Written by: Alice Xie
Bread is deceivingly simple. At its core, it is merely a mixture of flour and water, with some salt or an extra egg to improve its texture or leavening qualities. There are only a few ingredients and the directions seem straightforward enough - so why is it that we sometimes end up with a dense, hard bread?
Many recipes often include informational tidbits on gluten development: “rest the dough to allow proper gluten development”, or “bread flour contains higher protein content, promoting gluten formation”. But what exactly is gluten? From the perspectives of food chemistry and material science, gluten is the structural network that is crucial to the rise, texture, and grain of our daily bread. Often, a weak gluten structure is the culprit behind a bad loaf.
What is Gluten?
Gluten is not a singular molecule but rather a network of multiple water-insoluble proteins that bond and interact with each other as water and flour are mixed and kneaded. This network acts as a foundation that gives the dough structure. Dan Souza, from America’s Test Kitchen, demonstrated how this network behaves by washing away the starch from a well developed dough, leaving behind a bare mass of gluten. When air is blown into the gluten, it expands without tearing, much like a balloon.
Gluten largely consists of two groups of proteins: gliadins and glutenins. When bakers refer to ‘gluten development’, it is in reference to the entanglement of these proteins.
Gliadins, represented in the above diagram by circles, affect the wetness and stretch of the dough - increasing their concentration within a flour results in a firmer bread. Unlike other proteins within gluten, gliadins are compact. The positioning of cysteine residues at the ends of gliadin proteins permits the formation of intramolecular disulfide bonds, balling up the proteins and preventing bonding to other groups in the network. Thus, gliadins work as plasticizers - they act like beads between strings, limiting the strength of interactions between adjacent polymers.
Glutenins, on the other hand, are long chains of amino acids. They affect the strength and elasticity, or ability of the dough to return to its original shape. More glutenin gives a softer loaf with larger specific volumes. Compared to gliadins, glutenins are able to form both intra and intermolecular disulfide bonds - the positioning of cysteine residues here creates steric hindrance, promoting interactions between different proteins rather than within the same protein molecule. These intermolecular interactions help to hold the gluten network together.
When mixed with water, hydrogen bonds and additional disulfide bridges begin to form between glutenins. Gliadins become trapped within this network, and as kneading continues, old bonds break and new bonds form. The network undergoes dynamic changes as the dough progresses through different stages of gluten development.
Bakerpedia provides an informative table on the observable characteristics of dough during kneading. Dough that has not been kneaded adequately is considered “undermixed”, having not developed a proper gluten network. Over kneading, in comparison, occurs when fully developed dough is stressed too far, causing breakdown. A microscopic view, using scanning electron microscopy (SEM) images from Amend and Belitz allows us to see how physical manipulation of dough affects the final product and gluten structure.
Stages of Gluten Development
Pick-Up Bakers often describe the initial incorporation of ingredients as a “shaggy dough” - it is soft and limp, with some sticky, some dry areas. The SEM image reflects this, displaying strands of protein that hang loosely, interrupted with gaps of air. The direction of the strands are also rather nonuniform, creating a sagging web-like structure.
Initial Development Once kneading starts, tensile strength begins to form. While large gaps still exist in the network, the proteins themselves are in the process of realigning to form an entangled sheet. Uneven conglomeration creates thick and thin “ropes”.
During clean-up, the gluten network has developed to the point where strands are becoming more evenly distributed, with some gaps occupied by starch granules (seen as the ovoid shapes). However, the network is still lacking in strength. Applying pressure causes strands to break easily.
The dough is now ready to be proofed and baked! Compared to before where the gluten network consisted of delicate strands, it has now become a strong sheet that is able to withstand a larger degree of extension. Starch granules are uniformly distributed, held in place by gluten. To check for final development, many bakers use the “windowpane test” by stretching out a piece of dough into a thin film that allows light to pass through while not tearing. The test indicates the strength of the gluten as well as the formation of these “sheets”.
If the dough has been kneaded past final development, the sheets of gluten begin to break down, creating limp and tangled strands. This causes the dough to be easily torn. Overmixing also causes hydrogen bonds to break between proteins and water molecules, leading to an overly sticky dough that is difficult to work with.
Making bread is not all about kneading, however - bread recipes that rely on a risen dough also call for periods of “resting.” Why do we need to rest dough if kneading already develops a gluten network that can trap gases? The answer lies in the balloon analogy - if the material of the balloon is too tense or rigid, it will pop. Similarly, kneading the dough helps to create strength and elasticity - however, an overly “strong” network is inflexible. By allowing the dough to rest, gluten strands can relax and bakers improve the rise that occurs in the oven, leading to a lofty loaf. At the same time, yeast fermentation can occur to allow for some pre-baking rise.
As complex as gluten is, it is only one component of what makes bread, bread. Proteins other than glutenin and gliadin, starches, and of course, yeast, all affect the final product. While gluten plays a significant part in the structure and rise of breads, other compounds affect the viability of gluten and, on their own, impart unique flavors and characteristics to individual loaves.
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