From America's Test Kitchen Season 3: Lemon Meringue Pie
Most everybody loves lemon meringue pie—at least the bottom half of it. The most controversial part is the meringue. On any given day it can shrink, bead, puddle, deflate, burn, sweat, break down, or turn rubbery. We wanted a pie with a crisp, flaky crust and a rich filling that would balance the airy meringue, without blocking the clear lemon flavor. The filling should be soft but not runny; firm enough to cut but not stiff and gelatinous. Most important, we wanted a meringue that didn’t break down and puddle on the bottom or “tear” on top.
We consulted a food scientist, who told us that the puddling underneath the meringue is from undercooking. The beading on top of the pie is from overcooking. We discovered that if the filling is piping hot when the meringue is applied, the underside of the meringue will not undercook; if the oven temperature is relatively low, the top of the meringue won’t overcook. Baking the pie in a relatively cool (325-degree) oven also produces the best-looking, most evenly baked meringue. To further stabilize the meringue and keep it from weeping (even on hot, humid days), we beat in a small amount of cornstarch.
Makes one 9-inch pie