Why not keep the custard simmering?

Reader Nick asks:

[When making lemon meringue pie] why can’t the filling just be left on a low simmer while I’m making the meringue? Since there’s a lot of cornstarch in it the odds of it curdling seem remote. Wouldn’t that be a better solution for keeping it hot?

Hey Nick! I like that thinking! However I don’t think it would work for a couple of reasons. Firstly, despite the amount of cornstarch in the filling you still run the risk of curdling the filling the simmering goes on for more than a minute or so. The mixture is so thick there isn’t much convection in the pan, so the filling on the bottom absorbs a whole lot of heat if you aren’t whisking it constantly.

And even assuming you have a third hand to whisk with, you’ll have another issue: sustained heat and whisking causing the breakdown of the cornstarch network, which will cause the filling to thin back out again. More on that here. So unless you can have that meringue finished in under two or three minutes, my suggestion is to take the filling off the burner and give it a final heating just before assembly of the pie. You’ll be surprised at how little time it takes to get it boiling again, and that way you’re not racing against the clock. Thanks for the great question!