How can we melt ice in a microwave efficiently?

[I like Serena]

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.

 

[sophiecentaur]

while a glass with just water doesn't defrost at all.

But it's defrosted already !?
"Almost fully frozen" isn't 'deep frozen' (-20°C). You should aim at your experiment mimicking realistic conditions

Interestingly, within a couple of minutes the ice was defrosted, while the liquid water didn't really get warm.

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.

 

[I like Serena]

But it's defrosted already !?

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

"Almost fully frozen" isn't 'deep frozen' (-20°C). You should aim at your experiment mimicking realistic conditions.

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.

 

[sophiecentaur]

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

Right. I get it. 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.

 

[Nik_2213]

"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...

 

[sophiecentaur]

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. 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.

 

[rumborak]

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.

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 :)

 

[I like Serena]

Whilst I agree it's a very interesting Physics conversation topic

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.

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.

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.

 

[Herman Trivilino]

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.

 

[sophiecentaur]

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.

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.

 

[I like Serena]

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.

I'm trying to get as close as I can get to a replicator, which I consider to be pretty cool. ;)

In Star Trek a replicator is a machine capable of creating (and recycling) objects. Replicators were originally seen used to synthesize meals on demand, but in later series they took on many other uses.​

A microwave that prepares almost any meal for 1-2 persons in ~6 minutes is as close as I've been able get.
Unfortunately, I still have to put a selected set of raw materials into the machine myself, and I have to select the mode (micro, crisp, forced air), the power, and the time.
I've kind of accepted those limitations, but that I have to interrupt the process, massage it a bit, and then continue it, is a pretty bad user experience!

I do seem to recall that doctor McCoy was usually unhappy with the quality of the replicator. He somehow seemed to be convinced that everything had to be done old-school or it wouldn't be right.

Oh, and as for small portions of frozen meat, I found that hitting it with full power for about 2 minutes, followed by drying it, applying pepper to it, and putting it for 1 minute to each side into a baking pan (induction, the counterpart of microwave!), works pretty well!

 

[sophiecentaur]

I have to agree with 'Bones' on that point. Freezing never leaves food the same.
A replicator meal from the microwave could be a good fun item at a party of Startrek enthusiasts but not to a gourmet party, I' afraid.
You will just have to go out in the woods and shoot your own Wild Boar and then cook it on an open barbecue, I'm afraid.

 

[Spinnor]

Are you following the directions on the box?

We cook large amounts of food and then freeze it in quart size Chinese soup containers. Two minutes in the microwave at high and a full quart of frozen food, about 2 pounds, is softened enough to use a pointy knife to break out a meals worth of food into a bowl. Another 3 minutes on high and stew or meatballs and sauce or chili is ready to eat.

Ingredients:
Ingredients: Gravy (Water, Mushrooms, Onions, Modified Corn Starch, Cooking Sherry Wine[Sherry Wine, Salt], Flavor [Salt, Beef Stock, Beef Fat, Maltodextrin, Potassium Chloride, Hydrolized Soy & Corn & Wheat Gluten Protein, Yeast Extract, Vegetable Stock (Carrot, Onion, Celery), Natural and Artificial Flavors, Potassium Lactate, Onion Powder, Dextrose, Sugar, Disodium Inosinate, Disodium Guanylate, Soy Sauce (Soybeans, Salt, Wheat), Modified Corn Starch, Tomato Powder, Citric Acid], Enriched Wheat Flour [Wheat Flour, Niacin, Reduced Iron, Thiamine Mononitrate, Riboflavin, Folic Acid], Onion Powder, Sugar, Cream Powder Blend [Heavy Cream, Whey Protein Concentrate, Whey], Caramel Color, Salt, Yeast Extract, Spice), Cooked Salisbury Steak Patty (Beef, Water, Pork, Textured Soy Protein Concentrate with Caramel Color, Bread Crumbs [Enriched Bleached Wheat Flour (Enriched with Niacin, Ferrous Sulfate, Thiamine Mononitrate, Riboflavin, Folic Acid], Dextrose, Salt, Yeast], Soy Protein Concentrate, Salt, Dehydrated Onion, Grill Flavor [Grill Flavor (From Sunflower Oil)], Mesquite Smoke Flavor [Salt, Smoke Flavor], Caramel Color, Sodium Phosphates, Dried Garlic, Spice Extract, Eggs, Spice), Mashed Potatoes (Water, Dehydrated Potato Flakes (Potatoes, Mono and Diglycerides, Sodium Acid Pyrophosphate, Citric Acid), Seasoning Sauce (Soybean Oil, Water, Mono and Diglycerides with BHT and Citric Acid to protect flavor, Beta-Carotene for color (Corn Oil, DL-Alpha-Tocopherol)). Contains less than 2% of: Salt, Dried Dairy Blend (Whey, Calcium Caseinate)), Green Beans, Brownie (Sugar, Water, Enriched Wheat Flour [Wheat Flour, Niacin, Reduced Iron, Thiamine Mononitrate, Riboflavin, Folic Acid], Soybean Oil, Cocoa, Eggs, Seasoning Sauce [Soybean Oil, Water, Mono and Diglycerides with BHT and Citric Acid to protect flavor, Beta-Carotene for color (Corn Oil, DL-Alpha-Tocopherol)], Acacia and Xanthan Gums, Sodium Bicarbonate [Hydrogenated Vegetable Oil], Salt, Natural and Artificial Flavor [Water, Propylene Glycol, Ethanol, Natural and Artificial Flavor, Caramel Color, Vanilla Extractives]), Seasoning Sauce (Water, Sugar, Seasoning Sauce (Soybean Oil, Water, Mono and Diglycerides with BHT and Citric Acid to protect flavor, Beta-Carotene for color [Corn Oil, DL-Alpha-Tocopherol]), Salt, Soybean Oil), Contains 2% or less of: Seasoning Sauce (Soybean Oil, Water, Mono and Diglycerides with BHT and Citric Acid to protect flavor, Beta-Carotene for color [Corn Oil, DL-Alpha-Tocopherol])Directions:
Instructions: Cooking Instructions - General - For food safety & quality follow these instructions carefully: Keep frozen - Do not thaw. Cook thoroughly. Check product temperature. Internal temperature must reach at least 165F as measured by a food thermometer taken in several spots. Promptly refrigerate any leftovers. Do not use in toaster oven. Do not reuse tray. Microwave - From Frozen - The following cooking directions were developed using an 1100-watt microwave oven. Since microwave oven wattage varies, cooking times may require adjusting. Microwave oven. 1. Remove film from brownie. Cut slit in film over each remaining item. 2. Microwave on High 4 1/2 minutes. Carefully remove brownie with a fork; set aside. Turn back film; rearrange steak and stir potatoes. Replace film. Return tray to microwave oven. 3. Microwave on High 2 1/2 minutes. 4. Let stand 1 minute in microwave oven. Stir potatoes. Oven cook - From Frozen - Conventional oven. 1. Preheat oven to 350F. 2. Remove film from brownie. Cut slit in film over each remaining item. 3. Place tray on baking sheet on middle oven rack. Cook 35 to 40 minutes. 4. Carefully remove baking sheet with tray from oven; let stand on baking sheet 1 to 2 minutes. Stir potatoes. Cook thoroughly.

About 8 minutes, my dinner is ready quicker.

 

[scottdave]

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.

Interesting article.

 

[Herman Trivilino]

A microwave that prepares almost any meal for 1-2 persons in ~6 minutes is as close as I've been able get.

Have you given any thought to the quality of the food you're able to "replicate" in this way? Abandon such concerns altogether (not a big leap from where you are now) and a 3D printer seems a good option.

I have to agree with 'Bones' on that point. Freezing never leaves food the same.

Alton Brown dedicated an entire episode of "Good Eats" to this topic, and it seems the secrets to success are the fast-freezing industrial processes. He does a fair job of explaining why. I try to place the food on a wire rack shelf touching nothing else so it freezes faster. Foods that tend to get freezer burn can be double- or even triple-wrapped.

 

[sophiecentaur]

I know that many people hate cooking so much and say they are just too busy. They either buy all frozen meals or make enough for several meals at one go and freeze them. Frankly, even when I was working (teaching), I found that cooking the evening meal was a relaxing task and was an ideal way to wind down. The shop - freezer - microwave pathway is not particularly fulfilling. Food prep is a bit of a paper tiger, once you are used to it and it's particularly nice when someone else is benefitting from it. Freezers are a brilliant invention but a bit over-used imo.

 

[Spinnor]

make enough for several meals at one go and freeze them

To each his or her own! Last year we cooked an extra turkey. Deboned the meat, cooked a large batch of gravy and stored about 15 pounds of turkey and gravy. When reheated tasted better then Thanksgivings meal. I like to save time for other things and my taste buds and belly are quite satisfied. Did the same with lasagna, cooked several large batches and made individual meal sized portions, great for lunch or dinner. Efficient use of time in my opinion. And of course everything was reheated quickly with microwave photons.

Lately, I do the same thing with carrots. Carrots are cheaper when bought in 5 pound bags. I put aside enough raw carrots for a week of crunching and chop and cook the rest, cool and bag them. Just added frozen carrots to today's frozen lunch.

 

[sophiecentaur]

To each his or her own!

Absolutely.
There is a sort of dichotomy when it comes to the marketing of food. On the one hand, shoppers are obsessed with buying cosmetically perfect veg and fruit yet, at the same time, there are a vast amount of ready meals bought, which will almost certainly be made with ingredients which have been sourced on the basis of price and which won't include 'perfect' items. But the customers don't get to see that. I guess it takes care of all the stuff which the farmers can produce.
But buying ready meals was not what you were advocating, I know.

 

[Spinnor]

But buying ready meals was not what you were advocating.

Passed along what works for me. Only thing I buy in the frozen section is ice cream from time to time. My frozen lunch is quite warm now, could have used some frozen shreaded cheese.

 

[Delta2]

Where can I find in more detail as to why the ice molecules do not absorb microwave energy as good as the liquid water molecules?

 

[Herman Trivilino]

Where can I find in more detail as to why the ice molecules do not absorb microwave energy as good as the liquid water molecules?

A molecule that's bonded to other molecules is not as free to move as one that isn't.

 

[I like Serena]

A molecule that's bonded to other molecules is not as free to move as one that isn't.

Alcohol molecules do bond to water molecules in liquid form, which is why they don't easily separate when boiling or freezing.
However, when freezing, these polar molecules become 'stuck' in a grid, which limits their degrees of freedom (their ability to rotate).
Consequently, the microwave radiation has significantly less effect.

 

[Delta2]

However, when freezing, these polar molecules become 'stuck' in a grid, which limits their degrees of freedom (their ability to rotate).
Consequently, the microwave radiation has significantly less effect.

The way you stating this makes me think like "Oh the microwave photons transfer less energy to the polar molecules" but I don't think its that ( I am not sure anyway if you mean this), it is that the polar molecules need more energy (more photons or more energy per photon) to get them moving. And because microwave ovens at the current era are not sophisticated enough to focus more energy in the frozen areas(they would have to have some sort of image analysing to spot the frozen areas and laser beams to focus the energy in the frozen areas), the energy goes randomly inside the oven, so if you try to offer high energy you might defrost the frozen areas but you ll burn the rest of the food, if you don't offer high energy, the frozen doesn't melt good but the rest of the food gets warmed..

 

[collinsmark]

Perhaps a simple way of looking at it is that ice (solid water) is somewhat translucent in the microwave frequencies, while liquid water is very opaque. So why are some things opaque (at some particular frequency) while other things are transparent/translucent? That's a big can of worms. Here is my attempt at a simple explanation.

Microwave waves will create torques on the ice molecules causing them to temporarily absorb energy (imagine a molecule to rotate a little bit on a tight spring), but because they are trapped in a restricted structure, they then release that energy coherently with the microwave wave on the rebound. All the other molecules do this too, in unison (mostly). Energy goes in coherently and comes out (mostly) coherently because most of the ice molecules are acting together. (And, for the most part, the motion of the ice molecules in the presence of the microwave energy don't seem to matter if you view it forward or backward in time. More on this later.)

Imagine an array of buoys floating on the water. As a water wave travel's under it, the buoys rise up absorbing some energy (gravitational potential energy), but then they give that energy back coherently (mostly) as they fall on the other side of the wave. In a sense, the buoy array was transparent to the water wave. If you were to film the wave passing under the buoys, and then play it backwards, it would look pretty much identical to a wave traveling in the other direction. If you didn't know any better, you wouldn't be able to tell which way was forward in time and which way was backwards.

On the other hand, if the wave imparts energy in a non-coherent fashion, like a wave crashing on the rocks, it destroys the coherence, and thus the wave. Play that one backwards and its obvious which is forward and which is not. This water crashing onto the rocks is a thermodynamically irreversible process.

Liquid water molecules are free to rotate. That alone wouldn't cause decoherence; but higher-energy, rotating molecules bumping into other molecules certainly would. The chaos involved in spinning molecules bumping into each other, and imparting all sorts of chaotic behavior as a result, is most certainly thermodynamically irreversible. And that kills the coherency of the microwave wave. Thus energy that was once in the form of microwaves is converted into more random thermal energy in the water.

Ignoring reflections, this ratio of coherence to decoherence of a given material to a particular wave is one way to conceptualize transparency.

---

I should mention that liquids do not need to be polar to absorb microwaves. There are other modes of energy transfer. For example, oil molecules are not polar. But those molecules are long and complicated and they can vibrate in several modes, many of which are at microwave frequencies. Given the complicated nature of these molecules in relationship to nearby oil molecules, and even other atoms in a given molecule, their vibrational modes are rarely coherent with each other. Since the long and twisted oil molecules don't all vibrate in unison, they are bound to absorb the microwave energy in a thermodynamically irreversible process too.

 

[I like Serena]

The way you stating this makes me think like "Oh the microwave photons transfer less energy to the polar molecules" but I don't think its that ( I am not sure anyway if you mean this), it is that the polar molecules need more energy (more photons or more energy per photon) to get them moving.

I imagine that it's like this.

Consider a block on the ground.
When we push against it, it starts moving.
That is, we applied work on it, and now it has more energy.
Now consider a block that is stuck somehow.
We push against it, but it cannot move. Therefore we did not apply work and it did not gain energy.

Hmm... I guess we'll have to push real hard so that something breaks before we can actually apply work to it. And then it should get the full energy. ;)
Still, continuing the analogy, if we simultaneously push both loose blocks and stuck blocks, the loose blocks will get all the energy won't they?

I should mention that liquids do not need to be polar to absorb microwaves. There are other modes of energy transfer. For example, oil molecules are not polar. But those molecules are long and complicated and they can vibrate in several modes, many of which are at microwave frequencies. Given the complicated nature of these molecules in relationship to nearby oil molecules, and even other atoms in a given molecule, their vibrational modes are rarely coherent with each other. Since the long and twisted oil molecules don't all vibrate in unison, they are bound to absorb the microwave energy in a thermodynamically irreversible process too.

Aha! So mixing the water with a couple of drops of oil might have the same effect as mixing it with alcohol?
That sounds as if that won't hit the brick wall of food and tax regulations.
If hope it freezes without fully separating.
(Running to the fridge and putting 3 new glasses in it.)

EDIT: Yep. That seems to be a problem. I can't seem to mix oil and water without it separating again within seconds.

 

[sophiecentaur]

I can't seem to mix oil and water without it separating again within seconds

There are Emulsifiers which can help there. Look on the contents labels of many prepared foods (sauces in particular) and the word "Emulsifiers" will be there. It won't be straightforward because they need to be appropriate. They can taste and may separate out during the deep freezing. Looks like you may have suddenly found another vast area of research you need to do.

 

[rumborak]

I find the "ice doesn't heat easily because the molecules can't move as much" theory unconvincing. After all, this isn't too unlike a spring system; even if the spring is more rigid it can still absorb just as much energy. Just because something moves less doesn't mean it contains less energy.

One creeping suspicion I have is that people think ice has just one temperature, whereas an ice cube can be -1C or -100C. They look the same, but the time to melt it will be vastly different.

 

[jerromyjon]

Aha! So mixing the water with a couple of drops of oil might have the same effect as mixing it with alcohol?

Oil and water don't mix but a layer of oil (as in under the food container) might create a buffer to disperse the energy better. I think that is the main problem is that the energy is distributed unevenly.

 

[collinsmark]

Glass apparently has similar behavior to water in the microwave (transparent at microwave frequencies when solid, opaque when liquid).

Steve Mould has a pretty good video on this -- except for the part where he insists that molecules must be polar to be heated, which isn't always a necessary requirement.

 

[Tom.G]

Where can I find in more detail as to why the ice molecules do not absorb microwave energy as good as the liquid water molecules?

The loss factor (higher for water) and penetration depth (deeper as temperature rises) seem to be the significant variables. Note how they differ between water an ice and how they vary with temperature.

https://arxiv.org/ftp/arxiv/papers/1308/1308.1229.pdf
http://www1.lsbu.ac.uk/water/microwave_water.html
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC285670/pdf/pnas00146-0047.pdf

EDIT:[/color] One of those references mentioned that the heating mechanism can be explained entirely by Proton exchange between atoms, for both Ice and Water.