Besides a flaky crust, getting the filling in fruit pies to thicken up but not turn into a gluey mess requires some know-how about different thickeners, how they work, and what works best for the particular pie that you’re making. Here’s a guide to five common thickeners so you can make the right choice when deciding how to thicken your classic apple pie or triple-berry summer pie. So how does starch thicken pie filling anyway?