The droplets of water that appear on a baked pumpkin pie come from overcooking the eggs in the custard base. A custard is a liquid (in this case, milk and pumpkin purée) that is thickened by eggs. “When you bake a custard,” says Piper Davis, co-owner of the Grand Central Baking Company, which has locations in Portland and Seattle, “you want it to set at the lowest heat possible.”
Wayne Gisslen explains in Professional Baking that “an overbaked custard becomes watery because the moisture separates from the toughened protein.”
Davis says the easiest way to avoid the risk of weeping is to blind-bake (or prebake) your crust, because “then you can bake the pie at a much lower temperature; instead of 350 you can do it at 325 or even 300.” It’s also important to remember that the filling should be a little jiggly when you take the pie out—don’t cook it until it’s totally solid. The filling will continue to cook a little as it sits.
Read more on custards and their relation to pudding. And check out CHOW’s easy pumpkin pie recipe.