Older recipes often call for scalding the milk before adding it to the roux. Is this step really necessary?
A white sauce or béchamel is one of the French mother sauces. Classically, equal amounts of flour and fat (usually butter) are cooked to allow the fat to encase the starch granules before milk is whisked in and the sauce is brought to a boil to thicken it. Older recipes often call for scalding the milk before adding it to the roux. Theories abound regarding the reason for doing this. Some sources insist that it prevents heat shock, a phenomenon in which milk proteins separate and curdle when the cold milk comes in contact with hot roux. Others suggest that it’s done to protect the starch in the flour from losing its thickening potential. Julia Child claimed that hot milk plus vigorous whisking is the only way to get a lump-free white sauce.
To test this claim, we made Child’s recipe for béchamel using cold milk (straight from the refrigerator), room-temperature milk (about 65 degrees), and just-boiling milk. We streamed the milk into each sample while whisking vigorously, brought the mixture to a boil, and then cooked the mixture (while whisking frequently) for a full 2 minutes, as per her instructions. We then strained the sauce through a fine-mesh strainer to catch any lumps of flour.
In multiple tests, we found very little difference in the amount of clumping that occurred in the cold and room--temperature milks—both were fairly smooth, with minimal lumping. The hot milk, however, yielded significantly more lumps despite the same amount of vigorous whisking. Could the classic recipe be wrong?
Our science editor explained that in order to form a really smooth, lump-free sauce, the starch granules in the roux must disperse completely in the liquid milk before they start to absorb water and gelatinize. Adding cold milk to the roux gives the fat-coated starch granules time to disperse before the fat melts away from the granules and they begin to absorb water and gelatinize. Adding hot milk quickly melts some of the fat, so some of the granules become exposed to the hot liquid before they are completely dispersed, causing them to become wet on the surface and stick together to form lumps.
THE BOTTOM LINE: There’s no reason to scald the milk for white sauce unless you’re trying to infuse it with other flavors (onion or herbs, for instance). Not only does the scalding process take extra time, but using cold or room-temperature milk actually gives you a smoother sauce—and one less pot to wash.