The Science of Thanksgiving dinner
November 17, 2016
Let’s talk turkey. One of the great meals of the American calendar year is approaching, and as you go about prepping, here’s a side of science: four facts about the Thanksgiving meal to aid in the digestion.
The Tryptophan in Turkey is not Responsible for the Food Coma
Perhaps the most commonly recited scientific “fact” about Thanksgiving is not a fact at all. The amount of tryptophan in turkey is not enough to force a post-dinner nap.
Tryptophan is one of the 20 amino acids we need to make proteins, and since the human body cannot manufacture it, we need to get it from our diet. Besides building proteins, however, tryptophan signals the release of serotonin, a powerful hormone that among other things, helps control the brain’s sleep/wake cycle.
The reason it was thought that turkey could be the cause of the sleepiness, was that tryptophan enters the brain through the same channel as a number of other large neutral amino acids. And by eating foods rich in tryptophan, tryptophan would outcompete those other amino acids and flood the brain, creating the serotonin for sleepiness.
The problem with Thanksgiving dinner is that we eat so much of so many different foods that plenty of other amino acids get into our bloodstreams along with carbohydrates and fats. It turns out that insulin helps the tissues soak up carbohydrates and many amino acids, but does not affect tryptophan as much. So the brain does get a slight surge in tryptophan after a big meal, but food comas are the larger phenomenon.
Food comas, also known as postprandial somnolence, come from the stimulation of the parasympathetic nervous system—the opposite of the “fight-or-flight” sympathetic nervous system. Stretching of the stomach and small intestines, as well as detection of glucose in the blood tells the body to “rest-and-digest.”
So while turkey and its tryptophan do contribute to the yearly food coma, you should also blame the stuffing, potatoes and pumpkin pie.
Watch out for Salmonella when Stuffing the Turkey
On turkey day, chefs across the country will closely watch meat thermometers and built-in poppers to make sure the turkey gets cooked all the way through. The reason: salmonella. Those chefs, however, need to give stuffing the same attention.
Stuffing sits at the center of the turkey, and since it cooks from the outside inward, the stuffing is the last part of the turkey to cook.
It has three potential ingredients that could spell trouble. First is the bread, which soaks up juices from the turkey. Tasty when cooked, potentially dangerous when not. Second are eggs, which, like the turkey, need to be cooked thoroughly to stop salmonella. Third are other raw meat ingredients like sausage and oysters that can carry their own harmful bacteria.
Personally, I have eaten stuffing that was cooked in the bird every year since I discovered how delicious stuffing was, and it has never made me sick. That said, people get sick every year from undercooked turkey and stuffing, so this one is more of a caution than a fact or directive.
The USDA has a factsheet with tips to make sure the stuffing is cooked completely. Among the highlights are using a meat thermometer to make sure the stuffing reaches 165 degrees Fahrenheit, leaving it inside the turkey for 20 minutes after the turkey comes out of the oven and precooking all the meat ingredients before stuffing the turkey.
Some chefs, including the Food Network’s Alton Brown, recommend cooking the stuffing outside the bird—which technically makes it dressing instead of stuffing, but who's counting—and for those who like that cooked-inside taste, you can put the cooked stuffing inside the cooked bird for 20 minutes after the turkey comes out of the oven.
Why Frozen Turkeys go Boom
The great thing about turkey is there are so many ways to prepare it to get a unique flavor. Brines and spice rubs, smoking and deep frying can all make great turkeys, but deep frying also come with some risks: namely the fryer erupting into a huge ball of flames.
Hot oil and open flame combine to make more open flame, so when deep frying a turkey, it is important to keep the fry oil inside the fryer and away from the burner.
There are two notable situations where fried turkey chefs get into trouble. The first is when there is too much oil in the fryer to start with. Adding the turkey overflows the pot and spills oil onto the burner.
The second occurs when the turkey is still partially frozen. The ice in the turkey rapidly turns to the steam, a process called flash-boiling. When that steam takes up more room than ice or water, and since oil and water don’t mix, it creates bubbles in the oil that can splash over the edge of the fryer and into the flame. The turkey itself does not explode, but the oil boils over so violently that it can resemble an explosion.
The good news is, there are a few simple steps you can take to stay safe when frying a turkey. You can find them here, but chief among them are to make sure not to overfill the pot with oil, and always turn the burner off when transferring the turkey into and out of the pot.
The Science of a Thick Thanksgiving
We have covered turkey and stuffing, so now it is time to branch out to the rest of the meal. Gravy, cranberries and pumpkin pie all have one thing in common: they need to be thickened. The chemistry of how they do so, however, is different for each food.
Gravy thickens with starch. Starch is a carbohydrate polymer that exists in tiny granules. When put into a hot liquid, where the liquid molecules are moving very quickly, those molecules smash and soak into the starch granules. The trapped molecules are no longer as free to move around, creating what we call a thick gravy.
Heating the gravy too much will cause the starch to break down, making it unable to trap water, so you can actually break a gravy by overheating it. If it does break though, adding more starches like flour or corn starch at a lower temperature will thicken that gravy once again.
Cranberries rely instead on the process of protein polymerization to transform from berries and sugar into a thick sauce or even a jelly. Plants have a protein on their cell walls called pectin, which helps hold a plant’s cells to one another. As cranberries heat up in sugar and water, they start to break down, and as they do, they release their pectin into the bubbling mixture. The pectin proteins in the mixture start to bind with each other, creating a web that traps sugar molecules, giving the sauce a more rigid structure.
You can actually tune the consistency of your cranberry sauce by how long it cooks. Cranberries will release more pectin the longer they cook, so only cooking the sauce for a few minutes will give you a more liquid sauce while letting it cook for longer will create something closer to a Jello.
Finally, pumpkin pie relies on a different sort of protein interaction to set up a nice silky filling. Proteins take on very specific shapes when they are functioning properly, but once you heat them, they lose their shapes in a process called denaturation. The proteins in an egg clump together when you denature them, which is how a broken liquid egg can turn into solid scrambled eggs.
Pumpkin pie filling, like any custard, relies on the clumping of egg proteins to set. The trick with custards is making sure that clumping does not happen too quickly. The milk, sugar and pumpkin help with that, keeping the egg spread out, but custards also require patience. Turn the heat up too quickly and your pie will look more like scrambled eggs floating in pumpkin milk. You can learn more about making the perfect custard here.
No matter how you create your Thanksgiving dinner, be it baking, smoking, deep-frying, thickening or jellying, there is a pinch of science in every recipe. Someone at some point had to understand these food science details, to make sure we can make safe, delicious food and that’s something we can all be thankful for.
Daniel Lane covers science, medicine, engineering and the environment in North Carolina.