The Secret of Bread - excerpt

[Guest guest]

Molecular Nutrition and Genomics

Nutrition and the Ascent of Humankind

Mark Lucock, JOHN WILEY & SONS

 

 

Chapter 35- excerpt

 

 

The Secret of Bread

 

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 440û680 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 Guegen 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 for- mation 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?