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My mom uses frozen sausages for a soup. She defrosts frozen sausages by submerging them in water (room temperature I believe). She claims this makes them defrost faster. She learned this from some article in a magazine and now swears by it. She is of no scientific background, so no accurate experiments have been made. Is there any physics why this might be true? Why would they defrost quicker in water? I cannot think of a reason why this would happen.

Moreover, she sometimes keeps the sausages wrapped in aluminum foil to keep them from absorbing moisture. Would this surely slow down the melting because of this extra layer?

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  • $\begingroup$ There is an SE site for cooking related topics and they do deal with technical aspects of cooking. It's called Seasoned Advice SE, which is frankly a name I am surprised anyone finds. :-) $\endgroup$ – StephenG May 30 at 12:40
  • $\begingroup$ I think it would be useful to note that you mean defrost, not melt, since the sausages are not turning to liquid (even if there may be some liquid water as part of the sausage's internal structure). $\endgroup$ – dbmag9 May 30 at 22:00
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    $\begingroup$ @dbmag9 I heartily agree with your suggestion, so I've done it in an edit. (I would say you could have done the same yourself.) S. Rotos, if you disagree you can edit it back, but I really do think "defrost" is a much more appropriate word here than "melt". $\endgroup$ – David Z May 30 at 23:01
  • $\begingroup$ @DavidZ - (no, they couldn't; only suggest an edit) question still has a second question in it. Does wrapped in aluminum foil keep them from absorbing moisture? $\endgroup$ – Mazura May 30 at 23:34
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    $\begingroup$ @Mazura (1) Yes suggesting an edit is exactly what I meant. (2) I thought about editing out the second question, and I ordinarily might have done so, but several of the answers already address it and I thought it was more important to not invalidate them. $\endgroup$ – David Z May 30 at 23:53
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I do exactly the same. It is a very effective way of defrosting food fast.

Compared to air water has a much higher heat capacity and a much higher thermal conductivity. That means heat flows from the water into the sausages much faster than it would in air and the water cools less than air would as it heats the sausages.

Aluminium foil has a much, much higher thermal conductivity that either water or sausage meat. Provided the foil is tightly wrapped around the food, so there isn't a large air gap between the food and the foil, wrapping the sausages in aluminium foil will have almost no effect of the rate of thawing. Any small gaps are quickly filled with water as the frost on the surface of the food melts.

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    $\begingroup$ wrapping the sausages in aluminium foil will have almost no effect of the rate of thawing. Iff you manage to pack them so tight that less than 68nm of air is trapped between the foil and the sausage. Otherwise you are bound to see a (maybe just slightly) longer time. $\endgroup$ – Adrian Colomitchi May 30 at 23:00
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    $\begingroup$ To add a little intuition for the “thermal conductivity”: Think of the difference between standing outside on a cold winter day, and then jumping into a lake. The lake may be the same physical temperature, but it will feel much colder, because it’s cooling your body down much more quickly. That’s what “water has a higher thermal conductivity than air” means in practice; and that’s essentially the same mechanism that makes the sausages defrost quicker. $\endgroup$ – PLL May 31 at 0:25
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    $\begingroup$ @AdrianColomitchi I have done the experiment many times. What happens is that the film of frost on the surface of the food melts and the gap between the foil and food fills with water. $\endgroup$ – John Rennie May 31 at 4:07
  • $\begingroup$ I agree on thermal conductivity, but I'm not so sure the heat capacity matters here. The heat capacity of the air in a room would be effectively infinite compared to the heat capacity of a sausage; that is, the room shouldn't cool down by any perceptible amount when defrosting a sausage. $\endgroup$ – gardenhead May 31 at 14:27
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    $\begingroup$ @gardenhead there might be an effect given by the fact that the air around the sausage stays locally colder than the average temperature. I have no idea if it matters. $\endgroup$ – fqq May 31 at 23:57
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The heat transfer from 'hot' air or water to 'cold' sausages is roughly determined by Newton's law of cooling/heating:

$$\boxed{\frac{\text{d}Q}{\text{d}t}=hA[T_{\infty}-T(t)]}\tag{1}$$

where, for heating:

  • $\frac{\text{d}Q}{\text{d}t}$ is the rate of heat transfer into of the body, which determines how quickly the body's temperature rises,

  • $h$ is the heat transfer coefficient (assumed independent of T and averaged over the surface),

  • $A$ is the heat transfer surface area,

  • $T(t)$ is the temperature of the object's surface, as a function of time $t$,

  • $T_{\infty}$ is the temperature of the environment; i.e. the temperature suitably far from the surface.

For now we'll assume that $T_{\infty}=\text{constant}$, throughout the heating process.

In that case, acc. $(1)$, the rate of heating depends strongly on $h$, the convection coefficient. It's well known that broadly speaking $h$ is much larger for liquids than for gases, all other things being equal. This table of $h$ values bears that out. So based on this it's reasonable to assume that defrosting sausages in water will be quicker than in air.

There is however a caveat. With water heating not only will the temperature of the sausages increase but also will the temperature of the water decrease. The latter, according to $(1)$, will decrease the heating rate. The trick to avoid this is to simply uses a large ratio of water to sausages or simply run the sausages under the cold faucet, continuously.

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I believe it has to do with the speed of heat exchange between sausage and air in one case and sausage and water in the other. Also, frozen sausage essentially means that the water in this sausage is turned into ice - you can test that a block of ice in water melts faster than in air.

The most efficient technique is microwave in low power mode (if the power is set to high, it may cook the food prematurely), but many people don't like it since it seems slow.

Finally, another tip is simply to take the food out of the freezer 24 hours in advance and leave it in frigo.

I suppose my answer looks more like cooking advice than physics discussion :)

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True. It's very easy to explain. Thermal conductivity ratio between water and air is about 23 times ! This means water transmits heat 23 times better than air - you can do nothing about about. So if you want to cool matter fast - you need to search for a maximum thermal conductivity materials.

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