Brioche is one of those base pastry components that most of us take for granted, and that’s a mistake because brioche is miraculous in itself. It’s a bread, but more than that a golden bread, and more than that a very rich but still feather light golden bread, full of egg yolks and butter (sometimes brown butter, which cranks the deliciousness factor still higher). But how can a bread contain so much egg and milk fat yet remain so light? Friends, it’s all in the mixing.
The story of pastry is one of war between the forces of Up and the forces of Down. The Up brigade consists of all the materials that give a baked item its volume and lightness: mainly white, gluten-rich wheat flour but also water, egg whites and leavers like yeast and baking powder. Together these components form what you might think of as the beams and girders of a bread or cake layer. They are the structure that surrounds and contains the empty space in the crumb.
The opposition consists of everything that gives bread its flavor and tenderness: fats of all kinds, sugars and syrups, wheat bran and germ, and any sort of inclusion you can think of from pieces of fruits and vegetables to purées like pumpkin, cocoa powder, chocolate chips, nuts, seeds and so on. All these make breads and cakes interesting. But they’re also all downers, so to speak. They undermine structure, either by interfering with gluten formation or by simply weighing the dough down to the point that the leavening can’t lift it.
The trick to good brioche is manipulating the forces Up and Down in such a way that they deliver the best of both. One could in theory knead the heck out of a flour-and-water paste to get a good strong gluten network formed, then add fat, since once a gluten network is formed fat can’t easily disrupt it. But that’s not how brioche dough is made. The fat is added steadily, some in the initial sponge, more in the first stage of mixing and then a whole lot in the second stage of mixing. The result being that a strong network of gluten molecules never quite has a chance to form. Some of the gluten molecules end up getting lubricated with fat along the way, and that forever denies them the opportunity to get the hook up, as it were.
Yet to ensure that this gluten-compromised dough can get as strong as it reasonably can before baking it is mixed, mixed, mixed — up to 25 minutes for some formulas. All that mixing develops any remaining gluten but also creates a very uniform dough which bakes up with a tight, even crumb. That fine texture is itself a source of strength for the finished brioche. In place of a system of thick beams and supports you have a fine lattice-like structure which is surprisingly strong but still a delight to chew. High ratio cake layers are built according to this same principle. There again, more fat and sugar doesn’t necessarily translate to less height or fall-apart texture, because it’s all in how you mix it.