I could spend quite a long time expounding on the microbiology of yeast breads (and in fact I often do that, as long-time readers of joepastry.com know). But if the Straight Dough Method had to be reduced to any one thing, that thing would be gluten. For bread making is all about gluten.
And why is that? Gluten, you see, is what’s ultimately responsible for raising bread. Oh sure, yeast organisms produce the CO2 gas that gets all those little leavening bubbles started, but if it wasn’t for the elastic mesh that is a gluten network, all that gas would just dissipate out the top and sides of the loaf, leaving a crumbly brick of moistened grain behind.
And so we knead, for as I’m constantly repeating, flour + moisture + agitation = gluten. In the case of the Muffin Method, the Biscuit Method and the Creaming Method this is a bad thing for the most part. But where the Straight Dough is concerned, it is a very, very good thing.
Gluten, you see is made up of two types of wheat protein: glutenin and gliadin. Glutenin is the workhorse of the two, being a long-chain molecule that is capable of bonding very tightly to other glutenin molecules — both end-to-end and side-to-side. Gliadin, by comparison, is a curlicue-shaped chain that forms only weak chemical bonds, though it nevertheless has its place in the Straight Dough universe, being a sort of wadded-up spoil-sport that keeps glutenin molecules from bonding to one another too tightly and/or in too many places. Together these two proteins create was is essentially a net that catches the CO2 bubbles the yeast create.
A common misconception among bread bakers is that it’s this CO2 that’s primarily responsible for raising bread. In fact what really leavens a loaf is steam. Similar to the creaming method, where sugar is driven into fat to create “seed” bubbles, CO2 creates the initial spaces that fill up with steam as the loaf bakes. Not being a physicist (or any type of real scientist for that matter), I can’t say I know for sure if CO2 gas expands with heat. It probably does to some degree, though definitely not more than water, which, when converted to steam, occupies about 1600 times more space that it does when it’s in liquid form. And so as the loaf heats the bubbles expand, with the stretchy gluten network expanding right along with them until the starch in the dough gelatinizes, the structure sets up, and much of the steam is finally forced out the surface of the loaf. The end result is the light fluffy substance we know as bread.