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Heat Transfer is for Turkeys!

This week, many Americans will be sitting down to Thanksgiving dinner, which is as good a time as any to Think Thermally™! Whether you are the cook, the “bottle washer” or just hungry for turkey, useful thermal lessons abound during Thanksgiving.

Most of us will buy our turkey frozen—so dig deep in the freezer to make sure they are fully frozen when you purchase them. Once you get the bird home, carefully follow the thawing directions. The potential problem with careless food handling is the bacterial growth, which occurs at temperatures above 40oF (4oC).

The real issue, however, is one of thermal capacitance and the time it takes to transfer heat! Let’s look a bit more closely. A given amount of energy, based on the bird’s weight and the specific heat of turkey, is required to get it from the freezer to the table. How we get that heat into the bird is also a factor to consider.

First, the bird must be warmed to its freezing point, a fairly easy job in terms of energy—thawing frozen turkey only takes about half the energy required to warm the thawed turkey. But getting it fully thawed requires a great deal of energy by comparison, on the order of ten times (per unit mass or volume) what it took to bring it up to the freezing point! You can thaw the bird in the refrigerator, but this takes about five days. You could also use a large pan of cold water at 40oF (4oC)—this will take about 10 hours if you change the water frequently.

Why does it take so much time to fully thaw the bird once it has reached 32oF (0oC)? Basically, it takes a lot of energy and, more importantly, we can’t speed up the process with hot water or by leaving it at room temperature because the bird should not get warmer than 40oF (4oC) in the process. Therefore, the temperature difference between the bird and the refrigerator or the bird and the water is never very great—and as a result, the heat transfers slowly.

Once thawed, cooking the turkey to the recommended minimum temperature of 170-180°F (77-82oC) is pretty straightforward, but again, in an oven it is not a rapid process. This is because the heat is transferred to the meat by convection (via air) and radiation—neither of which is particularly fast at the recommended temperatures. Once heat has been transferred into the meat, it travels to the core by conduction (also a fairly slow process), so relax, you can’t rush things in the oven. But deep frying the critter is another story!

Even though the temperatures are about the same as in the oven (350oF), the deep frying process is much faster because the heat is transferred by convection from the oil which has a much higher specific heat than the oven’s air. The bird will be fully cooked in one hour!

Regardless of how you cook your turkey, ensuring that the whole bird is cooked is essential. To determine this, we could use our thermal imager, but what is the thermal gradient between the nicely browned (and warmer) outside of the bird we see and the juicy (if cooler) inside? That is hard to predict! Most of us have come to rely on the little “pop up” thermometer we find in many birds! In fact, this simple device relies on the melting point (state change) of a soft metal inside the thermometer to release the indicator. The melting point is calibrated so that when the indicator pops up the bird is done, but not overdone.

If you are measuring temperatures with your own insertion thermometer, you need to place it deep in the bird, against a bone in the thigh. That is typically the last part affected by the heat transfer process and, therefore, if it is hot enough, the bird should be properly cooked.

For the most part, the exterior of the turkey is warmer than the interior but we can see the wing tip already cooling after a few minutes on the table. Thanks to my colleague, Matt Schwoegler, for the great thermal image, and to his lovely wife for cooking the bird. Let’s eat!

The cooked bird should generally be allowed to sit anywhere from 10-30 minutes before it is served. In that time, the heat continues to transfer into the core of the turkey, causing the internal temperature to rise by as much as 10oF (5oC) The outside of the bird, of course, begins to cool toward room temperature, so by the time it is carved it should be just the perfect temperature to eat!

All this talk of heat transfer and turkey has made me hungry! Enjoy your bird and be thankful for all things thermal!

Thinking Thermally,

John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner

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