From delicate cheese soufflés to fondues and comforting casseroles, many of our favorite dishes get flavor, texture, richness, and nutrients from cheese. When properly handled, melted cheese can be creamy, smooth, and fully integrated into the rest of the dish. But sometimes even a simple dish like macaroni and cheese can go wrong, and you end up with tight curds atop an oily puddle or with a stringy mess. These two hazards, curdling and stringiness, are easily avoided with the right technique or with a slight modification to your recipe.
Cheese can curdle when proteins get overheated
Cheese is an extremely high protein food—as much as 30% protein in Gruyère, 36% in Parmesan. A protein molecule in any food is normally tightly wound, like a spring. When heated, the bonds holding the coil break, and the protein unwinds. The unwound protein molecule is now eager to bond with other unwound proteins, forming a loose mesh. This bonding process is called coagulation. If the proteins are subjected to too much heat, the mesh will tighten, coagulating further into clumps or curds. In cheese, many milk proteins were coagulated as the cheese was being made, but there are still uncoagulated proteins and loosely coagulated proteins that can tighten to form curds.
When you stir grated cheese into boiling liquid or cook a cheese casserole, this curdling can easily occur. Curdled proteins separate from the fat and other components of the cheese, turning your smooth sauce into an unsightly mix of rubbery curds and an oily puddle.
Pay attention to temperature and time. Many cheeses will tolerate only brief, gentle heat. It’s best to use grated cheese, which requires less heat and less time to melt. Often the residual heat in the rest of the dish—freshly cooked pasta, cream soup, or scrambled eggs, for example—is enough to melt cheese smoothly. Brief stirring, off the heat, will disperse the cheese evenly through the dish and will generally prevent the overheating that causes curdling.
A little starch can rescue a sauce
In some dishes that require longer heating, such as a potato gratin or a baked casserole with cheese, starch can prevent curdling. Incorporate a little flour or cornstarch—in a roux or slurry, for example—at the start of the recipe before adding the cheese. The exact mechanism by which starch prevents proteins from curdling is unknown. It may be simply that when the heated starch granules soak in liquid and swell, they become large enough to physically keep proteins apart. Whatever the science, we know from experience that starches —flour, cornstarch, potato starch, tapioca—can keep cheese sauces smooth and allow us to heat them without worry.
Some cheeses get stringy
A second problem when cooking with cheese is that it can get stringy when stirred into sauces or soups. Dr. Norman Olson of the University of Wisconsin explains that some cheese, particularly Swiss and mozzarella, contain calcium phosphate, a compound that tends to link cheese proteins together to form long strings. Even when heated gently, the stringy character of these cheeses makes them unmanageable and almost impossible to stir or serve.
This can be prevented by adding a bit of wine or lemon juice to the cheese before melting it. Traditional cheese fondue recipes contain white wine to prevent stringiness.
Dr. Anthony Blake, director of food science and technology for an international flavor and fragrance company, explains that the tartaric acid in wine helps prevent the calcium phosphate from linking cheese proteins together, thus preventing stringiness; he adds, however, that the citric acid in lemon juice is much more effective. Citric acid actually binds with calcium and can overcome stringiness even in the extreme case of mozzarella.
I now make fettuccine with a mozzarella, prosciutto, mushroom, and tomato sauce that I love. Try sprinkling a little lemon juice on the grated mozzarella before adding it to the cream sauce (a béchamel) and then stir it in over low heat. You’ll be astounded at the way it prevents stringiness.