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Preparation
Step 1
For a lemon filling that's glossy and translucent, and firm enough to cut yet melts in your mouth, cornstarch is the thickener of choice. It's also a risky one because heat and acid (in the lemon juice) can destroy cornstarch's ability to thicken or stay thick.
Happily, the most foolproof way to make a perfect lemon filling is also the easiest. Just dump in the ingredients ― in a specific order, with a combination of cold, then hot water ― and cook them together. As the starch granules swell with the heating liquid and begin to form a thickening network, the sugar and egg yolk proteins join forces, like suits of armor, to coat and protect the starch at its vulnerable bonding points from the effects of heat and acid.
If the meringue is swirled onto a cool filling and baked, steam in the reheating filling just reaches the meringue. As the pie cools, the steam condenses to form the sweet weeping (sometimes a pool) under the meringue. And when the pie is cut, the meringue is inclined to slip off the wedges.
As egg whites are beaten, they stretch and trap air bubbles. The size and strength of the bubbles determine the durability
of the meringue. Small, even bubbles are more lasting than big, uneven ones.
An acid, such as cream of tartar, makes egg-white bubbles stronger. Add it before beating the whites into a coarse foam. To further strengthen the protein in the whites and force the bubbles to break into firmer, tiny ones, gradually beat in the
sugar. When the foamy mass is glossy and holds distinct peaks, the bubbles are still flexible enough to swell when heating
air expands in them as the meringue bakes. If sugar is added before a coarse foam is established, the whites get too stretchy to make a stiff foam. If you add the sugar too fast, the granules won't dissolve and the bubbles will be uneven. If you overbeat the whites, the meringue loses its gloss and the bubbles are stretched to their maximum; when heated, they pop and drain liquid. In underbeaten egg whites, the insufficiently developed bubbles break and drain.
However, even perfectly whipped meringues are relatively short-lived (unlike hard meringues, which are made with twice as
much sugar and linger forever). Heat firms the pie meringue, and one that is baked longer at a lower temperature will hold
up better than one baked briefly at a high temperature. As the meringue cools, the air in the bubbles contracts and causes
slight shrinkage. Eventually, the bubbles sag, moisture drains from them, and the mixture gets gummy and chewy.