Chances are the cook heated the cheese at too high a temperature or for too long. Such treatment separates the protein from the fat and makes it tough, rubbery, and stringy. A cook who continues to heat the sauce in the hope of melting the unsightly protein lumps is pursuing an impossible dream. Once the damage has been done, further cooking only makes matters worse.
A cook's two primary goals therefore are to minimize the level of heat and the length of time that will be necessary to melt a particular cheese. The first goal is reached by keeping the heat source at a low setting and, for even heat distribution, using a thick-bottomed pan or double boiler. To achieve the second goal, bring the cheese to room temperature and cut it into small pieces. If the cheese is dry - as, for instance, a Parmesan - it should be finely grated. Add the cheese just before you have finished cooking the sauce, stirring all the time.
Different types of cheeses have different melting characteristics. Processed cheeses like American cheese melt more quickly and easily than most natural cheeses because they have low meting points. Among the natural cheese, the driest ones - if finely grated - tend to melt better than their moist counterparts because their protein is less likely to separate from the emulsion and coagulate into tough, chewy strands that diminish the appearance and texture of the dish. This is why knowledgeable cooks prefer to use "cooking cheeses" like Parmigiano-Reggiano (authentic Parmesan) for preparations like sauces, where the cheese must become well integrated. The longer this cheese has been aged, the better its cooking qualities.
For some culinary specialties, a cheese's tendency to become stringy is a sought-after quality. A case in point is mozzarella, a protein-rich cheese. Can you imagine eating the typical Italian-American-style pizza without having to bite off those strands of melted cheese that stretch between your mouth and the slice?
** Cooking Recipes **