Why do double-crust fruit pie recipes call for dotting the top of the filling with butter?
Some say that scattering small bits of butter over a fruit filling keeps the juices from bubbling over in the same way that adding a bit of fat to simmering jam keeps it from foaming up in the preserving pan. The theory is that the fat disrupts the formation of bubbles on the surface of the viscous fruit mixture. Others claim that the butter simply enriches the flavor and texture of the pie filling.
To see which of these theories might be true, we baked six particularly juicy double-crust fruit pies—two each of blueberry, strawberry-rhubarb, and cherry—and dotted the filling of one of each pair with 2 tablespoons of butter. The butter neither encouraged nor hindered boilovers: Both the plain and the butter-dotted strawberry-rhubarb pies overflowed, but none of the blueberry and cherry pies did. As far as flavor and texture, tasters could not consistently identify which pies had added fat.
THE BOTTOM LINE: If your favorite family recipe calls for dotting the filling with butter, you can certainly continue to do so out of deference to tradition, but it probably won’t have much of an effect.