Wednesday, November 23, 2011

Food Science 101: The Perfect Pie Crust

My unofficial statistic (as in, totally made up but I'm sticking with it) is that about a billion pies will be made in America over this Thanksgiving weekend.  Some of those pies will have a beautiful, flaky, tender crust with a perfect crispiness.  And some will be dry, tough, crumbly, and/or soggy.  You want yours to be the former.  But how does one acheive such pastry perfection?  I've heard many times that making the perfect pie crust is an art.  That's incorrect.  The perfect pie crust is actually a science!  

A standard recipe for pie crust is actually a pretty simple.  It only contains flour, fat (generally butter and/or shortening), salt, sugar, and water.  The thing that makes pie crust scary to the casual cook is that those simple ingredients can turn into something terribly wrong if not prepared properly.  To make the perfect pie crust, you must first have a basic understanding of how the different ingredients help a pie crust get its tender, flaky, crispy characteristics.  

Flour contributes our old friend gluten.  Recall from Food Science 101: Over Mixing that gluten is what gives a baked good structure, and that too much of it cause toughness.  Adding too much flour can be problematic, especially during rolling.  Professional bakers typically use special pastry flour because it is lower in protein which leads to less gluten development and a more tender crust.  So if you want to really impress your guests this year you can pick up a bag, but all-purpose flour can be used as well.

Water is needed in pie crust dough in order for the gluten to properly develop.  But you want to add only enough to keep the dough together.  Adding too much water leads to too much gluten development, and a tough crust.

Salt and sugar are easy.  They are there to give the crust some flavor.  Because let's face it, flour and fat do not taste good on their own!

Now on to the crown jewel of pie crust--fat.  There's a hefty amount of fat in pie crust, and as much as I wold like to banish it, it does serve some pretty important functions.  Fat is how pie crust achieves that signature flakiness we all love so much.  The objective is to get small pieces of fat distributed evenly through the dough.  During baking, the water in the dough turns into steam and the layers of fat create an impervious layer.  The steam can't escape so it stays trapped inside each layer of dough.  This forces the dough to expand.  Then the fat melts into the dough and is absorbed.  And of course, we all like the mouth feel of fatty foods.  The pockets of air left by the melted fat allows steam to leaven the dough and create the characteristic of flaky crust.  Got it?

Different recipes use different types of fat for different reasons.  Shortening is the best fat for creating a flaky crust because it is 100% fat and is softer and more pliable than butter so it coats the flour more easily.  But most recipes call for some butter to improve the flavor of the crust.  Liquid oil is not recommended for pie crusts because it coats each flour particle which results in a grainy texture.  I would also not recommend margarine because of its high water content.  So if you are eating pie for Thanksgiving this year, don't tell your cardiologist!

Now that you know more than you thought you would ever know about the properties of a good pie crust, let's go through the steps of making a perfect pie crust and I'll share a few more secrets.  
These are your ingredients.  Like I said, simple enough!
Start by mixing the sugar, flour, and salt.  Then cut in small pieces of chilled butter and shortening.  You want some chunks about the size of peas to remain when you mix the fat with the flour.  The pea-sized pieces are what melt off during baking.  It is also important during the whole process that the fat is cold.  Chilled fat means less fat is absorbed by the flour and more pea-sized balls are distributed through the dough. 
A food processor makes preparing pie crust dough quick and easy, but if you are without, a pastry cutter or two knives can be used.
Add the water a tablespoon at a time just until the dough is moistened.  Remember that adding too much water will make the crust tougher.  Mix the dough until it no longer clings to the side of the bowl.  That is the sign that you have enough gluten development, but not too much.
When the dough comes together, form it into a ball then lightly flatten it into a disk and refrigerate it to let the fat chill again.
Now it's time to roll that dough out.  Here's a few tips for rolling out dough that can affect the final product:
Go easy on the flour.  Because excess flour leads to tough crust, I like to roll the dough out on waxed paper instead of the usual "floured surface."  However, if your dough is super sticky at this point, you might still have to flour the waxed paper a little bit.  
Chill out!  You don't want the fat to melt during rolling because it will coat too much of the flour, which will actually lead to too little gluten formation, and a grainy texture.  So if it is warm in your kitchen it is probably a good idea to put a pan of ice and water on your counter top before rolling the dough to cool the surface and prevent fat from melting during the rolling process.
Work quickly!  The more time allowed between adding water to the dough and baking, the more the gluten is hydrated and the tougher the crust will be.
Don't roll it to death!  Over-manipulating the dough causes excess gluten formation and a tough crust.  Avoid rolling the same area too many times, and use light pressure.  If the dough is too hard, allow it to rest at room temperature until it is softer.
You want your dough to be about 1/8 inch thickness.  Use short strokes starting at the center of the dough.  Ease up the pressure along the edges of the circle so that they don't get excessively thin.  Roll the dough until it is wide enough to overhang a 9-inch pie plate by about 4 inches.  

To prevent soggy crusts, do not add filling until the pie is ready to be baked, and make sure there are no holes in the bottom crust that would allow moisture to run through.  If you are making a pie with a juicy filling you can prevent it from soaking into the bottom crust by coating the bottom of the crust with a thin layer of melted butter, or place the pie plate on a baking sheet that is preheated along with the oven.  That allows the fat to melt quickly which sort of water-proofs the crust.

Here is the recipe for pie crust that I like to use.  This makes 2 crusts.  If you are making a single crust pie you can cut the recipe in half, or prepare the whole recipe and freeze the unused crust for up to six months.

Double Pie Crust
3 cups all purpose flour
2 1/2 tsp sugar
3/4 tsp salt
2/3 cup chilled shortening, cut into small pieces
1/2 cup plus 2 Tbsp (1 1/4 sticks) chilled unsalted butter, cut into small pieces
10 Tbsp ice water (approximately)
Combine flour, sugar, and salt in food processor.  Cut in cold shortening and butter until a coarse meal forms.  Blend in water a tablespoon at a time just until dough is moistened.  
Gather dough into a ball; cut in half.  Flatten each half into a disk.  Wrap separately in plastic.  Refrigerate until firm, at least 15 minutes.  
Let dough soften slightly at room temperature before rolling.  Roll out one disk at a time on floured work surface or waxed paper into about a 13-inch round.  Transfer dough to 9-inch glass pie plate.  Trim excess dough, leaving 3/4-inch overhang.
Bake according to recipe directions.

Reference: Understanding Food: Principles and Preparation 2nd Edition by Amy Brown.   Sometimes it pays to keep your college textbooks!

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