Reader Laura asks this about lemon bars:
Why can’t you just cook the curd in the pan to 196 degrees and then spread the fully cooked curd on the baked crust?
That’s an excellent question that’s going to allow me to get good and geeky. It’s a fair thing to wonder. I mean, if I’m going to cook custard (which is what curd is, a fruit juice custard) why not just do it on the stovetop where I have more control, then apply it to the crust?
The thing about custards is that there are two types: “still” custards and “stirred” custards. Both rely on networks of bonded egg proteins to give them their texture. Still custards include things like quiches, and are thickened with whole eggs for the firmest possible texture. They’re typically baked. An example of a stirred custard is pastry cream. It’s prepared on the stovetop and employs yolks which create looser, more flowing consistency. Both contains networks of intertwined proteins, but where the proteins in one are continuous and rigid, in the other they’re broken up and (semi-)flowing.
A pan of lemon bars occupies a strange middle ground between still and stirred custard. It contains lemon curd which is normally a stirred custard, but it finishes in the oven like a still custard. Why? The reason is because baking the curd for the last 25 degrees of its cooking time creates the uniform protein network that you need in an item you want to hold its shape when it’s sliced. The thing about egg protein networks is, once you disturb them, they’re broken for good. Once it’s hit 196 degrees, a stirred custard will not become a still custard just because it’s allowed to sit for a time. It will remain a disconnected, flowing mass that won’t stay on the shortbread after it’s been cut.
For more on custards, see “About Custards” in the menu on the right.