We wanted a pie that had a firm, juicy filling full of fresh blueberry flavour with still plump berries, and we also wanted a crisp, flaky crust.
To thicken the pie, we tried cornstarch as well as our gluten-free flour blend but preferred tapioca starch, which was subtle enough to allow the berry flavour to shine through. Too much of it, though, created a congealed mess.
Cooking some of the blueberries down to a saucy consistency helped us reduce the amount of tapioca required, as did adding a peeled Granny Smith apple that we shredded on the large holes of a box grater.
Rich in pectin, the apple helped thicken the berries naturally. Since gluten-free pie crusts can easily turn soggy, we found that preheating a sheet pan in the oven and baking the pie on the lower rack helped keep the crust crisp. (We also offer a gluten-free flour blend).
It’s not safe to place a glass (Pyrex) pie plate on a preheated baking sheet. If you must use a glass pie plate, do not preheat the baking sheet; note, however, that your crust will not be as crisp. This pie is best served the day it is made.
Start to finish: 3 hours
30 ounces (6 cups) blueberries
1 Granny Smith apple, peeled, cored, and shredded
5 1/4 ounces (3/4 cup) sugar
2 tablespoons tapioca starch
2 teaspoons grated lemon zest plus 2 teaspoons juice
1 recipe Double-Crust Pie Dough (recipe below)
1 large egg white, lightly beaten
Cook 3 cups blueberries in medium saucepan over medium heat, mashing occasionally with potato masher to help release juices, until half of berries have broken down and mixture is thickened and measures 1 1/2 cups, about 8 minutes. Let cool slightly.
Place shredded apple in clean kitchen towel and wring dry. Combine apple, cooked berry mixture, remaining 3 cups uncooked berries, sugar, tapioca starch, lemon zest and juice, and salt in large bowl.
Adjust oven rack to lowest position, place foil-lined rimmed baking sheet on rack, and heat oven to 425 F. Roll 1 disk of dough into 12-inch circle between 2 large sheets of plastic wrap. Remove top plastic, gently invert dough over 9-inch metal pie plate, and ease dough into plate; remove remaining plastic. Roll other disk of dough into 12-inch circle between 2 large sheets of plastic. Remove top plastic. Using 1 1/4-inch round cookie cutter, cut hole in centre of dough, then cut out 6 more holes, about 1 1/2 inches from hole in centre, evenly spaced around centre hole.
Spread blueberry mixture evenly into dough-lined pie plate. Gently invert top crust over filling and remove remaining plastic. Trim dough 1/2 inch beyond lip of pie plate, pinch dough edges together, and tuck under itself to be flush with edge of pie plate. Crimp dough evenly around edge using your fingers. Brush pie with egg white.
Place pie on preheated baking sheet and bake until crust is light golden brown, about 25 minutes. Reduce oven temperature to 350 F, rotate baking sheet, and continue to bake until juices are bubbling and crust is deep golden brown, 30 to 40 minutes longer. Let pie cool on wire rack to room temperature, about 4 hours. Serve.
DOUBLE-CRUST PIE DOUGH
Makes enough for one 9-inch pie
Perfect pie dough has just the right balance of tenderness and structure. The former comes from fat, the latter from the long protein chains, called gluten, that form when flour mixes with water. Too little gluten and the dough won’t stick together; too much and the crust turns tough.
So presumably we would face mostly a structural issue with a gluten-free dough, since gluten-free flours are naturally low in protein. As our first step, we swapped in our gluten-free flour blend for the wheat flour in all the pie dough recipes the test kitchen has developed over the years. We produced workable doughs in every case, but an all-butter dough (which includes sour cream for tenderness) had the necessary richness to stand up to the starchiness of the gluten-free flour blend and was clearly the best starting point.
Although we weren’t surprised to find that the dough was still too soft and lacked structure, we were taken aback by how tough it was; on its own, the sour cream was not sufficient to tenderize a gluten-free dough. We solved the structural problem easily with the addition of a modest amount of xanthan gum, but flakiness and tenderness were still elusive.
In an effort to further tenderize our dough, we tested ingredients that are known to tenderize: baking soda, lemon juice, and vinegar. Vinegar was the clear winner, producing a pie crust that was not only tender, but also light and flaky. Like conventional recipes, this pie dough can be prepared in advance and refrigerated for two days; however, it is not sturdy enough to withstand freezing.
6 tablespoons ice water
3 tablespoons sour cream
1 tablespoon rice vinegar
13 ounces (2 3/4 cups plus 2 tablespoons) ATK Gluten-Free Flour Blend (recipe below)
1 tablespoon sugar
1 teaspoon salt
1/2 teaspoon xanthan gum
16 tablespoons unsalted butter, cut into 1/4-inch pieces and frozen for 10 to 15 minutes
Combine ice water, sour cream, and vinegar together in bowl. Process flour blend, sugar, salt, and xanthan gum together in food processor until combined, about 5 seconds. Scatter butter over top and pulse until crumbs look uniform and distinct pieces of butter are no longer visible, 20 to 30 pulses.
Pour half of sour cream mixture over flour mixture and pulse to incorporate, about three pulses. Add remaining sour cream mixture and pulse until dough comes together in large pieces around blade, about 20 pulses.
Divide dough into two even pieces. Turn each piece of dough onto sheet of plastic wrap and flatten each into 5-inch disk. Wrap each piece tightly in plastic and refrigerate for 1 hour. Before rolling out dough, let it sit on counter to soften slightly, about 30 minutes. (Dough can be wrapped tightly in plastic and refrigerated for up to 2 days.)
THE AMERICA’S TEST KITCHEN GLUTEN-FREE FLOUR BLEND
Makes 42 ounces (about 9 1/3 cups)
It is important to bring the mix to room temperature before using it in a recipe. Be sure to use potato starch, not potato flour. Tapioca starch is also sold as tapioca flour; they are interchangeable. See notes at right about shopping for rice flours and substitutes for potato starch and nonfat dry milk powder.
24 ounces (4 1/2 cups plus 1/3 cup) white rice flour
7 1/2 ounces (1 2/3 cups) brown rice flour
7 ounces (1 1/3 cups) potato starch
3 ounces (3/4 cup) tapioca starch
3/4 ounce (3 tablespoons) nonfat milk powder
Whisk all ingredients together in large bowl until well combined. Transfer to airtight container and refrigerate for up to 3 months.