Gluten is a controversial subject these days, so lets start with the basics and move on from there. Gluten is protein found in wheat. (Some other grains have very similar proteins which for convenience are also referred to as ‘gluten’.)
This protein creates the gluey texture of flour mixed with water. As the gluten is developed in bread dough (usually by kneading, though sometimes through allowing a wet dough to sit for long periods of time as in Jim Lahey’s 24 hour bread) it creates the elasticity of the dough.
It is these gluey, elastic fibers of gluten that hold the bread together and trap the small bubbles of ‘air’ (gases created in the dough, usually by yeast) that create the lightness and fluffiness of a good piece of bread. The developed gluten also contributes to the chewiness of the bread fibers around those air pockets.
I have never found a really good explanation of the difference between the actions of yeast and the actions of baking soda or powder, and why one requires the development of gluten more than the other, but here are the differences as best I understand. (If anyone has more complete information please comment and let me know!)
Yeast works slowly, releasing the ‘air’ bubbles over time. A developed gluten (kneaded or very long rising dough) holds those bubbles in as they develop and contributes to the more solid and chewy texture we expect from yeast breads. Insufficiently developed gluten will allow these bubbles to escape or merge, creating a denser bread with larger and less regular holes, instead of an even textured spongy network of holes.
Baking soda (and baking powder, which is baking soda mixed with an activating agent) works very quickly. The undeveloped gluten holds the batter together, but isn’t needed to trap bubbles, as the dough or batter is generally mixed and baked immediately as the bubbles are forming. The lack of developed gluten allows the texture to be softer and more tender than the chewier yeast breads. (This is why biscuit and muffin recipes warn against over mixing, to prevent accidental development of the gluten.)
In my next installment of this series I’ll tackle some of the more controversial aspects of gluten, allergies and digestion.