Gluten is one of the most important molecules in baking. Anything that contains wheat flour, be it a cake, cookie, or bread, relies on gluten for structure. Gluten is also responsible for the difference in texture between a light, fluffy cake and a dense, chewy pizza crust. Thus, understanding what gluten is and how to work with it will help us prepare doughs and batters that come out of the oven as scrumptious treats with the perfect texture.
Gluten is a protein.
Gluten is a protein, which is a type of large molecule that serves diverse, crucial functions in every organism. I will discuss proteins in more detail in a separate post, but in the kitchen, many ingredients that came from living things, including dairy products, fruits, and flour, contain proteins.
Scientists think of proteins as molecular strings of beads called amino acids. Take a look at the “Proteins” illustration below. Living organisms make their proteins from twenty amino acids, which we can represent with twenty different colors of beads. Protein strings contain dozens to millions of amino acid beads in any order, and the specific pattern of amino acids determines the protein’s shape. If, in our string of beads, there are detailed rules about which colors must touch and which must be as far away from each other as possible, we can imagine, even with a few colors, how the string might take on a three-dimensional structure. And if we add more strings to this first one, beads across different strings can interact to form a protein complex. This is what gluten is: a complex of the proteins gliadin and glutenin that are naturally present in flour.
Gliadin protein strings are anywhere from 28,000 to 50,000 amino acids long, and the strings fold into spherical structures. Glutenins are larger, from 500,000 to 10 million amino acids long, and shaped like long coils. I like to think of them as those coiled Slinky toys. Glutenins are actually some of the largest proteins found in nature, and the coils bond with each other to form aggregates we can think of as a springy mesh of several Slinkies locked together.
In dry flour, the spheres of gliadin and meshes of glutenin are isolated and frozen. It’s like they’re critters in frozen tide pool ice cubes. But as the tide rises, the ice melts and the critters thaw and scrabble out, and the separate tide pools become one big ocean again and the critters all stretch and float around and give each other big warm hugs and hang on tightly because they’ve been frozen for so long.
This is exactly what happens when we add water to flour. (Well, not exactly, but kinda?) The glutenin and gliadin reach for water in a process called hydration to become flexible, mobile proteins in one big mixing bowl ocean. When the proteins get close enough to each other, their amino acids form chemical bonds. The resulting protein, gluten, is a web of glutenin Slinkies interspersed with gliadin spheres as shown in the illustration below. When flour first meets water, the gluten web is sparse because the proteins just bond with their tide pool neighbors. As the gluten develops, however, more proteins join together, more chemical bonds form between the proteins, and the web of gluten thickens. This changes the properties of the dough and the texture of the final baked product.
Gluten holds air.
A denser gluten web traps more air than a sparser web. In this case, we can think about gluten as a rubber balloon. A less developed, sparse gluten network is like a balloon made from old, brittle rubber. As we blow up the balloon, it expands a little, but it soon breaks, and the balloon fragments remain stretched. A more developed gluten network is like a latex balloon. As we pump more air into the balloon, it stretches to a few times its original size before popping. After the balloon pops, the pieces of latex snap back to their original shape.
These balloons demonstrate three properties of gluten that are summarized in the illustration below: strength, extensibility, and elasticity. Strength is the ability to withstand force. With the balloons, a low air pressure pops the weak, brittle balloon; in baking, air escapes weak gluten networks more easily. This can influence the rise of our baked goods. For example, over-mixed muffins, which have a strong gluten network, trap more air and rise higher in the oven than their properly mixed, weak-gluten counterparts. (The texture of the over-mixed muffins isn’t worth the rise, though. I’ll be sharing this soon!) The second property of gluten, extensibility, is flexibility: how much does the balloon stretch before it pops? The latex balloon expands more and is thus more extensible. In bread dough, extensibility is illustrated by the windowpane test. Sufficiently kneaded dough should stretch so thin that light passes through it. If the dough breaks before it reaches that point, it needs more kneading, which produces stronger gluten with greater extensibility. Elasticity, the third property of gluten, is the ability of the material to return to its original shape. With the balloons, the more elastic latex regains its shape, but the old rubber does not. In the kitchen, overworked pie crusts shrink in the oven because they contain too much gluten, and the elasticity of the gluten pulls the edges of the crust back toward the center.
These three properties of gluten are partially determined by the amounts of the proteins in the flour. Remember that gluten is a network of gliadin spheres interspersed in a springy mesh of glutenin Slinkies. Because of its shape, glutenin is elastic on its own, but gliadin disrupts the interactions between individual glutenin loops, allowing the loops to move past each other without locking. If we think about glutenin as several Slinky toys mashed together, it would be hard to imagine stretching a Slinky very far before it got caught in another one. Gliadin essentially prevents the loops from locking together, which lets us stretch the Slinkies further. Thus, gliadin adds flexibility, or extensibility, to the glutenin mesh and the resulting gluten network, and glutenin is largely responsible for the elasticity of the gluten.
It then makes sense that doughs made from flours with more gliadin are more extensible, while those with more glutenin are more elastic. Furthermore, flours that contain more glutenin and gliadin overall develop more gluten than flours with less of these proteins. The amount of each protein in the flour depends on many factors, including the environmental conditions of the original wheat and when the wheat was harvested, but the total protein content generally falls within a range of seven to twelve percent of the total flour weight. I will be writing more about different flours and their protein contents in a future post.
Gluten affects texture.
The structure of the gluten network also affects the texture of the final baked product. Think of the difference between a single cooked spaghetti strand and a clump of cooked spaghetti that’s stuck together. The single strand, sparse like minimally developed gluten, is thin and easily broken. On the other hand, the clump, like the thick, developed gluten, has more bite to it. So when we prepare cake batter, we don’t want to develop the gluten any more than we have to. We want to maintain a sparse gluten network for a light, fluffy texture. For chewy pizza crust, however, we want a dense network of gluten, so we encourage gluten development.
But how can we control the amount of gluten that forms in our mixing bowls? Stay tuned! In the next post, we’ll be talking about how we work with gluten in the kitchen in everything from breads to cakes to salt dough.
References
Corriher, S. O. Bakewise; Scribner: New York, 2008.
Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.
Gardel, E. Materials Science in the Kitchen: Basic Bread Revisited. Science in the News, 2010.
Hill, M. Attack of the Gluten. ChemMatters, 2012, 9-11.
Wieser, H. Chemistry of gluten proteins. Food Microbiology, 2007, 24, 115-119.