- The Washington Times - Tuesday, July 19, 2005

Cheese is an all-time, anytime great ingredient. We love cheese sprinkled on salads, soups, entrees, casseroles, eggs, and on and on.

Cooking with cheese, however, can have an occasional surprising result. Curdling and stringiness are two possible problems, but never fear because science is here to help us avoid these cooking mishaps.

Why does cheese curdle? Cheese is made by curdling milk with an enzyme (rennet) or with heat or both. Pecorino and ricotta, for example, can come from the same batch of sheep’s milk. The milk is treated with rennet to make pecorino. Then the whey, which is left after the pecorino curds rise to the top, is heated to make ricotta.

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.

Almost immediately, an unwound (or denatured) protein with its bonds sticking out will run into another unwound protein, and they will join together. Now there is no longer room for light to go between them. (We see this change when the egg white in a heated frying pan turns solid white or raw shrimp or fish or chicken goes from glassy-looking to white.)

Initially, when these proteins bond, they are moist and tender because a lot of water is trapped between them.

If you continue to heat or expose them to acid or air, the bonds tighten and squeeze out the moisture, and the proteins become dry and tough.

You may remember having had overcooked shrimp that was so tough that you practically needed a steak knife for cutting. Likewise, with many proteins, there is a narrow range between cooked and overcooked.

A stirred custard properly cooked is creamy and thick, but overcook it and it is transformed into knotty curds (curdled egg and dairy proteins) floating in a liquid. We refer to this as curdled.

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.

A macaroni-and-cheese dish that has a little starch such as flour in the sauce won’t curdle, while one that contains only grated cheese and milk may. Time and time again in cooking, starch will prevent proteins from curdling, whether it is the flour in a pastry cream, a little cornstarch in a yogurt quiche or cornstarch in a low-fat sauce.

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.

Whatever the science, we have learned from experience that most home-use starches, including flour, cornstarch, potato starch and tapioca, can prevent proteins from curdling.

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.

As you can see, cooking with cheese is easy. Just remember our trusty ingredients — a little starch and a little lemon juice — and cook with confidence. The result will certainly be a cheese sauce that will please.

Fettuccini with mozzarella, ham, mushrooms and tomatoes

6 tablespoons dried mushrooms such as cepes or porcini


2 tablespoons butter

3 tablespoons flour

1/8 teaspoon white pepper

2 cups milk

2 tablespoons lemon juice

3/4 cup grated mozzarella

½ teaspoon hot red pepper flakes

4 tablespoons olive oil

3/4 cup country ham, chopped

½ pound fresh mushrooms, sliced

4 tomatoes, peeled, seeded and coarsely chopped

18 ounces fettuccini

In a small bowl, soak dried mushrooms in 1/3 cup hot water to soften. Set aside.

Bring 2 gallons of water with 1 tablespoon salt to a boil. While water is heating, melt butter in a large skillet over medium heat. Stir in flour, ½ teaspoon salt and white pepper, and simmer on low for 2 minutes.

Remove from heat and whisk in milk. Cook over medium heat, stirring constantly, until smooth. Sprinkle lemon juice over the mozzarella, toss to coat, and then whisk cheese into sauce. Stir pepper flakes into olive oil in a large skillet over medium heat. Stir in ham and mushrooms. Saute briefly and remove from heat.

Remove dried mushrooms from soaking water with a slotted spoon; add to pan. Pour all but the last tablespoon of soaking liquid (which may contain sand) into pan.

Stir mushroom mixture and tomatoes into the mozzarella sauce. Add fettuccini to boiling water. Stir for the first 2 minutes of cooking. Cook for the time recommended on the package. Drain and add pasta to skillet containing cheese sauce. Cover and let stand for 1 minute. Serve immediately. Makes 6 servings.

• Food scientist Shirley Corriher is the author of “CookWise: The Hows and Whys of Successful Cooking” (William Morrow).

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