In many cultures, including our own, bread constitutes a significant portion of the daily diet. However, how often do we stop to think about the unique material properties of the food that we eat and how those properties influence how bread (or any other food) is made?
The two basic components of bread are water and wheat flour. Most powdery ingredients used in the kitchen, like baking powder or cornstarch, form a paste when mixed with water. In contrast, bread dough made by mixing wheat flour with water, in a 5 to 3 ratio, forms a cohesive material that retains its shape and is stiff enough to be manipulated by hand, unlike a paste. If you punch down on a piece of dough, you will notice that the dough deforms to the shape of your hand. Once you remove your hand, the dough will return to its original shape. Two material properties allow bread dough to respond in this way – “plasticity” and “elasticity”. The ability of the dough to deform under the pressure of your hand is due to its plasticity, whereas the ability of the dough to move back to its original shape is due to its elasticity. Both of these properties are important characteristics that contribute to the texture of the bread.
Wheat flour contains two main proteins – glutenin and gliadin – that make up what is known as the “gluten network,” which gives flour mixed with water its unique properties. Glutenin consists of long amino acid chains that can form strong bonds with other glutenin molecules. This enables chains of hundreds of glutenin molecules to form the extensive interconnected gluten network. Gluten is elastic because the glutenin molecules have a coiled and kinked structure that allows them to act like tiny springs. When bread dough is stretched, these glutenin molecules straighten out, but when this pressure is released, the dough pulls back to its original state. Plasticity comes from another protein, gliadin, which resides among the glutenins and allows glutenins to slide past each other without bonding.
What Makes Bread Fluffy?
Kneading the dough is a critical step in the bread making process because it helps to stretch and organize the glutenin proteins into an elastic network. Adding a small amount of salt can further strengthen the gluten network; ions from the salt cluster around the charged portions of the glutenin proteins and prevent them from repelling each other, which allows glutenin molecules to cluster more closely together. Kneading the dough also introduces air bubbles that expand when heated in the oven. These air pockets can also serve as starter bubbles that will expand when gas is produced in the rising dough by yeast. In the absence of yeast, these bubbles help produce a fluffy flatbread. The more bubbles generated through kneading, the finer and more tender the final bread product.
Mechanics of Yeast
Although it is not necessary to use yeast when making bread, yeast does impart certain flavors and aromas. Humans have been eating raised breads made with yeast for 6000 years, but it wasn’t until about 150 years ago that we started to develop our scientific understanding of microbes such as yeast – thanks to researchers like Louis Pasteur. Yeast are single-celled fungi with more than 1500 different known species. One very popular type of yeast is Saccharomyces cerevisiae, which is better known as “baker’s yeast” or “brewing yeast”. In the past, yeast was simply supplied from an earlier piece of dough or from the surface of beer-brewing vats; but today, yeast strains specifically selected for bread-making can be purchased commercially. Yeast produce spores that are ubiquitous in the air as well as on grain surfaces, and these can readily grow in moist materials, so it is likely that the first yeast-raised bread was probably a delicious accident!
Yeast metabolize simple sugars such as glucose, fructose, and maltose – which are produced in flour by enzymes in the wheat plant that break down complex sugars like starch. The yeast ferments these simple sugars to form alcohol and carbon dioxide. The carbon dioxide is then released into the bread dough, where it diffuses into and enlarges the tiny air bubbles that were formed during the kneading process. Amazingly, as much as 80% of the bread’s final volume can be empty air space due to these bubbles! The expansion of the bubbles interrupts and weakens the gluten network, dividing it into millions of very thin delicate sheets that form the bubble walls. The production of carbon dioxide by yeast can be further increased by adding a small amount of additional sugar and salt and by maintaining the bread dough at 95ºF/35ºC – an optimal temperature for yeast. Yeast also release a variety of other compounds that strengthen and improve the elasticity of the gluten network, in addition to enhancing the flavor of the final product.
Wheat flour consists of about 10% protein and 70% starch. Starch is necessary for bread to reach its final rigidity. During the baking process, starch granules become incorporated into the gluten network and absorb water, causing them to swell and form the rigid bulk of the walls that surround the carbon dioxide bubbles. At the same time, the rigidity limits the expansion of the bubbles due to heat and expanding gases. The gases trapped within the bubbles eventually pop through the bubble walls, thereby turning the foam of separate bubbles into a continuous spongy network of interconnected holes. This also allows for the release of water vapor from the bread – a crucial step, since if the water vapor is not released, it would condense and contract as the bread cools and cause the bread to collapse.
The unique balance of properties of the materials present within bread, the formation and expansion of gas bubbles, and the rigidity provided by starch all act together to produce the tasty marvel we call bread. So next time you bite into your light and fluffy slice or crusty and chewy loaf, consider the science of the materials that makes it possible!
Emily Gardel is a graduate student in the School of Engineering and Applied Sciences
McGee, John. On Food and Cooking. New York: Scribner, 2004.
Ruhlman, Michael. Ratio. New York: Scribner, 2009.
Links of Interest
America’s Test Kitchen: Multigrain Bread