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The Food Lab's Apple Pie, Part 2: Perfect Apple Pie Filling

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[Photographs: J. Kenji Lopez-Alt]

This is the final entry in my three-part primer on Apple Pie.

The first dealt with the science of pie crust, along with a recipe for foolproof pie dough. TL/DR version: you don't have to cut butter into your flour very carefully. The key is to add the flour in two batches, forming a flour/butter paste with the first.

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The second article was about choosing the best apple cultivar for pies. Results? Golden Delicious or Braeburns are the way to go, displaying the best single-apple flavor balance of all the varieties tested. The only problem is that their texture is not perfect.

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See how they get just a bit too soft in the pie? I want apple slices that retain their structure as they bake, fully softening so that there is no crispness remaining, but still remaining firm enough that an individual slice retains most of its initial shape. That's exactly what we're going to address this week.

Apple Anatomy, 101

The basic structure of an apple is simple. Like all plants and animals, it consists of teeny tiny cells—imagine them as tiny water balloons all bundled up really close to each other. Inside these water balloons is a number of things. The cytoplasm makes up the majority of their filling and is mostly comprised of water. This is the part that houses all of the cell's active mechanical parts, as well as all of the apple's flavorful compounds. In addition to the cytoplasm, there's a small air bubble called the vacuole.

The exterior of these water balloons—the rubber part, if you will—is called the cell wall, and it's mostly comprised of cellulose and pectin. These two compounds behave differently from each other, and the simplest way to think of them is to imagine cellulose as the bricks and pectin as the mortar that's holding them together. Destroy either one of them and the apple collapses, rapidly losing volume, and releasing its liquid in a torrent of juice that saturates the bottom crust.

You end up with a pie with a huge space between the upper crust and the apples, the bottom crust swimming in soupy apply liquid, the slices reduced to mush. This is not my idea of fun, and I seriously hope that it's not yours either.

So the question is, why does this happen?

Pie Times

Let's consider the case of an apple pie made by placing raw apple slices tossed with sugar, a bit of cornstarch, and a touch of cinnamon directly into the empty pie crust. Before the pie even hits the oven, the sugar will begin to draw out some of the moisture from the apples via the process of osmosis—that's the tendency of a liquid to travel across a permeable membrane (in this case the cell wall) from an area of low solute level (the interior of the cells) to an area of high solute level (the exterior of the apple slices—sugar being the solute).

This process is mainly a surface treatment. The exterior of the apple slices may soften ever so slightly as the balloon-like cells partially collapse when they're robbed of liquid, but the cells on the interior are still totally intact. So the apple slices will remain firm and crunchy.

As the pie starts to bake, the first thing to happen to the cells is that the air inside them (the vacuole) will begin to expand as it heats up (a given volume of air will expand greatly when heated, while a volume of water will stay the same size). This expansion puts a lot of pressure on the cell walls. The balloons are literally ready to burst, and indeed some of them do. The apples soften a bit and turn from opaque and crisp to slightly translucent and softer. The only thing holding them together at all is their pectin and collagen-based walls.

Now, it'd be great is we could keep our apples in this exact state—they're soft, but they still have some structure. Unfortunately, at this stage, your pie crust will still be completely pale. You have no choice but to continue baking.

As the filling reaches 183°F, we reach a critical point. This is the temperature at which pectin begins to break down. When the mortar in a brick building beings softening, there's suddenly nothing holding the bricks together. Cells collapse, liquid gushes out, and you've got apple sauce on your hands.

How do you prevent this from happening?

Well, we've already learned that lowering the pH of an apple pie filling (that is, making it more acidic) can help matters a bit. Pectin is stronger in more acidic environments, which is why tart apples like Granny Smith tend to hold their shape better than purely sweet apples like Red Delicious. Many recipes call for adding lemon juice to pie filling for this very reason.

Problem is, I'm 100% happy with the flavor of my pie filling as-is. It's intensely apple-y, and already has a good balance of sweetness and tartness. Adding lemon juice or another acid would throw this balance off. There must be a different way.

Half Baked

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Par-cooked apple slices

Pop quiz: you start with two identical batches of apples. One of them you slice, season, and throw raw into a pie shell and bake. The other you slice, season, par-cook in a skillet for a bit, then throw into a pie shell and bake. Which comes out softer?

Obviously the one that was cooked more should come out softer, right? Wrong!

Well, really, it depends. Cook a pot of apples on the stove too hot and indeed, they'll eventually break down into apple sauce. But cook them more carefully, and a pretty awesome thing happens: an enzyme naturally present in the apples will convert the pectin in the cell walls into a heat-stable form, very much like curing the cement mortar in between the bricks and allowing them to fully harden.

This process takes both careful temperature control and time. Ideally, you want to hold your apples in the 140° to 160°F temperature range for around 10 minutes before allowing them to cool. There are a few ways to accomplish this.

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You can see air coming out of the apples as the vacuoles expand and expel some of their air.

The beauty of this method is that by using a pre-determined amount of hot water for a certain amount of apples, you don't even need to use a thermometer to check on their temperature (of course, I do anyway). Just pour, wait, and proceed.

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After ten minutes in the bowl, your slices will have softened ever so slightly, but still retain most of their form. After this point, even if you bake them into a pie, they still pretty much retain their basic shape and a bit of pleasant al dente firmness.

Check out this side-by-side of apples treated with hot water vs. those put into the oven raw.

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160°F vs pour over hot water

Okay, it's not the most illustrative picture. But please do look at this photo of a pie made with treated Golden Delicious apples vs. the one further up made with baked raw slices:

Now that is one good-looking pie.

And of course, now that you have this method of producing perfectly textured apple chunks, you aren't limited to simply making perfect pies with it. There's a whole world of apple-stuffed desserts that can benefit from the hot water pour-over. How about...

Apple Pie Alternatives

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Don't feel like making a full-on double crust with all the requisite rolling, folding, trimming, and crimping? Well, for times a double crust just ain't worth the effort, a Skillet Apple Pie is clutch. I like to take my treated apple slices, toss them with sugar, cinnamon, and a bit of cornstarch to thicken their juices (just follow the proportions in the actual apple pie recipe), pour the mixture into a cast iron skillet or pie plate, then top it off with a single pie crust. It bakes just like a regular pie.

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If you want to get extra special, try adding a layer of nice nutty aged cheese, like good cheddar, or perhaps some Comté, to the top of the apples. You'd be amazed by how well cheese and apples go together.

Or, turn your skillet pie into a perfectly fine pandowdy by breaking up the crust half way through baking and pushing the edges directly into the filling to soften up a bit as it finishes baking.

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Apple crisps and apple crumbles are perhaps the easiest variations to make. Just make your apple filling, then combine 3/4 cup all-purpose flour with 3/4 cup old fashioned oats, 3/4 cups chopped or slivered nuts (I like almonds or pecans), 3/4 cup brown sugar, a pinch of salt, and 1/2 a cup of butter. Work the mixture together with your fingers, spread if over your apple filling, and bake at around 375°F until nicely browned.

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If you've got a thing for biscuits, then a cobbler is your dessert of choice. All you've got to do is make your apple filling, drop it into a deep sided pot (I use my enameled cast iron Dutch oven), drop a few balls of biscuit dough on top, and bake.

Anyhow, you get the point. This filling will work for really any sort of pie-like apple creation. Heck, if you wanted, you can just go ahead and eat it on its own; it's that good.

For the record, I go super light on cinnamon because I want my apples to taste like—shock!—apples, but feel free to add extra cinnamon, other spices, more sugar, lemon juice, whatever the heck you feel like. After all, it's only apple pie.

Get The Recipe

Perfect Apple Pie »

More on Pie

For extra credit, check out The Food Lab's pie guides:
Selecting the Right Apple For Pies
The Science of Pie Dough

About the author: J. Kenji Lopez-Alt is the Managing Editor of Serious Eats where he likes to explore the science of home cooking in his weekly column The Food Lab. You can follow him at @thefoodlab on Twitter, or at The Food Lab on Facebook.

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