Ask typical home bakers about their experience with lemon meringue pie and the response is likely to be something along the lines of a haunted thousand-yard stare. The weeping…the weeping…
The things just ooze. Most infuriatingly, they do it in two directions: up (as a result over over-heated meringue) and down (caused by under-heated meringue and/or a broken custard). And what happens when meringues and custards weep? Soggy crusts, sticky, gummy toppings, grainy fillings…the whole custard pie bag of horrors.
The root of the problem is the lemon meringue pie’s size. It’s a thick mass composed of two fussy things: a light frothy meringue layer and delicate citrus curd layer, both of which need a judicious application of heat but which cannot be over-heated.
The most vulnerable region of a lemon meringue pie is the very center, where the lower middle of the meringue meets the upper middle of the custard. The meringue there frequently doesn’t get warm enough for the simple reason that heat has a hard time penetrating that far before the outer regions of the meringue start to over-cook. Meringue, you see, is a frustratingly good insulator (see: baked Alaska).
What happens when a light meringue doesn’t heat to the point that it sets? The bubbles in it pop and the foam collapses back into a liquid, the result being a little puddle between the layers. Dang.
The solution: inside-out baking. That is, using the heat from the filling — which is cooked separately ahead of time — to set the center of the meringue before it ever goes into the oven. It’s a slightly tricky procedure that requires crackerjack timing. But when done well yields a perfect pie.