That’s from reader Paul. It’s an often-asked question and one you can’t talk about enough, in my opinion. The one word answer to the question is: gluten. Gluten is a catch-all word for the various proteins found in flour. A great blessing to the bread baker, they’re frequently a curse for the pastry maker.
American bakers have a particular problem with gluten because the gluten found in American flour is very stretchy and elastic. Not so with most European gluten, which tends to be hard. That means Europeans have a much easier time with things like pie doughs, which don’t shrink up in the oven like American doughs do.
Gluten becomes “developed” when it’s combined with water. Which is to say: with the addition of water the spring-like protein molecules present in the flour start to bond to one another end-to-end. What you get then is an entire network of springs that stretches and pulls back whenever you apply any sort of pressure to the dough. In fact the whole process is greatly accelerated whenever you knead or agitate a mass of moistened flour.
This is why pretty much every dough made with North American flour wants to snap back whenever it’s rolled or stretched. Even more frustrating: even if a dough isn’t visibly snapping back as you’re working with it — say when you’re rolling out a rested pie crust — it will later in the oven. For heat causes protein molecules to ball up. If all those protein molecules are all connected one one another, the overall effect will be a mass drawing-in. Which means the crust will shrink.
So how to combat the curse of elastic gluten? Answer: with sustained pressure. Steady pressure eventually causes the taught networks of molecules to break. A rolled sheet of pie dough looks pretty relaxed just sitting there. But if you could somehow go inside it, you’d see titanic forces at work: legions of hyper-extended proteins, hand-in-hand, struggling heroically to pull a flat piece of enriched dough back into a ball. After a few minutes of this fruitless activity, one of the molecules can’t take it anymore and lets go. Then another, and another and another until after an hour or so there’s very little of the original network left.
This is what it means to “relax” dough. You let it sit until all the little proteins tire out and release each other. It takes time, and it needs to be done every time you apply any sort of pressure to a crust — because even the slightest touch or pressure will activate more gluten that must then be relaxed. This is why double crust pies need to be relaxed both after they are rolled and then again after they’re filled and assembled.
Yes that means adding a lot of time to a process that happens in the blink of an eye on the Continent. But what can I say, it’s one of the prices we pay for living in North America versus Europe. Console yourself with the thought that we play much better basketball.