Choux is a truly ingenious invention, though it almost certainly wasn’t “invented” in the classic sense of the word. It evolved, probably through decades, maybe even centuries, of trial and error. The secret of choux is that it’s “double cooked”, a process that imbues it with some very special properties.
If you’ve made éclair or cream puff shells before, you probably recall the process. Water and butter are combined in a sauce pan and heated to the boil, at which point enough flour is added to turn the mixture into a fairly stiff paste. That paste is then cooked over low heat until it forms a ball, which is then finished by beating in several eggs, one after the other. It’s then piped and baked (the second “cooking”). Interesting. Curious. But what does it all mean?
Choux is a batter which by design contains a lot of activated gluten, and that’s unusual. Normally batters, regardless of how thick they are, are low-gluten affairs. Recall how one is time and again admonished not to stir a pancake batter for fear of toughening the cakes. The same goes for muffin batters, where agitation leads to big bubbles or “tunnels”, crowning and a gummy texture. That logic is reversed with choux, which is beaten vigorously in an attempt to activated the gluten and turn the entire thing into one giant bubble.
But then what does the double cooking accomplish? For one it causes the flour to gelatinize, which creates a starch mesh that helps reinforce the gluten. When baked, these dual networks help the bubble of expanding batter to stretch (they also help prevent steam from escaping). But the cooking also does something else. It partially denatures (science-speak for “messes up”) the gluten molecules so that they lose some of their elasticity, helping to ensure that when the single bubble is fully expanded it doesn’t snap back before the pastry hardens.
Amazing, n’est pas?