The Science Behind Baking Your Perfect Pie (Happy Pi Day)

Editor's note: Last fall, NPR's Maanvi Singh embarked on a months-long quest to find her ideal pumpkin pie recipe. As she discovered, there's a lot of science involved in getting the crust and filling just the way you like it. To celebrate Pi Day, we reprise this story, first published last December.

It was the best of pies, it was the worst of pies. I have baked many, many, many pies.

And when I first began making pumpkin pies this autumn, my results were at best inconsistent and, at worst, disastrous.

"One of the problems is most people make it once, maybe twice a year, right around the holidays, so they're not very practiced at it," says Dan Souza at America's Test Kitchen. "And pumpkin pie is a little bit different in that it's essentially a custard baked into a pie shell." In trying to juggle both components, even experienced bakers may end up with a grainy, overcooked filling or a soggy crust, Souza says. "There's a lot of places you can slip up."

So Souza and J. Kenji Lopez-Alt, a chef and author of The Food Lab: Better Home Cooking Through Science, coached me through the process.

The Ultimate Custard

"Mainly it's the texture that can go very wrong," Lopez-Alt says. "It can end up grainy, or kind of broken up like scrambled eggs."

Blergh.

See, a classic pumpkin pie filling includes a few eggs, as well as pumpkin puree, some cream, sugar and spice.

"As the custard cooks, there's a loose network of the egg proteins that intertwine with each other and set to give the filling structure," Lopez-Alt explains. Ideally, you want to cook the custard just enough to form a soft, gel-like texture. "But as the custard keeps cooking, the eggs start to set so tightly that they squeeze moisture out," he says. That's when you get an icky, lumpy pie.

Both Lopez-Alt and Souza recommend cooking the filling quickly, but gently: The center of the pie should never get any hotter than about 175 degrees Fahrenheit.

And you've got to try to cook the pie as evenly as possible. Some cracking at the center of the pie is inevitable, Lopez-Alt says. It happens because the edges of the pie tend to cook and set faster, and pull at the center, which takes longer to cook.

Souza's pro tip is to bake the pie at about 400 degrees Fahrenheit for about 10 minutes, until the filling just starts to set, and then lower the oven temperature to about 300 degrees until it cooks through. "By lowering the overall temperature, you get less of a disparity between temperature at the perimeter of the pie, and the temperature at the center," he says. He also recommends heating the filling on the stove for a bit, so it has to spend even less time cooking unevenly inside the oven.

Oh, and padding the custard with lots of fat helps as well. Souza's recipe calls for three eggs plus two egg yolks. "By ditching some of the egg whites, you're reducing some of the water content, so the filling takes less time to firm up," he explains. "And the fat in the egg yolks also serves as a buffer to keep the proteins from over cooking."

Lopez-Alt suggests substituting the cream in the filling with cream cheese for a similar reason: It helps the custard cook more evenly for a smooth, creamy end result.

Brilliant.

The Ideal Crust

Over the past few months, I tried nearly everything to avoid ending up with soggy, mushy pie crusts. I pre-baked and crisped up my crusts before pouring in the filling. I tried brushing the bottom of my crust with some egg wash — to waterproof it. I pleaded with the culinary gods and complained to my friends and cried to my mother.

All of this, to no avail.

And then Lopez-Alt suggested something wacky and wonderful: vodka. Not to drink (though a shot or two have served to console me after a culinary disaster). Lopez-Alt suggests substituting some of the water used to pull the pie dough together with a tablespoon or two of vodka. "When you mix flour with water, it produces gluten," he explains. "And gluten is what makes your pie dough tough rather than flaky."

But gluten doesn't form in the presence of alcohol. "Plus the vodka evaporates a bit more quickly when you bake the crust, so you're less likely to end up with a soggy crust," Lopez-Alt says.

He also suggests blending flour and butter together in the food processor. "That way you need to add barely any water at the end to bring the dough together," he says. "But the dough is still very soft, and easy to roll out."

Pre-bake the crust until it's golden brown, and then, Souza says, fill it with a custard that you've already heated and thickened up a bit on the stove. "That way," he says, "the filling is less likely to seep and soak through the crust, and ruin that buttery, flaky goodness."

The Richest Flavors

Canned or fresh squash?

The experts agree it doesn't really matter.

If you are using the fresh stuff, Lopez-Alt recommends roasting it very slowly in the oven — at around 275 degrees Fahrenheit. "Pumpkins and squashes have these enzymes that will break down some of their starches and convert them to sugar," he says. "Cooking them slowly can actually accelerate that process." Slow-roasting also allows for more water to evaporate from the squash, concentrating its flavor.

And feel free to experiment with different squashes. Sugar pie pumpkins are classic, but butternut and kabocha squash can often taste sweeter and more "pumpkin-y" than the traditional orange gourds. "Just never use one of those Jack-O-Lantern pumpkins," Lopez-Alt warns. "Those are meant to look pretty, but they taste nasty."

But Souza says people can rarely taste the difference: "I think you may as well save yourself the effort and use a can."

Of course, a judicious sprinkle of spices, some maple syrup and even a bit of sweet potato can deepen and enhance the flavor.

The Taste Tests

My journey to learn the art and science of the ideal pie has been long. Very, very long. I've baked a total of 10 pies over the past three months. I wanted to perfect my pie by Thanksgiving — but that didn't happen. Instead, I berated my Thanksgiving guests: "You're saying it's good, but what does that even mean? Be more constructive with your criticism!"

I faced many setbacks: I badly burned a couple of crusts. And, despite all the coaching, I curdled some custard. Take note, folks: Timers and thermometers are key.

But finally, in mid-December, I brought in two pies that combined all that I had learned to face my toughest critics yet: NPR's Science Desk.

For one pie, I used freshly roasted squash and cream cheese. I made the other with canned pumpkin. One of the crusts had some shortening, whereas the other was pure butter. And I used a good shot of vodka in both, of course.

Each pie had its fans.

My editor, Maria Godoy*, liked the cream cheese. "It's like creamy pumpkin... Like, if pumpkin were milk."

NPR Skunk Bear's Adam Cole disagreed: "Not my cup of tea." He liked the canned pumpkin better. [*Editor's note: Adam has an admitted preference for bland food.]

Photo editor Ryan Kellman said he was divided, insisting "I need to probably to eat a lot more of this" in order to decide.

And science correspondent Geoff Brumfield was no help at all: "I mean, I'm just here to eat some pie."

I'm still not satisfied. A bit more ginger may have given the pies more kick. The cream cheese filling didn't taste pumpkin-y enough, and the other one tasted perhaps too pumpkin-y. Maybe I should have baked the crusts a bit longer. Maybe I should have cooked the custard a bit less.

True perfection — and success — remain just beyond reach. Guess I'll have to pie, pie again.