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Amy Rowat, a biophysicist and professor at the University of California, Los Angeles, understands the nuances of pie-making better than anyone. For four years, she’s tinkered with the quintessential American dessert to teach chemistry and physics to students. Here, Amy explains the science behind the art.

Popular Science: Do you feel like you have the pie all figured out, or does it sometimes surprise you?

Amy Rowat: I’m always experimenting with pie, have been for decades. Rarely do I make the same recipe. My students are always experimenting with optimizing piecrust and filling, like using avocado in the crust instead of butter.

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Do you advocate for butter, shortening, or lard?

Normally I do all butter but in some cases, like savory pies, a lard crust can be really delicious. Butter makes for a flakier crust because it has higher water content, and I also prefer the flavor. Lard and shortening make a shorter crumb, so there are fewer pockets of air.

What’s the deal with substituting alcohol for water when making pie dough? Does it really work?

Water interacts with the protein in the flour to form the gluten protein network. Unlike bagels, you don’t want these networks to develop as much, you want tender flake, not chew. Alcohol, since it has lower water content, can’t participate in the bonding process. The idea is that you can handle the dough better without adding more water molecules. Vodka is tasteless, obviously, but we’ve used rum and bourbon to add flavor.

“Flake is very complex.”

Explain to us how flake forms.

Flake is very complex. But one of the factors is that you have these chunks of cold fat that you distribute throughout your flour. Once it’s in the oven, that butter goes from solid to liquid, and the water that separates out turns to gas, which forms air pockets to create flake. I like mostly pea-sized chunks of gritty flour, but I keep some bigger chunks is as well. There’s evidence from our student projects that American butter, which has slightly higher water content than European butter, makes a more porous crust.

What are your thoughts on gluten-free crusts?

They’re very challenging to make. You’re adding in different starch molecules that entangle with each other to form networks. But a lot of these molecules suck up a lot of water and don’t develop the same type of interconnected bonds. That being said, adding an egg to the dough really helps to bind it. And an egg wash helps with browning on gluten-free crusts.

How do you know when a pie is done baking?

With pies that have a top crust, visual cues are key. You’re looking for a toasty brown color. Don’t worry about the time so much. One of our chef contributors says you need to bake your pies until that “oh shit” moment when you think it’s gone too long.

As a baseline, Dr. Rowat recommends this recipe for crust from Martha Stewart.

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