The Problems with Flour

• Flour works well as a thickener, but it's not always your best choice in a pie. For one thing, it is not a pure starch, so you need to use a relatively large amount of it to thicken the berries' juices. It also takes a relatively long time to thicken, meaning the juices will have plenty of opportunity to make your crust soggy. If the flour doesn't gel fully, it will also add a pasty, uncooked flavor to your pie filling. Finally, flour will make your filling opaque and take away from the beautiful color of the berries.

Choice of Flour

• You can minimize those difficulties to some extend through your choice of flour. Quick-mixing or "instant" flours are made specifically for thickening sauces, and they're much better for pies than regular all-purpose flour. When quick-mixing flour is manufactured, it's moistened and the flour is par-cooked to the point that its starch granules begin to gel. Then it's dried again and milled to a fine powder. This type of flour thickens almost as quickly as cornstarch, minimizing the risk of sogginess in your crust. Since it is precooked, it doesn't add a starchy flavor to the filling. However, it still won't give your pie the glossy appearance you'd get with other starches.

A Workaround

• If you're working with a limited pantry and regular all-purpose flour is your only choice of thickeners, you can get a satisfactory result through a workaround method. Combine the berries and sugar your recipe calls for, and heat them slightly until the berries begin to give up their juices. Place a colander over a bowl or small saucepan, and let the berries drain for an hour. Make a slurry with the flour and some cold water, and stir it into the strawberry juices. Cook until the juices thicken, then let the mixture cool. Combine it with the berries, and then fill and bake the pie, as usual.

Other Starches

• Although it is possible to get a passable result with flour, it's better to use a more suitable starch if you can. The most readily available is cornstarch, which gels more quickly than flour and begins to thicken at a lower temperature. Cornstarch has about four times the thickening power of flour, so you can use much less. It's also clear when it gels, which gives your pie a prettier appearance when it's finished. If you're avoiding the use of cornstarch or corn products in general, you can get a similar result by using arrowroot powder or powdered tapioca starch.