Better living through chemistry

I'm sure we've all suffered through the experience of trying to melt cheese, only to have it turn stringy. (I once had the fun of trying to salvage an attempt at making cheese dip at a SF convention, with a crock pot and a microwave as my heat sources.)

Part of the problem is that with cheese, we're already starting with a curdled product.

Cheese is made by curdling milk with an enzyme (rennet) or with heat or both. ... Because we are already dealing with curds, it shouldn't be surprising that further curdling can occur. The natural proteins in cheese are individual units like little wads of string. They float around totally separated, so there is room for light to go between them. When you heat these proteins – or add acid or, with some, just expose them to air – the bonds in the natural wadded-up protein release and partially unwind. Since it has changed from its natural form, it is now called a "denatured" protein.

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Because the proteins in cheese are already linked, they can easily overcook and curdle. The temperature and ingredients with which a cheese is combined, as well as the nature of the cheese itself, influence how easily curds will form. Simply preventing overheating may save a dish. In many recipes, the directions tell you to remove the dish from heat and then stir in the grated cheese. This helps you avoid the overheating that can cause curdling. Cheese sauces that contain starch also help us avoid curdling. Most are made by adding grated cheese to a cream sauce. Just make sure that any dish to which you are adding grated cheese contains some starch. Scientists do not know exactly how starch prevents proteins from joining at the temperature they normally do when heated. It may simply be that when the heated starch granules soak in liquid and swell, they become large objects that physically keep proteins apart. That leaves us with the battle against stringy cheese. If you stir a cheese sauce much after the cheese is added, particularly with certain cheeses such as Swiss, it may become stringy. Because mozzarella can become so stringy, many cooks would not dream of putting it into a sauce. The reason, as explained by Norman Olson of the University of Wisconsin, is that some cheeses, including mozzarella and Swiss, contain calcium phosphate, a compound that can link cheese proteins in long strings. To reduce this, cooks have traditionally added dry white wine to such foods as fondue. Anthony Blake, director of food science and technology for the international flavor and fragrance company Firmenich SA, lives in Switzerland and is an enthusiastic cheese lover. He says wine contains tartaric acid, which helps prevent calcium phosphate from linking the cheese proteins. This can prevent stringiness, but citric acid in lemon juice is actually much more effective. Citric acid binds with calcium and can overcome stringiness, even in the extreme case of mozzarella. I love to make fettuccini with a sauce of mozzarella, prosciutto or country ham, mushrooms, and tomatoes. To prevent it from becoming stringy, I sprinkle a little lemon juice on the grated mozzarella before adding it to a cream sauce, then stir it in over low heat. You will be astounded by how well this works.

Of course, you can buy tartaric acid, and citric acid, if you don't want the lemon flavor, or alcohol from wine, in your dish.