Bread-making is a scientific symphony of physical, chemical, and rheological processes. Beyond the apparent simplicity of mixing flour, water, and yeast lies an intricate web of molecular and mechanical changes that transform these raw materials into bread.
Dough mixing is regarded as the most crucial step in the bread making process. The primary goals of mixing are :
+ Ingredient homogenization
+ Flour hydration
+ Gluten network formation
+ Air bubble incorporation
This series of interconnected physical and chemical phenomena make mixing an indispensable step, impacting everything from dough texture to bread volume and flavor.
The initial stage: hydration and protein activation
The cornerstone of dough formation is the hydration process. Water activates the flour’s primary proteins – gliadins and glutenins – enabling them to uncoil from their compact, globular shape. During this stage, water molecules break through the flour particles, facilitating the swelling and dissolution of proteins and starch granules. The water absorption thus initiates the gluten network’s formation, albeit in a rudimentary state.
Non-covalent interactions and gluten matrix formation
Once the proteins are activated, the dough undergoes a series of transformations facilitated by non-covalent interactions, such as hydrogen and hydrophobic bonds. The proteins begin to elongate and stretch, weaving an organized matrix that effectively traps water and other molecules, thereby gaining structural integrity. The Farinogram (Figure 1), a tool used for quantifying dough consistency, shows a progression from an inchoate, lumpy mass to a cohesive, elastic structure, hitting a ‘peak development‘ stage where the dough exhibits optimal rheological properties for baking.
Figure 1: Farinograph and Farinogram
Molecular cross-linking: the role of sulfhydryl groups
In the world of proteins, the formation of disulfide bonds (SS) between sulfhydryl groups (SH) of amino acids is a pivotal molecular event. These SH groups exist as individual chains in their reduced form. When oxidized, the SH groups link to form SS bonds. This bonding is particularly significant for glutenin molecules, as it enables them to cross-link and create a larger, stronger network (Figure 2). This molecular cross-linking contributes to the dough’s elasticity, stability, and viscosity, which are crucial properties for bread-making.
Figure 2: The development of the gluten network according to Shewry et al. (1987), Belton (1999), and Veraverbeke and Delcour (2002)
Gliadins Vs. Glutenins: complementary roles in gluten formation
While both gliadins and glutenins are instrumental in creating the gluten network, they serve distinct roles. Gliadins offer extensibility and viscosity to the dough but lack the ability to form intermolecular SS bonds. Their role is confined to intramolecular bonding due to the structure of their cysteine amino acids. Glutenins, however, possess an extra cysteine residue at the end of their chain that can form intermolecular SS bonds, thereby serving as the backbone of the gluten network (Figure 3).
Figure 3: Photographs demonstrating the extensibility of gluten (left) and its components gliadin (center) and glutenin (right)
Mechanical forces: shear and elongation
Mechanically, dough mixing can be broken down into two key forces: shear and elongation. These forces cause extreme strains on the dough, sometimes as high as 500,000%. The high shear rates of 10 to 100 s^-1 are adequate to disrupt the hydrating flour particles‘ outer layers. This shearing action exposes more surface area for hydration, further enhancing the potential for gluten network formation and refinement.
Rheological shifts: elasticity and viscosity
In terms of rheology, the dough transitions from a sticky, highly viscous mass to a more stable and elastic form as mixing proceeds. These changes are marked by shifts in key rheological attributes, such as elasticity, plasticity, and viscosity. The Farinogram captures this evolution, reaching a peak where both elasticity and viscosity are at their optimal balance.
The impact of aeration and starch interactions
The dough’s rheological characteristics are not merely a function of protein interactions. They are also influenced by the air content and starch structure within the dough. Air bubbles entrapped during mixing, and the foam structure from yeast fermentation, play roles in dough rise during baking. Additionally, the interaction between the protein network and starch granules also contributes to the dough‘s stiffness and rheological behavior.
Figure 4: Dough structure after mixing
Overmixing: a molecular and mechanical downfall
The perils of overmixing manifest as a breakdown in both molecular and mechanical properties. Excessive mixing disrupts the delicate balance of SS bonds and non-covalent interactions, leading to a loss of structure. This is signaled by a reduction in the dough’s torque and an increase in plasticity, as captured by the Farinogram.
Grainar
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Conclusion
Bread-making is a complex interplay of chemistry, physics, and mechanics, each contributing to the dough’s final characteristics. Understanding these intricacies requires an integrated perspective that bridges molecular interactions with mechanical forces and rheological properties. By harmonizing these elements, one gains not only a deeper scientific understanding of what happens during dough mixing but also the key to optimizing the age-old art of bread-making.
Key points summary:
+ Water absorption activates flour proteins, gliadins and glutenins, initiating the gluten network formation.
+ Proteins stretch and elongate, forming an organized matrix via hydrogen and hydrophobic bonds.
+ Disulfide bonds (SS) form between sulfhydryl groups (SH) of amino acids, significantly contributing to dough’s elasticity and stability.
+ Gliadins add extensibility but can only form intramolecular bonds. Glutenins serve as the backbone of the gluten network due to their ability to form intermolecular bonds.
+ High shear rates and elongation during mixing disrupt flour particles and enhance the potential for gluten network formation.
+ Dough transitions from being sticky and highly viscous to more elastic and stable as mixing proceeds. This is tracked using a Farinogram.
+ Air content and starch structure contribute to dough stiffness and influence its ability to rise during baking.
+ Excessive mixing breaks down molecular and mechanical properties, leading to loss of structure and increased plasticity.
Note: The terms ‘kneading’ and ‘mixing’ are often used as synonyms in scientific literature, even though they actually explain a temporal sequence of physico-chemical phenomena.
Dough mixing: Refers to the initial phenomena of homogenization and hydration of the ingredients
Dough kneading: Refers to the subsequent development of the gluten network
In this review, both kneading and mixing terms are used to refer to all the above phenomena.
References
1. Dimler, R. J. 1963. The key to wheat’s utility. Baker’s Dig. 37(1):52-57.
2. Shewry, P. R., & Tatham, A. S. (1997). Disulphide bonds in wheat gluten proteins. Journal of Cereal Science, 25(3), 207–227.
3. Veraverbeke, W. S., & Delcour, J. A. (2002). Wheat protein composition and properties of wheat glutenin in relation to breadmaking functionality. Critical
Reviews in Food Science
4. and Nutrition, 42(3), 179–208.
5. Belton, P. S. (1999). Mini review: on the elasticity of wheat gluten.
6. Journal of Cereal Science, 29(2), 103–107.
7. CM. Courtin: Journal of Cereal Science35(2002) 225–243
