Chemists look to improve bread dough by investigating the protein bonds that form its glutenous network.
THE BEHAVIOR OF WHEAT FLOUR can be understood by analyzing the properties of its two main components: starch granules, which swell up in the presence of water, and proteins, which form a glutenous network as dough is kneaded. How do the forces among proteins contribute to the mechanical properties of the dough? It has long been known that bonds between the sulfur atoms found in wheat proteins play a role in structuring gluten. Other forces have been discovered as well.
Gluten is a viscoelastic network of proteins that becomes elongated by pulling and then partially reverts to its initial form when the tension is relaxed. The quality of bread depends on the quality of its gluten. Indeed, gluten is what makes breadmaking possible: Yeast produces carbon dioxide bubbles, with the result that the volume of the dough increases, and the protein network of the gluten preserves the dough’s spherical shape by retaining these gas bubbles. It therefore becomes necessary to understand the reticulation of wheat proteins, that is, how bonds are established between them.
As early as 1745 the Italian chemist Jacopo Becarri showed that gluten can be extracted by kneading flour with a bit of water and then placing the lump formed in this way under a thin stream of water. Rinsing washes away the white starch granules, leaving the gluten between one’s fingers. It was later demonstrated that only certain insoluble wheat proteins called prolamins make up the glutenous network of bread.
These prolamins are of two types: gliadins, which are composed of only a single protein chain (a sequence of amino acids), and glutenins, which are large structures composed of several protein subunits linked by disulfide bridges (that is, the subunits are connected by two covalently bound sulfur atoms). Do these disulfide bridges also link the glutenins to one another? The traditional view is that kneading establishes supplementary disulfide bridges between the various prolamins that break and almost immediately reform as the baker works the dough.
The glutenins have a central domain (containing 440–680 amino acids) formed of short repeated sequences and flanked by two terminal domains. The size of the central domain determines the molecular mass of the glutenins; the terminal domains contain cysteines, amino acids that bear sulfur atoms capable of forming disulfide bridges. Nonetheless, the chemical characteristics of glutenins do not completely explain their capacity to make gluten.
A World Made of Dough
In 1998, a team led by Jacques Guégen at the Institut National de la Recherche Agronomique station in Nantes showed that prolamins can bond with one another by means of dityrosine bonds. Tyrosine is an amino acid whose lateral chain is composed of a CH2 group, a benzene nucleus, and an –OH hydroxyl group. Shortly afterward, on the basis of this research, Katherine Tilley and her colleagues at the University of Kansas demonstrated the importance of dityrosine bonds in gluten. From bread dough at various stages of kneading, they extracted, dissociated, and chemically analyzed the gluten of the kneaded flour and found that concentrations of dityrosine increased during kneading. This raised the question of what role dityrosine plays in the formation of gluten.
Further analysis disclosed the existence of two types of dityrosine bonds: dityrosine, in which two benzene groups are linked by a neighboring carbon atom of the –OH hydroxyl group; and isodityrosine, in which an oxygen atom belonging to a hydroxyl group on one tyrosine binds to its carbon neighbor in the hydroxyl group on the other tyrosine.
This discovery caused a stir among gluten chemists, for dityrosine bonds are commonly found in plant proteins, whose sequences and structures resemble those of glutens, as well as in resilin proteins, found in insects and arthropods, and in elastin and collagen, both found in vertebrates. In forming dityrosine bonds by kneading dough, the baker reproduces the living world.
On the other hand, the Nantes team observed that dityrosine bonds occur in the presence of a type of enzyme known as peroxidase, which is naturally present in flour. Does the long working of the dough needed to make bread cause the enzymes to react with the glutenins by giving them time to establish dityrosine bonds? What roles do dityrosines and disulfide bridges play in the formation of gluten?
The ability to identify these bonds raises a further question. Improved additives can be used to facilitate kneading or intensify the production of gluten. When oxidant compounds such as ascorbic acid and potassium bromate are added to bread dough, for example, the number of dityrosine bonds that are formed increases. It used to be thought that additives of this sort favor the formation of disulfide bridges, but it may be that they also cause dityrosine bonds to be created. In that case one may imagine new methods for selecting wheat on the basis of its gluten content. Would it be enough simply to measure the quantity of dityrosines in a certain kind of dough in order to assess the quality of its gluten?