I was working on a podcast for A Year From Scratch, and my co-host, Ben, asked me what the mechanism was that caused dough to become so elastic that it's hard to roll out, and what happens to cause it to relax when you let it rest. That's when I realized that my understanding of gluten was incomplete. I hit the web, opened up On Food and Cooking, and determined to correct my ignorance.
Gluten is a network of proteins. It's what you get when you combine two molecules, glutenin and gliadin, with water. It's what holds doughs and batters together, makes muffins tough if you overwork it, and allows crusty French bread to have magnificent holes the size of a quarter while still allowing the bread to hold up.
Glutenin looks like a long spring, and essentially that's what it is. At either end of the spring, it can connect with other glutenin molecules. This allows the glutenin to form really long chains that stretch out really far. When you're done stretching, though, the springs keep most of their springiness, and the dough slides back mostly into the shape it was before your stretched it. This is gluten's elasitc nature.
Gliadin is a tightly wound ball with sticky points all over it. If glutenin were a metal spring like a slinky, then gliadin would be a tennis ball with little rubber hooks every 1/8" or so. Each hook can easily be bent and escaped from, but if you have enough latched onto the spring, it's hard for it to get out. As you knead the dough, more and more of these hooks get caught on the springs of the glutenin molecules. This keeps the gluten together into the shape you put it, which is considered gluten's plastic nature. It's also this gliadin that causes dough to become hard to knead after a while. All those hooks get caught in all those springs, and there's no way that you can stretch the dough where everything isn't pretty well caught up.
Individually, all of those hooks in the gliadin molecules are very weak. They can't maintain their grip forever. All the tension from the springs of the glutenin and all of the other gliadin molecules pulling in different directions will, over time, cause individual hooks to let go. Each time a hook loses its grip, the other hooks have more of a load to carry, and so they will begin to fail more quickly. After about 20 minutes of this, most of the gliadin has lost its grip on the glutenin, and all that remains holding everything together are the strong bonds between glutenin molecules. At this point, the dough is "relaxed," and it is easy to knead or form.
Some other notes about kneading and gluten formation. When glutenin molecules link up, they do so with the help of oxidization. Oxidization is common in our everyday lives. Most popularly, it causes iron to rust, but it's really everywhere. If you break a glutenin-glutenin bond and reform it, the dough is going to become a bit more oxidized. This is going to take a toll on the dough. The more you knead a dough, the more you take out of its unique color, flavor, and character. For example, look at this picture:
Those are two pieces of pasta dough from the same batch. The dough on the left has been kneaded a lot more than the dough on the right. All of the oxidization in the dough has essentially bleached out the dough, which is less than ideal. Kneading isn't a bad thing, certainly, but kneading unnecessarily will reduce the uniqueness from your end product. If you're making something like an enriched white bread, or cinnamon rolls, or something where the flavor is coming from something more than just flour, water, salt, and yeast, then it's not a big deal. But if you're making a lean dough where you're trying to bring out the most of the flavor of those four ingredients, then knead thoughtfully.
What if you didn't knead at all? If you've been following the food news for the past couple of years, you probably heard about the revival of the No Knead Bread. Essentially, you take a relatively wet dough, stir the ingredients together in an enamel-covered Dutch oven (a.k.a. a French oven) and let that sit overnight. Then you bake the whole thing, French oven and all, in the proper oven. There's plenty of gluten formed, with no mechanical agitation.
What happens to cause the gluten formation is that the glutenin molecules are floating around in the water willy-nilly. As their edges come into contact, they form bonds. Because there's no mechanical agitation (i.e. kneading), these molecules don't get broken apart. Consequently, the bare minimum amount of bonds get formed between the glutenin molecules, which means the bare minimum amount of oxidization happens to do the dough. This is not the only way, nor necessarily the best way, to make a bread, but it is certainly an important way.
Learning to cook, as with most things in life, is a constant re-evaluation of what you think you know vs. what you really know. We start at one level of understanding, then, when we're ready, we move on to the next stage. I look forward to finding out what else I don't really understand about what's happening in my gluten, and embracing it that much more fully when the time comes.