Pies and soufflés: these are two dishes that can try even the most experienced cook. Berry pies can be especially challenging, since the high water content of cherries, strawberries, blueberries, and blackberries often leads to a big, leaky mess once you cut into your beautiful pie.
However, the badasses at Cook's Illustrated have come up with a happy, natural solution to making a berry pie with a filling that is firm and slices nicely. The answer definitely isn't more flour—while it will thicken the filling, it can also leave behind a dry, dusty aftertaste and a rubbery texture.
Many apples are naturally high in pectin, which is a substance that creates the gel-like quality in jams and makes them spreadable, too. Cook's Illustrated peeled and grated one Granny Smith apple (using the large hole side of a box grater), wrung out the grated apple in a kitchen dish towel to make sure there wasn't too much moisture, and added the still-raw apple to the traditional cooked blueberry pie filling before it was added to the crust.
To ensure the perfect, firm blueberry filling, they also added two tablespoons of ground-up tapioca. Tapioca flour has a milder taste than wheat flour or cornstarch and is a little more forgiving than other types of starch-based thickeners. You might want to avoid using regular cornstarch as a thickener period; the folks over at Chowhound say that it tends not to do well at certain temperatures and loses its thickening abilities.
While Cook's Illustrated used quick-cooking tapioca and ground it themselves, try going to an Asian or Brazilian market if you happen to have one near by. Almost all of them will carry tapioca flour or starch (they're basically the same thing), and you won't have to do any grinding yourself. The results will be picture-perfect.
If you're cooking a berry pie out of season and have to use frozen fruit, using a grated apple and tapioca starch will come in handy, since thawed berries tend to be extra-watery.
I also think this trick would work with just about any kind of berry pie, although I'd be careful about pairing different kinds of apples with specific berries. Strawberries, which are more delicate in flavor, might require a more delicate-tasting apple, like a Gala or Pink Lady. A robust, tart apple like the Pink Lady would also work with the strong flavors of blackberry, while you might want to use a Golden Delicious with a cherry pie so as not to overwhelm the taste of the berry itself.
Never fear. Professional bakers have been using a product called Instant ClearJel for ages. This corn-based thickener has been treated so it thickens even at very high temperatures and won't break down in the presence of high-acid foods, like berries.
While it can be tempting to slice into your magnificent creation after only a few moments on the cooling rack, PJ Hamel at King Arthur Flours tells you definitively why that's a no-no:
"And finally—DO NOT cut into a fruit pie while it's hot! The vast majority of warm fruit pie fillings will collapse into the breach you've created by lifting out the first slice. Just to be safe, I like to let a pie rest overnight before cutting."
What tips do you use to create the perfect pie filling?