Among the breads, rolls and baked goods on many tables this holiday season is an unusual ingredient: gluten. The unique chemistry of gluten makes foods airy and elastic.
I’m a chemist who teaches cooking chemistry, and every year I ask my students: “What is gluten?” Common answers are “sugar” or “carbohydrates”. But rarely does anyone get it right.
So what is gluten?
Gluten is a complex mixture of proteins. It makes up 85%-90% of flour protein. Proteins are naturally occurring biological macromolecules composed of chains of amino acids that fold upon themselves into various shapes.
Gluten is obtained from the endosperm of wheat, rye, barley and related plants. The endosperm is a plant seed tissue that serves as a storage site for starch and protein. As a result of the flour formation process, the contents of the endosperm, including gluten, are released.
The main proteins in the gluten mixture are gliadin and glutenin. These proteins make up most of the structure of flour-based foods. During the kneading or mixing of dough preparation, these proteins form an elastic network, often called a gluten network.
Creating a gluten network
The formation of a network of gluten is key for the dough to rise. The mesh acts as a bubble to trap gases during the rising, setting and baking processes. During proofing and proofing, when the dough is given time to expand, the yeast releases carbon dioxide as it eats and digests the sugars present. This process is called fermentation.
A number of different gases are produced during the baking process, such as carbon dioxide, water vapor, ethanol vapors, and nitrogen. The gluten mesh traps these gases and the dough expands like a balloon. If the gluten network is too tight, the gases will not create enough pressure to make the dough rise. If it is too weak, the bubble will burst and the dough will not remain swollen. How strong the gluten network becomes depends on how long you knead and mix the dough.
In order for the gluten network to form, you need to knead the dough or mix it with a little water, which evens out the proteins.
Glutenin proteins come in long and short chains that adopt coiled structures. These coils are held together by attractive forces between the loops of the helices known as intramolecular hydrogen bonds. Kneading and mixing breaks some of the attractive forces and evens out the gluten proteins.
Bonds between individual glutenin chains are formed through sulfur atoms on some of the amino acids that make up gluten. When these amino acids, called cysteines, come into contact with each other, the sulfur atoms bond together to form a bond called a disulfide bond.
As more and more cysteines form disulfide bonds with cysteines on neighboring proteins, the network grows larger. So, the more proteins there are and the longer the kneading process, the stronger the gluten network. Bread flour has higher protein concentrations of 12%-14% than other flours, so bread flour results in a stronger gluten network and greater growth.
Gliadin proteins are smaller and more compact than glutenin proteins. During the kneading process, the gliadin is dispersed on the glutenin polymers. While glutenin gives dough elasticity and firmness, gliadin proteins make dough viscous or runny and dense.
Strengthening and reduction
Adding salt neutralizes any charge the proteins may have. This minimizes any repulsion between the proteins and brings them closer together. This process forces water out of the proteins, which both brings the proteins closer together and stabilizes the network. So adding salt will create a tighter network that increases the amount of stretch and stretch the dough has.
Fats like butter or margarine will weaken or “shorten” the gluten network. Usually, recipes call for the fats to be mixed with the flour before adding water or milk. So the fats cover the flour. And because fats are hydrophobic, or water-repellent, this process prevents the water that helps form the gluten network from reaching the proteins. This results in a softer, more tender cake.
Without the formation of a gluten network, baked goods will not turn into the light and fluffy delicacies we love.
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