Miracle Thaws are utensils for rapidly thawing foods. Put an ice cube on one and it melts before your eyes, yet the whole thing stays cool to the touch. How do they work?
(Inspired by this question: Utensil to thaw meat)
Asked
Active
Viewed 1.6e+01k times
31
-
You can always save some money and just thaw in cold tap water, provided your items are in plastic bags – Huangism Jan 21 '15 at 21:33
2 Answers
38
reference: http://www.engineeringtoolbox.com/conductive-heat-transfer-d_428.html
Let's take a moment to look at the heat transfer equation. Looking at it, we can see the ways to get more efficient heat transfer
q / A = k dT / s
q / A = heat transfer per unit area (W/m²)
k = thermal conductivity (W/(m·K))
dT = temperature difference (°C)
s = wall thickness (m)
- use a material with a high thermal conductivity constant (like copper)
- thinner (!) material
- maintain a higher difference in temperature
The way these thawers work should now be easy to understand. (1) They are made of a material that has a very high thermal conductivity constant, like copper. The higher a material's thermal conductivity, the faster it can equalize it's temperature with that of the surrounding material.
Things that touch each other want to be the same temperature. When you put an ice cube on a sheet of room temperature copper, they are very different temperatures. But as soon as they touch, they want to be the same temperature, so heat transfer begins. Heat "flows" from the copper to the ice, increasing the temperature of the ice (melting it), and decreasing the temperature of the copper. Heat also flows throughout the copper itself, meaning that even the parts of the copper that are far away from the ice are losing heat.
With the copper losing heat, it quickly falls out of temperature equilibrium with the surrounding air. But the air and copper also want to be the same temperature, and so heat from the air "flows" into the copper, bringing it back closer to room temperature, which in turn allows the copper to heat up the ice some more.... But of course there aren't distinct steps to this process: all of these heat transfers happen simultaneously and continuously. And as long as the air has some circulation, you can consider it to be an unlimited supply of room temperature heat.
The top of the copper plate is probably flat, to increase the amount of surface area in contact with the ice. The bottom of the copper plate, however, is probably ribbed or finned, to increase the surface area with the surrounding air, but without (2) creating more thickness!
We could also address (3) and heat the copper electrically, above room temperature, but then we run the risk of heating part of the food to that temperature as well. The benefit of using a passive copper heatsink is that the temperature will never rise above room temperature!
-
2The only thing I would say to improve (as far as SA expectations) is to word it more like a question in the question section. So instead of "I will explain", ask "How does?" You're totally allowed a bit of goofiness, but only questions are allowed in the question section; only answers to that question are allowed in the answer section. – Jolenealaska Aug 04 '14 at 10:15
-
-
Interesting! I suppose this also explains how those copper spherical ice makers work so quickly. – Archagon Aug 04 '14 at 11:03
-
If you wanted even more heat conduction than copper gives you, you could always embed some [heat pipes](http://en.wikipedia.org/wiki/Heat_pipe) in it. – Ilmari Karonen Aug 04 '14 at 12:56
-
2I'd suggest adding that a device like this works great for an ice cube because as the ice melts, it makes water which makes excellent contact with the tray, and spreads over a larger area. A piece of meat, on the other hand, doesn't melt, doesn't spread out, and makes less than perfect contact with the tray (because it's not a liquid), and the frozen part of the meat will have a thawed piece of meat between it and the tray, thus significantly increasing the thermal resistance, and thus decreasing the thawing rate. – Phil Frost Aug 04 '14 at 15:31
-
4I find using the underside of an extremely heavy pan just as effective as thawing plates. The same physics apply. – Chuu Aug 04 '14 at 18:14
-
Adding to this; essentially it's a heat sink for food; although with the opposite effect since the food is cooler than the environment (an odd way to think of it is it's a heat sink for the environment that dissipates the heat into your food). If you were to put *hot* food on it, the food would *cool* more quickly. Decent pans are designed for good thermal conductivity as well, which is why Chuu's use of pans in the above comment is also effective. – Jason C Jun 06 '15 at 00:59
5
Miracle thaws, also known as defrosting trays, are simply chunks of metal with a high coefficient of heat transfer. Materials that have a high thermal conductivity transfer heat more efficiently than those with a low conductivity. Aluminum is cheap and has a high thermal conductivity relative to other materials, so most are made of that material.
However, defrosting trays don't really work as well as the commercials would have you believe. The same reason that your food thaws slowly without a defrosting tray limits how quickly it will thaw with one, and that is because air is a poor conductor of heat. When you thaw something what is happening is that heat is being transferred from the environment (air, the counter surface, etc) to the object until the environment and the object are in equilibrium, that is the temperatures of both are the same. A thawing tray still has to get heat from the environment to transfer, and how quickly it can do this is limited by the fact it still has to get heat from the air. When you put a cold object on a thawing tray the tray will quickly transfer it's heat to the object, but once the tray gets as cold as the object the rapid thawing stops and it's all down to how quickly the environment can transfer heat, which isn't that fast.
So thawing trays are great at making ice cubes melt quickly, and they will speed up thawing a bit, just not that much.
GdD
- 74,019
- 3
- 128
- 240
-
Am I correct in assuming that you'll get a similar, but less potent effect from putting your frozen food in the fridge for a few hours? – Nzall Aug 04 '14 at 12:44
-
2A refrigerator would slow down thawing as there's less of a heat differential between the environment (the inside of the fridge) and the food. Maybe I don't understand your question. – GdD Aug 04 '14 at 14:01
-
1The Miracle Thaw speeds up your thawing a little bit, because it helps heat conductivity between the thing you're thawing and the air. A refridgerator does have colder air, but it's still warmer than the freezer. And because it's usually colder than 4 degrees in the fridge, your food still thaws (albeit it slower, thus less potent), but it will be safer than a Miracle Thaw for food safety (thawing aids are not FDA approved). – Nzall Aug 04 '14 at 14:04
-
-
I never said it would be more potent, on the contrary. I said it would be LESS potent. _you'll get a similar, but less potent effect from putting your frozen food in the fridge for a few hours._ My wording might be abit confusing, I should have said "when putting..." instead of "from putting". – Nzall Aug 04 '14 at 14:12
-
5If you are asking whether food will thaw in the refrigerator then the answer is yes. – GdD Aug 04 '14 at 14:18
-
3I think the risk of these isn't so much their exaggerated claims, but the fact that they are thawing at room temperature and may cause people to ignore the food safety guidelines (i.e. thawing in the refrigerator or under cold running water). I've never bought one of these, but I just seriously doubt that it could thaw a whole chicken in less than 2 hours, and maybe not even a thick cut of meat. – Aaronut Aug 04 '14 at 16:45
-
"but once the tray gets as cold as the object the rapid thawing stops" isn't strictly speaking true. As the cooling tray has a massively larger surface area (as so can transfer heat faster) that the object being thawed the thawing will remain faster than it otherwise would. Cooling fins work by the same principle – Richard Tingle Aug 05 '14 at 13:29
-
To add to @RichardTingle 's comment, even though air is a poor conductor, the greatly increased surface area of the tray has contact with a much larger volume of air... so the increased volume can collectively transfer heat faster than the small volume that is in direct contact with the (meat/ice cube/etc). – Doktor J Aug 05 '14 at 15:02
-
@DoktorJ, that depends on how much surface area is left after you put something on it. If the entire plate is covered there isn't much extra area to contact air. – GdD Aug 05 '14 at 15:48
-
@GdD you'd have to work hard to cover 100% of the surface area of the plate with meat. Even if you take the six steaks shown in the picture and squeeze two more on the plate, there is still more surface area of the plate in contact with air than in contact with the steak via the underside of the plate (assuming the underside is ribbed), the edges of the plate, and the small gaps on the plate between the steaks. Granted, it greatly reduces efficiency, but you still have conductive surface area you wouldn't have had with a steak sitting on a nonconductive surface (countertop, cutting board, etc) – Doktor J Aug 05 '14 at 19:25