Do you have a favorite Thanksgiving dinner dish? Maybe it's an aunt's special cranberry sauce or mashed potatoes combined with perfectly seasoned gravy. Or perhaps you enjoy sinking your teeth into a succulent roasted turkey the most. There are also often many baked goods to choose from: dinner rolls, biscuits, corn bread, muffins, pastries and pies! The foods in this group typically all contain a substance called gluten. In this science activity you'll explore why some foods, all made from wheat flour, have such different levels of tenderness and toughness based on their gluten content. Then, while enjoying the bready sides on Thanksgiving, you could impress your family with your gluten knowledge!
Breads, bagels, pastas, cakes, cookies, crackers, muffins, pastries, pies and pizza crusts. Are you hungry yet? What do all these foods have in common? Traditionally, they are all made from wheat flour, the fine powder made from grinding and sifting kernels of wheat, the edible part of that grain. Wheat belongs to the grass family, which contains some 8,000 species. Wheat was likely first cultivated in the Middle East about 10,000 to 12,000 years ago. Today, it is the third-most produced cereal grain in the world (corn is first and rice is second).
Why is wheat flour used in so many foods? When water is mixed with most other flours (such as those made from corn or rice), you get a ball of paste. Mix wheat flour with water, though, and it can make a ball that is both plastic (it can change shape) and elastic (it can return to its original shape)! Gluten, a composite of proteins within the wheat kernel, makes these valuable qualities possible. Because of these qualities wheat dough can trap gas (such as the carbon dioxide made by yeast) and expand, which helps breads and other baked goods rise to become light and fluffy!
- Two or three different flours made from wheat such as whole wheat bread flour, gluten flour, cake flour, pastry flour or all-purpose flour (You will need one cup, plus a couple pinches extra, of each flour type.)
- One small mixing bowl for each flour type
- Measuring cup with half- and quarter-cup measurements
- Tap water
- Sticky notes and pen or pencil (optional)
- Cutting board or other work surface
- Clock, watch or timer
- Strainer with small holes
- Measure out one cup of each type of flour into its own small mixing bowl. Keep track of which flour went into each bowl. You could use sticky notes to label the bowls. How do the different flours look and feel?
- Select one bowl to begin with and slowly add half to three quarters of a cup of tap water to the bowl while carefully stirring it with a fork. It may seem difficult to stir at first but will gradually begin to come together to form a rough ball. How does the flour feel as you stir it? How does it change over time?
- Sprinkle a spoonful of flour (of the type you are working with) onto a work surface, and onto your hands as well, so that both are lightly dusted with flour. Place the ball of flour onto your work surface and knead it for approximately five to seven minutes until it becomes smooth and elastic. (If you are unfamiliar with kneading, this is the process of pressing down on the dough ball with your palms and then pulling it back up again and rotating it slightly with your fingertips, then repeating this over and over again.) How does the flour change as you knead it? Does it easily form a ball over time?
- Repeat this process (of adding water and kneading) with the other flours you are testing. Be sure to knead all of the flours for the same amount of time. At the end of the kneading process you should have one smooth-looking elastic ball of dough made out of each type of flour you're testing. (Note that some flours will form better balls than others.)
- Place your balls of dough back in their small bowls and let them rest and relax for about 10 minutes.
- Now it's time to find out how much gluten is inside those dough balls. Place a strainer in the sink where the faucet can reach it, take one of the balls of dough to the sink and cup the dough in your hands over the strainer while you let cold water run on it. Gently pull and stretch the dough apart. The water will wash away the water-soluble parts, such as the carbohydrates, but will leave behind the insoluble protein, the gluten, that developed while you were kneading. The soluble parts being washed away will look like a milky liquid. Continue rinsing your ball of dough until very little milky liquid is coming out. How does the dough's texture and shape change as it is washed in the water? Place the remaining ball back in its original bowl to keep track of it.
- Rinse off your strainer and repeat the rinsing process with each of the other dough balls. At the end of the rinsing process, you should have a ball of gluten for each type of flour you used! (Note that, depending on the type of flour you used, a lot of the ball might have been washed away.)
- Compare the sizes of the gluten balls. Which type of flour produced the smallest diameter ball? Which produced the largest? Does this make sense based on the type of foods that each flour is used to create, and how much gluten may be in each?
- Extra: They won't make elegant pastries but you can continue the investigation and bake your gluten balls by putting them in the oven at 450 degrees Fahrenheit for 15 to 30 minutes. How do the gluten balls look, feel and taste after they've been baked?
- Extra: Repeat this using a wider variety of flours made from wheat and use a ruler to quantify your size comparisons between the gluten balls. Which flours make the largest gluten balls (the ones with the greatest diameters) and which make the smallest? How big are the differences?
- Extra: You could investigate how kneading affects the size of the gluten ball made. To do this, choose one type of flour and create three equal-size balls of dough from it using different degrees of kneading: no kneading, kneading one for five minutes and one for 10 minutes. Rinse the dough balls and evaluate the gluten balls that are created, either by measuring their diameter or weighing them. How does the amount of kneading affect the gluten ball's size?
- Extra: Select one type of flour and make dough with different additives, such as salt, sugar or oil. How do the additives affect the amount of gluten that develops?
Observations and results
Were the gluten balls made using bread flour or gluten flour the largest whereas ones made using cake flour or pastry flour the smallest—and all-purpose flour gluten balls intermediate in size?
Typically, bread and gluten flours have a relatively high amount of gluten (around 12 to 14 percent) whereas cake and pastry flours have the lowest (about 7 to 10 percent) and all-purpose flour has an intermediate amount (around 9 to 11 percent). The more gluten a flour type has, the larger the resulting gluten ball should be. "Hard wheat" flours, such as those for bread, are high in gluten and are good for baking things that need toughness and strength such as yeasty breads, bagels and puff pastries. "Soft wheat" flours, such as those for cake and pastry, are low in gluten and are better for baked goods that need to be tender such as pancakes, cookies and pastries.
Why is kneading important for measuring the gluten content in the flour? As the dough is kneaded, the chainlike molecules that make up gluten (glutenins and gliadins) link end to end to form superlong, springlike chains. This makes the tight, coiled mesh that is gluten, and this gives the dough its elasticity. All of those springs want to "spring back" when they're stretched!
More to explore
Discuss Gluten, from Oregon State University
Great Globs of Gluten! Which Wheat Flour Has the Most?, from Science Buddies
Goodness Gracious! Great Balls of Gluten!, from Exploratorium
The Scientific Secret of Fluffy Pancakes, from Scientific American
This activity brought to you in partnership with Science Buddies