Baking a pie? Go ahead and vent.
No, that's not an invitation to let loose kitchen rage, but rather, a recommendation to cut those seemingly insignificant holes or slits in the top crust of a double-crust pie. They might seem merely decorative, but they actually serve a critical purpose.
Pie vents open the top layer of a double-crust pie to allow the steam produced when the pie bakes to escape. Obviously, you don't have to worry about them if you're baking a single crust pie or making an unbaked pie such as a cream or chiffon pie.
Making the Case for Pie Vents
The perfect pie dough should be a delicate, almost crumbly pastry that just barely holds together. It's not particularly elastic, like pizza dough or bread dough. If you stretch a piece of pie dough, it should break.
Crumbly, breakable pie dough produces a light and flaky crust. But when a pie goes into the oven, the filling starts to boil and produces steam. When water turns to steam, its volume can increase up to 1,500 times. That means even a pie filling made only of fruit with no added liquid can produce a substantial amount of steam.
Baking a double-crust pie without vents leaves the top crust susceptible to rupturing as the building steam tries to escape. Bubbly pie filling seeks out the weakest point where you crimped the top and bottom crusts together.
Remember, too, that steam isn't merely a pressurized gas, it's super-heated water vapor. Thus, it can cause not only structural damage, but the excess liquid turns the top crust soggy while causing the filling to get soupy.
Preventing "Pie Gap"
"Pie gap" happens when you bake a pie with a filling such as apples, which shrink substantially when you cook them. To fill the pie, you have to start with a hefty mound of uncooked apples. Without pie vents, the apples shrink, but the crust retains its original shape, producing a wide gap filled with air between the crust and the filling. The top crust collapses when you cut into the pie. Venting lets the crust settle as the apples shrink, preventing "pie gap."
Cutting Pie Vents
The simplest way to add vents to a top crust is by cutting slits in it with a sharp knife. Four or five 2-inch slits, arranged circularly, radiating from the center toward the edges, should be plenty. You can use your finger to widen the slits slightly.
If you're more ambitious, use pastry cutters to cut decorative or geometric shapes—stars, hearts, flowers, diamonds, and so on—into the top crust before draping it over the pie. If you do this, you can arrange the cut-out pieces of dough attractively on top of the crust, affixing them with egg-wash.
Bonus: You can use these openings to test whether the apples in your pie are tender.
Using Pie Birds
Ceramic pie birds are a cute baking accessory that are perhaps more traditional for venting meat or chicken pies, but they work for fruit pies as well.
Shaped like a bird, with a hole in the opening of the bird's beak that allows steam to escape, a pie bird sits in the center of a pie, with the filling around it. It sticks through a hole cut in the center of the top crust.
The advantage of the pie bird is that in addition to venting steam, their tubular structure allows some of the liquid in the filling to bubble up into the bird instead of spilling out onto the pie. The downside is they can be a bit large and take up space in the pie.
You can DIY a pie bird using any of the following: