This idea came from working on my cacio e pepe recipe, the Roman dish with pasta, pecorino, and black pepper. In that recipe, I cook spaghetti in a small volume of water, using the starchy pasta water to emulsify the cheese into a creamy sauce. I wondered if the same thing would work for an American-style macaroni and cheese, using more cheese and cheddar in place of pecorino.
The first key was reducing the amount of water to a bare minimum, just enough cold water to cover the macaroni in its pot.
Less water yields more concentrated starch. As the pasta cooks, it sheds starch. You want that starch to stay in the pot, as it plays a crucial role in helping the sauce stay creamy and emulsified. Without it, the cheese turns tough and greasy.
The second key was using evaporated milk as the base liquid. Evaporated milk is a highly concentrated source of milk protein micelles—bundles of proteins that can act as powerful emulsifying agents—which help to keep the sauce creamy and smooth.
