Homemade macaroni and cheese is comforting and delicious, but intensely unattractive when the sauce curdles. The reaction of milk products to heat produces a sauce that is both gloppy and stringy at the same time. Taste may not be affected in the slightest, but curdled milk turns macaroni and cheese into a visual and textural disaster. You can prevent curdling at several stages during cooking, keeping your macaroni and cheese creamy.
Milk and Heat
Just like people in the sun, milk proteins can respond to warmth by relaxing. Tightly snarled protein fibers stretch and loosen their hold on each other. At the same time, fats in the milk soften and melt. Warmed over low or medium heat, milk heats smoothly and may seem even more creamy in texture than when cold. When heated too fast, milk separates or "breaks." Protein fibers form tight clusters, or curds, fats separate, and the remaining liquid becomes watery. The result is called curdling. Especially if you are combining milk with other ingredients, as in a cheese sauce, prevent curdling by keeping heat low enough that the sauce does not come to a boil.
Fat and Curdling
The fat content in milk appears directly related to the tendency to curdle. A higher proportion of fat makes it harder for proteins to cluster. Making macaroni and cheese with whole or full-fat milk, half-and-half, evaporated milk or cream may produce better results than using low-fat milk products.
Cheese and Curdling
You may find the sauce for your macaroni and cheese is stable until you add the cheese. Curdling is, after all, part of the cheese-making process. Even when the process is complete, re-exposing cheese to high heat can trigger curdling rather than melting. The prolonged heat needed to melt and incorporate large pieces of cheese can also cause separation of both cheese and milk. Finely grating cheese and thoroughly mixing it with other ingredients can help deter curdling.
One of the appeals of macaroni and cheese is its quick assembly. Traditional recipes may specify simply tossing cold milk, a pat of butter and a handful of grated cheese with hot drained macaroni noodles over a warm stove burner before serving. Making a sauce to distribute cheese throughout the dish slows this rapid assembly but is also the best assurance against a curdled dish. Thicken milk as it warms with a small quantity of roux -- butter and flour cooked briefly together -- then add cheese and cook over low heat until the cheese melts. The few minutes needed to create a stabilized cheese sauce to stir into the macaroni can ensure your completed dish will keep its creamy appeal.