The following post is a paraphrased version of chapter 10 in [1]
Table of Contents
Wheat
Sugars and Soluble Enzymes
The wheat grain is largely made up of starch, which is composed of two different types of glucose polysaccharides, amylose and amylopectin. In its polysaccharide form, the sugars are too large for yeast enzymes to access it for metabolism. Luckily, flour contains soluble enzymes called amylases. When activated with water, the amylases carry out hydrolysis reactions, using water to break the starch into: maltose, glucose, and other smaller glucose polysaccharides.
Gluten
The two classes of proteins that make up 75% of the protein content of the wheat grain are gliadin and glutenin. Gliadins are theorized to be small, tightly coiled proteins that fold into a tight, spherical structure. Glutenin proteins are helical structures which are much less tightly packed, and often bond with one another to form long protein complexes. In recently mixed flour/water mixtures, the gliadin molecules and glutenin protein complexes do not interact with anything. Once the mixture is folded or kneaded, the glutenin structures physically stretch, allowing them more opportunity and space to interact with one another, forming large, orderly, stretched protein sheets. The gliadins fill in the spaces in the network and limit the intermolecular interactions between the glutenins, giving the dough its plasticity and resulting in the final protein complex called gluten. Gluten accounts for the strength and elasticity of dough.
Yeast
Yeast uses the small sugars broken down by the amylase enzymes to carry out glycolosis, an anaerobic metabolic process which produces energy for the yeast and carbon dioxide and ethanol as byproducts. The CO2 is only captured in the bread if the gluten network is sufficiently strong enough.
Baking
During the baking process, the heat causes the pockets of trapped CO2 to expand, causing the bread to inflate and increasing the size of holes in the crumb. If the gluten network is too strong, however, the bread will refuse to expand. As the dough reaches 60 C, vaporization of the ethanol and water further contributes to the expansion of the dough. At 90 C, the crust becomes too firm and solidifies the final outer shape of the loaf. At 150-180 C, the starch gels into a more solid state and the denatured gluten proteins form even stronger bonds. The color and aroma of the crust is due to the Maillard reactions.
Sources
[1] Provost. The Science of Cooking. Wiley, 2016. Pages 343-365