Milk is familiar enough that cooks tend to take it for granted, but it's really a complex ingredient. Milk is a mixture of water, fat, minerals and several kinds of proteins held together in a stable suspension. Its fat content ranges from almost none in the case of non-fat milk to an average of more than 3 percent for whole milk. That low level of fat is the reason milk curdles easily when it's cooked while heavy cream does not.

When you heat milk, its proteins react the same way as the proteins in meats or eggs do. They begin to shrink and tighten, often forming a skin at the top or bottom of the pot if they're heated on your stovetop. If the heat is too intense, the protein molecules bond together and shrink into tight, grainy lumps of curdled protein. This doesn't happen in heavy cream, because there's enough fat in the cream to prevent the protein molecules from bonding tightly to each other.

When starches are heated in a liquid, their tight granules absorb moisture and begin to expand and unwind. As the temperature increases from approximately 135 to 180 degrees Fahrenheit, the chains of starch molecules gradually form a broad, three-dimensional mesh that immobilizes the liquid and forms a soft gel. When you use cornstarch or flour to thicken a sauce, this is the process that is occurring while you stir. If you use one of these starches to thicken your sauce, whether directly or in a flour-based roux, the mesh of starch will prevent the milk's proteins from bonding together to form curds.

Some macaroni recipes simply call for the ingredients to be combined and added to the pasta, but this isn't a foolproof technique. The milk in your sauce may begin to curdle before the starches have finished thickening the sauce. It's better to prepare the sauce ahead of time on your stovetop, stirring it constantly until it's well thickened. At that point, the milk will be fully stabilized, and can be baked without the risk of curdling.