussed the tax issues of selling liquor along with food, earlier. :smile:
     
  8. Dec 8, 2017 #28

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    Just found this wiki article about freeze distillation.
    • Freezing in this scenario begins at a temperature significantly below 0 °C.
    • The first material to freeze is not the water, but a dilute solution of alcohol in water.
    • The liquid left behind is richer in alcohol, and as a consequence, further freezing would take place at progressively lower temperatures. The frozen material, while always poorer in alcohol than the (increasingly rich) liquid, becomes progressively richer in alcohol.
    • ...

    So some of it will separate and some of the alcohol will get frozen inside the ice.
    And freezing only even starts well below 0 °C as I've observed.

    To be honest, I was also wondering why liquor doesn't spontaneously separate through diffusion.
    I remember an experiment where we pour water carefully on top of colored alcohol, after which the alcohol spirals through the water to the top.
    I'm guessing that in a solution the alcohol is bound to the water molecules through their polar structures.
    And that would be exactly why mixing water with another polar substance should help to defrost in a microwave.

    Indeed I haven't.
    I guess we'll have to buy it at the source if we can. :rolleyes:
     
  9. Dec 8, 2017 #29
    Here's an idea: given how the lack of liquid water is the problem in the beginning, maybe you could take a thin piece of cloth, soak it in water, and place it on top of the ice. The cloth would get hot very fast, melting the ice below it, thus jump-starting the overall heating process.
     
  10. Dec 9, 2017 #30

    sophiecentaur

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    That is only half the problem solved. When you try to thaw out a large piece of frozen food, the first regions of water that have melted will continue to absorb the microwaves preferentially and you risk over cooking those regions whilst the frozen bits are still frozen. The outside would defrost slightly quicker with a 'water jacket' but that doesn't help with the requirement for uniform defrosting throughout.
    There is a Health and Safety issue, too. There is a risk that the last few regions to be defrosted will never be taken beyond the safe cooking temperature yet this would go undetected because all the food would 'taste' warm enough. This is why the instructions are always to serve the food "piping hot" - just to be sure.
    Patience is required with all food preparation.
     
  11. Dec 9, 2017 #31

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    I've repeated my experiment a bit more carefully, and a glass with water and a bit of Vodka defrosts uniformly and pretty quick, while a glass with just water doesn't defrost at all.
    Then I thought, let's give that defrosting mode of my microwave another chance.
    So I set it to defrost 20 kilograms of bread and applied it to the glass that was still almost fully frozen. It showed that this would take 37 minutes.
    Interestingly, within a couple of minutes the ice was defrosted, while the liquid water didn't really get warm.
    For reference, my microwave is about 20 years old.
     
  12. Dec 9, 2017 #32

    sophiecentaur

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    But it's defrosted already !!?
    "Almost fully frozen" isn't 'deep frozen' (-20°C). You should aim at your experiment mimicking realistic conditions
    Look up the Latent Heat of fusion of water and compare it with the Specific Heat of water at different temperatures (above and below 0°C) for some interesting thoughts. This link is chatty but has some facts about specific heat.
     
  13. Dec 10, 2017 at 12:50 PM #33

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    Yeah, I just skipped over mentioning that both glasses were deep frozen before putting them in the microwave.

    Indeed. I only used the defrosting mode as an afterthought. And it takes a bit of time to deep freeze after all.

    Anyway. just now I've repeated the experiment with one glass of water with a bit of Vodka (~5% alcohol) and one glass with only water.
    First deep freeze, then put in the microwave and use its defrosting mode for 20 kilograms of bread.

    Now I see the exact same effect as just using the microwave at full power.
    The glass with frozen water/Vodka defrosts uniformly within 1.5 minutes and becomes warm afterwards.
    The glass with just ice still did not noticeably defrost after 6 minutes. Afterwards it starts melting from the outside in.
    Apparently we first had to wait for some water to form on the outside, which could then heat up.
     
  14. Dec 10, 2017 at 3:09 PM #34

    sophiecentaur

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    Right. I get it. :smile: Full marks for your attempt but, unfortunately you have raised another set of questions. Ain't that just the way?
    It's hard to make sense of the results, of course, because the amount of actual power that's transferred will depend on how well the Matching is achieved. The "full power" would have been the setting and not the actual amount of RF Power.
    It would be interesting if the cavity could be tuned to the individual load but, of course, a domestic oven is One Size Fits All for its tuning and it will also be tuned to avoid too high a standing wave ratio which could damage the Magnetron. I guess a possible improvement could be to have some form of automatic matching but that could be expensive and just another thing to go wrong. They are remarkably reliable devices and I have never (in several decades) never had a failed magnetron. It's silly things like dodgy door catches and turntable motors that have let them down.
     
  15. Dec 10, 2017 at 9:55 PM #35
    "The cavity would need bearings like a washing machine then." What about a modest turn-table ? Or the heating pad previously mentioned ??

    Our clever oven with the temperature probe has the problem that the probe will NOT go into hard-frozen food. Forward-planning required. Or a 'we deliver' meal...
     
  16. Dec 11, 2017 at 5:05 AM #36

    sophiecentaur

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    IMO, a microwave oven is most suitable for essentially small items which can be cooked evenly and faster than in a regular oven. If you want to cook something large, like a joint of meat, even when it is defrosted, the cooking time soon approaches that of a regular oven. This is because the microwaves don't actually penetrate all that far into a dense medium like meat. You still need time for the hot outer layers to heat up the inside. Also, the important 'resting time' after the heating will extend the real total time needed.
    Whilst I agree it's a very interesting Physics conversation topic, the problem of defrosting large items need never arise in a reasonably well run lifestyle. Taking a frozen leg of lamb out of the freezer twenty four hours before you need it is well within the capabilities of most PF contributors. :smile: If you are doing a meal for many people then it's not good to buy everything for it in a final rush so a bit of planning can take care of all this. A pack of bacon will defrost more evenly in a bowl of very hot water than with a "short burst in the Microwave"
    Our microwave oven is never used in our house for TV dinners but almost every day the oven is used for cooking veg or pre-cooking a hot pot or potatoes and it's better at that than the traditional pressure cooker.
     
  17. Dec 11, 2017 at 8:35 PM #37
    As you mention, the inhomogeneous thawing is always an issue though, right? That is, if you put frozen food into a microwave, some pocket will thaw first, and then you have a runaway effect in that pocket because it absorbs the microwaves much more efficient. So, some part will always be overcooked, and some part only barely.
    My suggestion simply jump-starts the "pockets" a bit faster :)
     
  18. Dec 11, 2017 at 8:55 PM #38

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    One more observation about defrosting ice in a glass in a microwave.
    It defrosts centrally from the top and simultaneously from the bottom, leaving an ice cylinder (with a slightly conic form) that has a reduced height.
    Apparently that's because some water is trapped centrally at the top, boring a hole.
    And the ice starts to float on the rest of the water that heats it from below.

    This does not happen if we previously added a drop of Vodka to it.
    Then it just defrosts uniformly.

    Indeed. Mine is the first one I have and is over 20 years old, and the only thing wrong with it, is that the plastic of the door has broken, which I duck taped.
    And the door tends to catch, probably because the plastic was broken.
    Otherwise it still works like a charm!
    Even the turn table shows no sign of mechanical wear (although I can't quite get it clean any more).

    Lessons learned:
    - be very careful when opening and closing the door, since this is the most fragile and only fragile part,
    - don't drop the turn table plate on the floor (I inherited a microwave that had a broken plate),
    - always cover the turn table with some kind of suitable paper to prevent it from getting unscrubbably dirty,
    - always cover whatever you heat up with a lid (like a ceramic plate) to prevent the insides of the microwave from getting dirty - it's not just eggs that can supposedly explode.
     
    Last edited: Dec 11, 2017 at 10:01 PM
  19. Dec 11, 2017 at 9:06 PM #39

    Mister T

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    My technique for fast thawing larger chunks of frozen meat.
    1. Place in microwave for a short time on the defrost cycle (I have an inverter) but remove while still frozen solid.
    2. Place in a shallow water bath.
    3. Place heat sinks (salvaged from old computers) on top of the meat.
    4. Flex the meat as it thaws.

    You can use any or all of these techniques depending on how fast you want the process to go. For example, place the frozen meat in the refrigerator the night before with the heat sinks resting on top. It will be thawed the next day and will never have gotten warm.
     
  20. Dec 12, 2017 at 12:10 PM #40

    sophiecentaur

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    There is no substitute for taking your time. There are not many meals that you cannot put off for another hour. If it's a problem then cook something else. The microwave oven is excellent for small portions.
     
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