Despite the lure of fresh berries in the produce department, frozen berries are almost always a better choice unless you plan to eat them while they're super fresh. Berries start growing mold in a matter of days, while their frozen counterparts can last in the freezer for weeks or months, and there's nothing sadder than dumping expensive berries in the trash. Frozen berries can also have better color than fresh.
If you're planning to make a pie or muffins with frozen fruit, however, they do tend to release more liquid when they're baked up, which is why adding a little flour to your frozen berries is key. When berries are frozen, they form ice crystals inside, which breaks down the cell walls that hold all the juice. When they're thawed, the frozen liquid is released, which can make a soupy mess of a blueberry pie or a batch of raspberry muffins if there's nothing to soak up all that juice.
If you toss frozen berries with a little bit of flour before you add them to the bake, the juice and flour will combine and thicken as your goodies cook. Even better, a little bit of flour will also keep the berries from sinking to the bottom of the pan in bakes like muffins and quick breads. Coat the berries while they're still frozen You don't need a lot of flour for desserts made with frozen berries.
About 1⁄2 cup is enough to thicken eight cups of berries (which will fill a standard 9-inch pie pan). There's no fancy technique required, either; simply scoop the flour right into the bo.