There are some very skilled cooks and bakers who swear by this technique, by which you heat milk (and/or cream) to just below the boiling point (to where a skin forms on the surface) then cool it and use it as the basis of a custard mix. People swear it results in a better-textured custard (it makes the interior creamier, the texture more delicate, the surface thinner, etc). The trouble is they can’t explain how or why. It reconfigures the proteins so that they, er…it inverts the um…
And in fact food scientists are clueless as to what they’re talking about. Not a one, it seems, can either prove what these chefs are saying, or even form a theory as to how scalding might bring these effects about.
Once upon a time scalding did serve a purpose, back before the days of pasteurization. Then, scalding served to kill bacteria and/or deactivate enzymes that could go on to compromise the safety and/or quality of the finished custard. Nowadays however, with ultra-pasteurization et al, there are neither live bacteria nor active enzymes in the milk we buy (or not many at any rate).
Do I scald my milk? Actually I do, but only because a baker and chef like Thomas Keller tells me to. He says it’s critical and who am I to argue? Sure, it’s possible that he’s carrying forward a piece of antiquated technique, one that he picked up from some French pastry chef and now does only out of habit. Maybe Bouchon uses local unpasteurized milk for its quiches and he must (and forgot to note that fact in his cookbook). Or perhaps he simply knows what he’s doing. Food science, helpful as it is, is a very young field. There’s still so much we don’t know about how food substances — especially substances as complex as milk — behave under certain conditions. The fact that science doesn’t know why scalding works isn’t proof that it doesn’t work. That said Thomas Keller is America’s chief practitioner of the food arts. I’m inclined to trust him.