Heat Transfer in the Kitchen, Part 2

Last post we were talking about heat transfer in the kitchen and we’ll continue exploring that topic today.
Another common kitchen process involves thawing food that is frozen.  At some point in your life, you might have thawed, say, a package of frozen meat by setting it inside the refrigerator, out on the counter, or in a sink full of warm water.  Heat transfer by natural convection (from the surroundings into the frozen food) is proportional to (1) the temperature difference, and (2) the convective heat transfer coefficient.  As a general rule, gases (air in this case) tend to have a much lower convective heat transfer coefficient than liquids.  So, applying these two principles, you can see that thawing in the refrigerator would be slowest (smaller temperature difference, and air as the heat transfer medium) and in a sink of warm water the fastest (larger temperature difference, and liquid as the heat transfer medium).  Thawing on a counter would fall somewhere in-between.

Cooling things down in the kitchen is almost as important as warming them up. The heat transfer processes that we’ve talked about are governed by the same equations and work the same way for both heating and cooling.  So, for example, if you had a package of hamburger that you wanted to freeze, the rate of cooling by convection is also proportional to the temperature difference and the convective heat transfer coefficient, just like we talked about for thawing.

We keep all kinds of things frozen in our freezers (ever saved a hailstone or a snowball?) but there is an important thing to keep in mind if you go to freeze liquid water.  Water, like most substances, gets more dense as it gets colder.  However, shortly before it actually freezes into a solid, this behavior reverses and it actually starts to get less dense as the temperature goes down.  Then, it expands a lot (relatively a lot: about 9%) upon solidifying.  In a sealed container without room to accommodate this expansion, large enough forces can be generated to break many types of containers. It is important to leave an air gap when freezing liquid water in a rigid container.