Why do pie crusts shrink?
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.
This is very interesting because I have a real problem with gluten-free pie crusts shrinking. Any thoughts on why that could be happening? (Prolly not, but worth a shot…)
Interesting. What sort of flour are you using?
I usually use a mix of starches and rice flour with a bit of millet flour for flavor. Starches I use are corn, tapioca, and potato…I have used mixes of all three, and sometimes just one. One very good reference for GF pie crust is here:http://www.artofglutenfreebaking.com/2009/11/pie-crust-gluten-free-refined-71210/
Although she doesn’t explain the shrinking either. Oh, and Xanthan gum is hard to come by where I live so I sometimes use guar gum instead.
UPDATE: Ann from It’s Gluten Free! (http://cestsansgluten.net/www-en) says:
One thing that I would say about is xanthan gum and/or gelatin: both might shrink during baking, which is why you have to use them wisely. Xanthan gum is heat stable, but it keeps shrinking the food before/during/after it’s baked. Psyllium has much better shelf life.
I also found out from a food scientist friend that xanthan gum’s heat stability increases in wetter mixtures. Which explains a lot: short crusts are extremely dry.
Hope this helps!
I was about to say this too. My gluten-free pie crusts shrink quite a bit. I use an equal blend of sorghum, white rice, brown rice, and cornstarch, plus xanthan gum.
Hey Mark! I’ve done some asking around and I can’t seem to get a solid answer as to why this would be. However it seems that gluten-free short crusts are notorious for shrinking in the oven. As to why that would be I don’t know. My first thought was that it had to do with the xanthan gum shrinking up in the heat. However the industry abstracts I looked at show that xanthan gum is quite heat stable. Furthermore, it seems that short crusts made without xanthan gum still shrink. So what’s causing the dough to consolidate? And what role does heat play in that consolidation? I have no idea. However from what I’ve been finding out, resting does help. I’ll keep trying to find out the answer to this. Meantime, anyone else out there with any idea, please weigh in!
– Joe
How long should you let it rest at each stage?
Hi Magi!
About an hour is best.
– Joe
So, why is our flour so snappy? Does it have advantages for certain applications, or are we some how incapable of creating European style flours?
Good question, V! It’s simply that the varieties of wheat that thrive in our climate produce proteins of a different character. I suppose the difference must reflect some sort of adaptation, but I’m not knowledgeable enough about wheat biology to be able to explain it. 😉
– Joe
Love your blog!
Thank you, Jessica! Come back!
– Joe
I’ve been searching for an answer to this question all morning and You are the only who one has actually answered it…Thank You!
So, what you’re saying is, let the pie dough rest for at least an hour after making, then let the assembled pie rest again for at least an hour before baking?
Hey Vicki! And yes, that’s what I’m saying. A full hour may be pushing it, but the truth is that any time you manipulate the crust in any way you’re activating gluten and creating the conditions for shrinking. The best bet is to allow lots and lots of resting so the gluten can fully relax.
Thanks for the question, Vickie, and good luck with the pie!
– Joe
I am trying to imagine allowing a double crust pie to sit / rest once it is filled with fresh apples or a blueberry pie filling? Do you recommend placing it in the refridgerator….on a counter?
Also, for a single crust to rest I imagine it should rest in the fridge so that the fat remains cold?
this is all intriguing to me, as I have always tried to keep pastry dough cold while working with it….