The science behind turkey brining, tryptophan and the perfect Thanksgiving roll

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    What gives baked goods their texture? How does turkey brining work? Does tryptophan really make us sleepy? 

    Ahead of Thanksgiving, The Pulse took a look at the science behind some quintessential holiday foods. We set out to answer questions about what gives baked goods their texture, how brining works, and whether the tryptophan in turkey really makes us sleepy.

    Q: A crusty dinner roll and a flaky pie dough are both made from flour and water (among other ingredients, of course.) What gives them such different textures?

    The short answer: Gluten.

    The science: The protein in wheat has gotten a bad rap lately as more and more people opt for gluten-free diets. But it’s gluten that gives bread and pastry products their structure, and it’s a major player in what makes a crusty artisan loaf different from a flaky, melt-in-your-mouth pastry.

    Jamie Landers, an aerospace engineer–turned-baker and owner of Philadelphia’s Luscious Bakery food truck, explains that gluten networks formed during the baking process give bread products their structure.

    “(Gluten molecules) are going to bond, and they’re going to create these structures and networks, and these networks create the structure or elasticity of the bread,” Landers said.

    Landers, who specializes in sweet and savory brunch fare, said choosing the right flour is the first step in giving any baked good the right texture.

    “The main difference is the protein content,” Landers said.

    A flour with more protein will yield a chewier dough.  A flour with less protein means airier, or flakier dough. Protein contents range from around six to eight percent in a cake flour to around 16 percent in a whole-wheat flour. The workhorse of the American home kitchen, all-purpose flour, ranks in the middle with 10-12 percent protein. 

    Gluten itself isn’t present in dry flour. It forms when flour is mixed with water, which coaxes together two wheat proteins called glutenin and gliadin to form gluten, what is called a composite protein. 

    After choosing the appropriate flour, the second variable giving a baked good its texture is how the dough is handled, Landers said.

    knead(Kimberly Paynter/WHYY)

    Kneading dough forces protein to come together and form longer gluten strands.

    While making pumpkin-seeded dinner rolls with The Pulse’s Carolyn Beeler, Landers mixed the wet ingredients with the dry to form a wet dough, rough like a pill-y wool sweater, and then turned the dough out onto a long stainless steel counter in her food truck to knead it.

    “You’ll notice I’ve just been kneading for about 90 seconds here, and it already looks different,” Landers said as she worked the dough with practiced hands.

    “It no longer looks shaggy or raggy, meaning it doesn’t look like a shag carpet anymore, there aren’t pieces sticking up,” she said. “It really is starting to come together.”

    Landers pulled at a piece of the dough, and the individual strands of gluten were visible as it stretched.

    Think of kneading as a workout for the protein in bread, just like lifting weights is a workout for your muscles.

    “We’re stretching it but not tearing it, just like you wouldn’t want to tear your muscle, and pushing it back together, almost like a rep of a bicep curl,” Landers said.

    Letting dough rise, Landers explains, further strengthens gluten networks. Those networks then trap gasses released when yeast eats the sugars and starches in dough and burps out carbon dioxide and alcohol.

    “Because we already formed a gluten network by kneading it for ten minutes, that’s going to create a mesh or a net within it, and it’s going to trap that carbon dioxide inside of it, and that’s why we’re getting that rise,” Landers said.

    Landers gave her dinner roll dough two major rises, and one lengthy knead. The resulting rolls were crusty on the outside and tender but slightly chewy on the inside.

    A crusty, artisan loaf would have required more working to build even stronger gluten networks, Landers said.

    A pie dough, in contrast, should be handled as little as possible for optimal flakiness.

    Landers uses cold butter and ice water in her pastry dough, to inhibit gluten formation, and mixes the dough with her hands just enough to incorporate the butter and flour together. She said some people use shortening, a gluten inhibitor, in place of some of the butter in a pie crust for a flakier result.

    Baking tips from Jamie Landers

    If you want crusty bread or rolls, throw ice cubes at the bottom of your oven to create steam while baking.
     To use all-purpose flour in a recipe instead of running to the store for pastry or cake flour, replace two tablespoons of each cup of all-purpose flour with cornstarch, a gluten inhibitor, to reduce its protein content.

    Follow @LusciousBakery on Twitter to find the location of Jamie Landers’ food truck. 

    Q: How does brining a turkey work?

    The short answer: Salt breaks down turkey proteins, which then suck in and bond with water molecules to create a more tender, juicy bird.

    The science: Brining, or soaking a turkey in salt water prior to baking it, is a popular way to keep turkey tender and juicy while baking. The added water keeps the breast meat moist, even if you have to cook it longer than necessary to ensure the thighs and legs are up to temperature.

    shutterstock 165835040(Photo via ShutterStock)

    Jacob Lahne, assistant professor of food science at Drexel University, explains that in a very strong brine, water first travels out of the bird and into the brine.

    “Osmosis is driven by the concentration of dissolved things on either side of a permeable barrier,” Lahne said. “And if you have a very high salt concentration on the other side, like if you just rub salt on meat, it will draw moisture out because then you have such a high concentration of solutes and such a low concentration of water.”

    When meat is submerged in a salty enough brine, water leaves the meat, which does not have much salt, and travels into the brine, which does. But some salt starts moving in the opposite direction, from the brine into the meat.

    “Once enough salt is taken up into the muscle fibers of the turkey, it causes those muscle fibers, at a molecular level, to start coming apart a little bit,” Lahne said.

    The long fibers that make up protein molecules start to un-knit, and want to flap around.

    “It actually causes the muscle fibers to swell,” Lahne said. “And that swelling pulls in water, because the muscle fibers don’t want to have a vacuum, they need to have something to be dissolved in.”

    That vacuum pulls the water from the brine into the turkey, where many proteins have already disassociated from each other.

    “When they dissociate from each other because of the salt, they can then associate with more water, and so it’s more tender because its holding more water more tightly,” Lahne said.

    The protein molecules breaking down makes a more tender bird, and the molecules bonding to more water make it juicier.

    Turkey-prep tips from Jacob Lahne

    The optimal salt concentration for increasing meat’s water-holding capacity is about one molar, or two and ¼ teaspoons of salt per cup of water.
    One criticism of brine is that it creates turkey drippings too salty to make a good gravy. Lahne’s work-around: make a stock with the turkey giblets, vegetables and herbs, and reduce it down to create a gravy.
    Lahne also butterflies, or spatchcocks, his turkey to allow for a faster cooking time.

    Q: Does turkey really make us sleepy after Thanksgiving dinner?

    The short answer: No

    The science: Just reading the word ‘tryptophan’ in a holiday story may be enough to put you to sleep, or at least roll your eyes. But the science behind why the ‘tryptophan makes you sleepy’ myth is more interesting than you may think.

    The myth goes like this: eating turkey increases tryptophan levels in the blood, which increases tryptophan in the brain. Tryptophan in the brain increases serotonin production, and serotonin can make you sleepy.

    Tryptophan is an amino acid, one of the nearly two dozen that form the building blocks of protein. The first problem with the turkey sleepiness explanation is that tryptophan is present at about the same levels in all protein, so a slice of turkey at Thanksgiving is no more sleep-inducing by itself than a piece of chicken any other day of the year.

    Any post-prandial doziness on Thanksgiving may be more to blame on that piece of pumpkin pie for dessert than the turkey, said Dr. Dick Wurtman, an MIT sleep researcher and professor emeritus.

    That is because increasing tryptophan in the blood by eating turkey doesn’t necessarily increase tryptophan in the brain.

    “The problem is when you eat the protein you’re not just increasing tryptophan,” Wurtman said. “You’re also increasing blood levels of half a dozen other amino acids, which compete with tryptophan for getting into the brain.”

    Tryptophan is the rarest amino acid, Wurtman said, so when it tries to cross the blood-brain barrier, it has a lot of competition. Wurtman likens it to a crowd of people pushing their way onto the same train in the Tokyo subway.

    “The doors open, maybe you have 10 or 15 people trying to get in, and they’re competing, they’re fighting with each other they’re not all going to make it,” Wurtman said. “Some of them are labeled tryptophan and more of them are not. So the more others you have competing, fewer of the tryptophan are going to get in.”

    If you eat more protein, more of both the tryptophan people and other people will crush around the doors, increasing gridlock.

    But if you eat a carbohydrate soon after eating a piece of turkey, say, for example, a slice of pumpkin pie, Wurtman said the scene changes.

    Carbohydrates increase the secretion of the hormone insulin. Insulin encourages most amino acids to be taken into tissues and absorbed by the body, reducing their levels in the blood.

    “But insulin doesn’t lower blood levels of tryptophan,” Wurtman said.

    Back to that Tokyo subway analogy, the insulin from that pumpkin pie acts like a bouncer, clearing all the competition off the platform, and leaving just the people labeled ‘tryptophan’ standing.

    “The number of other people fighting to get on the subway are going to go down,” Wurtman said, allowing more tryptophan to get on the subway, or cross the blood-brain barrier.

    “Then some cells in the brain use the tryptophan to make serotonin and more gets released,” Wurtman said, “and that’s why carbohydrates make a lot of people sleepy, but not turkey, that doesn’t do it.”

    Other potential causes of that Thanksgiving day nap: some research suggests that stretching the small intestine induces sleepiness. And a big meal will send more blood to the digestive system and divert it from other systems, like your brain and muscles.

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