Recall the discussion of how curds separate from whey. The introduction of rennet causes protein molecules to release water molecules and bond with fat molecules. Once the curd is cut, formed, and aged, the protein and fat link stays intact until the cheese is heated, and then the two separate. This is why cheese left in the sun develops an oily slick on the outside, or why cheese left too long in high temperatures begins to sweat with oil. Similarly, if you melt cheese quickly or cook it too long, the fats and proteins separate, which is why grilled-cheese sandwiches become so oily.

When you're cooking with cheese, however, you want the tastes of fat and protein to work together and the moisture to stay intact. In other words, you want to prevent heat from separating fat and protein molecules. Fortunately, there are a few things that help.

First, cook cheeses made with cooked curd. These cheeses melt at higher temperatures for longer periods of time than uncooked-curd cheeses. Cooked-curd cheese fats and proteins take longer to separate. This is because cooking the curds binds the fats and proteins together, before they are pressed and aged, and they retain their link for a longer period of time under higher stress. This is primarily why Swiss, Gruyere, and Appenzeller cheeses are often used in recipes that call for melted cheese; their curds are all cooked.

Second, introduce starch to your recipe. The starch of potatoes or pasta helps fats and proteins remain connected, which helps when melting Cheddar cheese, for example, or a Monterey jack, both cheeses with uncooked curds.

Third, introduce a bit of liquor to your recipe, which is why most fondue recipes call for a bit of Kirsch, wine, or cognac. The alcohol in liquor helps the fats and proteins stay together and prevents melted cheese from recurdling. Try it. Melt your fondue cheeses before adding the liquor and look at the granularity of the cheeses. They almost look like curds and whey. Then introduce the liquor and see how the texture becomes smooth again.

The fourth thing you can do is grate your cheese so that it melts quickly and doesn't need to be heated for very long. This way, the fats and proteins don't have time to separate.