Monday, April 14, 2014

Thaw food faster ... with the power of science


I have a chest freezer and I love it. But for some reason, it always takes at least 24 hours for stuff to thaw in my fridge -- and I am not really great about remembering to take stuff out of the freezer the night before. So, I generally take something out the morning I plan to use it, and come home from work to find it still frozen solid.  Then we end up eating cereal for dinner... 
I could always leave the item at room temperature to thaw but that is unsafe.  Or I could use the microwave defrost setting but I think that gives food an odd taste and texture (and besides, some of the containers that the food is frozen stuck to should probably not be microwaved).
So, cereal it is...
Except, the physicist in me knows better!

Background Science

Thawing food is really about trying to increase the amount of heat that is transferred to the food from its surroundings in a given amount of time.  This rate is dependent on surface area, difference in temperature between the food and its surroundings, and something called thermal conductivity.

Thermal conductivity is the ability of a material to transfer heat.  When two objects (including air) are in contact, they will eventually become the same temperature.  They do this by transferring thermal energy, which we call heat, from the warmer object to the cooler object.

Just like some materials are good electrical conductors (e.g. copper wire) and bad electrical conductors (e.g. the plastic coating on copper wires), some materials transfer heat better than others.  You can do a little experiment to see this:
Get a cookie sheet out and put it on your counter.  Now put one hand on the counter and the other on the cookie sheet.  Which one feels cooler?
As long as you use normal cookie sheets, and as long as you don't have metal counters, the cookie sheet should feel cooler.  But they are both at the same temperature!  (And I assume your hands are at the same temperature). What our skin measures best is not an absolute temperature but a flow of heat.  When we touch the cookie sheet, the heat from our hand flows away from the hand faster than it does with the counter because metal is a better conductor. (This principle is also why some people prefer wood to porcelain toilet seats, by the way -- because wood transfers heat less quickly than porcelain, it feels warmer on your bum (though it is actually the same temperature)).

Ok, so we can speed up thawing (transfer of heat to our frozen food) by increasing conductance, surface area, and difference in temperatures between the objects in contact.  The last point is a really important point - in fact, as it turns out, the most important of this lengthy post.   As our frozen food heats up, the objects it is in contact with will cool down, decreasing the difference in temperatures.  But different objects can give up or gain the same amount of heat and still have different temperature changes.  For example, if you give the same amount of heat to water and to sugar, the sugar will have a more than 3x larger change in temperature than the water because of a property called heat capacity.

So, I hypothesized two methods to speed up defrosting: Submerging the food in a bowl of water and putting the food on a heavy aluminum pan (designed for grilling) -- but any metal pan will work.  The idea is that both water and metal have higher conductivities than air/counter/porcelain.  Metals have MUCH higher conductivities than water, but they won't be able to touch as much food (surface area) and they have smaller heat capacities (i.e. their temperatures will change more drastically when in contact with objects of different temperature) than water.


I tested ice cubes, 1.5lb packages of ground beef, and blocks of cheese.

Ice cubes

I placed an ice cube, flat side down on my wood counter, on my aluminum pan, and in a zip-topped baggy in a bowl of water.  The aluminum pan melted its ice cube completely in 6 minutes.  The water took 12 min.  90min later, there is still unmelted ice in on the countertop.

This is expected since the thermal conductivity of the objects goes in order of aluminum, water, wood (with air being pretty much negligible).  However, aluminum conducts 350x faster than water (not just twice as fast), so we are already seeing effects from heat capacity.

Actual food

With similar experiments with actual food, the water bowl thawed the cheese in 45min and the beef in 1 1/2 hours.  The aluminum pan took 1 1/2 hr and 2 1/2 hours, respectively.  The cheese was not tested for the counter (I only had two blocks) but the beef took 4 1/2 hours sitting on a porcelain plate.


The effect of the aluminum pan cooling down a ton, compared to the water, caused the thawing of food to be much slower (this effect was not apparent with the ice, since the ice was relatively small and caused little cooling).  For appropriate (i.e. water-tight packaged) foods, submersion in water is by far the fastest route to thaw.  And the more water and the more frequently you can change the water (keeping the water from cooling down, keeping the difference in temperatures large), the better.  However, for other foods (e.g. non-water-resistant) or when having a big bowl of water in the fridge is inconvenient, using a heavy metal pan is a useful alternative.

Now what am I going to do with all this thawed beef and cheese??

I am linking up to these great parties!


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