“Christmas is coming, the goose is getting fat,” goes the old song, but these days, goose is a rare sight on the Christmas table. Instead, the place of honor goes to the turkey.
Turkeys are native to North American. In 1519 Spanish ships introduced them to Europe. By the 1700s turkey drives were being held from Norfolk to London. (If cattle drivers are cowboys, are turkey-drivers “turkeyboys?”) In 1851 Queen Victoria ate turkey instead of the traditional swan at Christmas, and by late Victorian times, the turkey had supplanted the goose at English Christmas feasts.
With this change came the annual battle to cook a turkey perfectly. The problem is that few people approach the cooking of turkey scientifically. Fortunately, Roger Highfield’s excellent new book The Physics of Christmas corrects that oversight.
Scientifically, a turkey is a combination of water, fat and protein: three-fifths the former, one-fifth each the latter two. The difference between white and dark meat is the presence of a compound called myoglobin, which retains and stores oxygen. Myoglobin includes an iron molecule which, when heated, combines with oxygen and turns dark–the same process that produces rust.
Muscle used for brief, rapid motion–draws its energy directly from the blood, and doesn’t need myoglobin. Since turkeys use their wings infrequently, this is the kind of muscle they have on their breasts. Muscle used for longer, slower, sustained motion needs the energy stored in both myoglobin and fat. That’s why drumsticks are darker and greasier than breasts.
When you roast a turkey, the proteins in its meat break down into fragments, creating volatile chemicals we can taste and smell. The two compounds most responsible for the taste are inosine monophosphate, which you’ve probably never heard of, and monosodium glutamate, which you probably have.
Roasting breaks down the molecular bonds that hold proteins tightly wound up in amino acids. As the proteins stretch out, the meat becomes tender. Keep up the heat, however, and the proteins form new bonds–a process called coagulation–turning the meat tough. Another cause of toughness is the connective tissue that holds the muscles to the bones. One of the proteins that make up this tissue is collagen. Heating it turns it into the much softer gelatin. To cook a perfect turkey, then, you need to heat it hot enough and long enough to turn collagen to gelatin while avoiding protein coagulation.
Peter Barham, a physicist at the University of Bristol, England, has found the longer the turkey remains at a high temperature, the more moisture it will lose and the more likely the muscle proteins will coagulate. He’s discovered the entire bird must be heated to 70 degrees Celsius (160 F) to ensure that all collagen turns to gelatin. In other words, the goal of cooking a turkey is to raise its central temperature to at least 70 degrees.
Thermodynamics is the study of the movement of heat. The basic equations that govern turkey cooking were formulated in 1947 by H. S. Carslaw and J. C. Jaeger, in a work called Conduction of Heat in Solids (not that turkeys were their primary concern).
To apply their equations, you assume that the oven is a constant temperature throughout, that the rate at which heat passes through the turkey is independent of both how long and how hot it is cooked, and that the turkey is perfectly spherical.
The time it takes heat to diffuse through this idealized turkey to raise the center to a certain temperature is proportional to the square of the radius. Since real turkeys aren’t spherical, you have to figure out how big a real turkey would be if it were converted into a sphere. You can do this based on mass; the mass of a sphere is proportional to the cube of its radius.
Comparing these two formulae lets you get rid of the radius and create a simplified equation that says the time required to cook the turkey is proportional to the mass of the turkey raised to a power of two-thirds.
Got that? Scientifically what this all boils down to is…um, well, roughly 20 minutes per pound plus 20 minutes for a small turkey, and 15 minutes per pound plus 15 minutes for a large turkey. Which is exactly what your grandmother would probably tell you.
In other words, you don’t need fancy mathematics to cook the perfect turkey–or to enjoy it. Which leaves me with only one thing to say:
Merry Christmas!
