A sauce is generally defined as a thickened, seasoned liquid that is served with a food to add flavor and moisture and to enhance its appearance. It can be thick or thin, hot or cold, savory or sweet. Many sauces, especially the classic French sauces, can be classified by the way in which they are thickened.

Flour-thickened sauces include two classics: white sauce and brown sauce. They are the basis for many variations. White sauce (known as béchamel sauce in France) is made by thickening milk or cream with a roux made of butter or margarine and flour. The roux is cooked for a minute or two until the raw taste of the flour has dissipated. Then the milk is gradually added and the mixture is brought to a boil while being constantly stirred. Boiling for 2 or 3 minutes will eliminate any residual raw flour taste. The white sauce can then be seasoned with salt and pepper. This rich, creamy sauce is the basis for cheese sauces, cream sauces, savory soufflés and cream soups. Variations of the white sauce are made by substituting chicken or fish stock for milk. Egg yolks and cream are sometimes added for extra richness and white wine for flavor.

White sauces can be made in different thicknesses. The thickness of the sauce is determined by the proportion of flour and butter to liquid. Consult the Sauce Chart (in Index) for appropriate ingredient proportions. Thin white sauce can be served as an accompaniment to vegetables or used as a thickening agent for delicately flavored soups. Medium white sauce is generally served over food or used as an ingredient in casseroles. Thick white sauce is combined with vegetables for creamed vegetables or used as the base for soufflés.

Brown sauce is made by thickening a rich meat stock with a roux made of flour and fat, such as butter, margarine, oil or meat drippings. The roux is cooked for 15 to 20 minutes until it is a rich brown color. Brown sauce is the basis for gravies, mushroom sauce, bordelaise sauce and Madeira sauce. It also acts as a thickener for stews, gumbos and meat soups. Like white sauce, brown sauce can be made in different thicknesses by altering ingredient proportions.

Egg-thickened sauces include hollandaise and béarnaise (savory sauces) and custard (dessert sauce). Hollandaise and béarnaise sauces are cooked emulsions of egg yolks, an acidic ingredient and butter. The egg yolks also act as a thickener. The acidic ingredient in hollandaise sauce is lemon juice. Béarnaise contains vinegar, white wine, shallots and tarragon. To prevent separation, these sauces should be whisked constantly over low heat. Egg-thickened sauces should never be allowed to boil, because this will coagulate the egg, resulting in a lumpy sauce. Egg sauces should not be prepared ahead. Hollandaise sauce is served with eggs, vegetables, especially asparagus, and poached fish. The rich flavor of béarnaise sauce is a good accompaniment to beef and lamb. Custard sauce is a cooked dessert sauce of milk, sugar and flavoring that is thickened with egg. Unlike emulsified sauces, it can be made ahead.

Butter sauces are rich emulsions of butter and liquid. They can be difficult to make if the butter separates from the liquid, causing the sauce to become watery and unattractive. Beurre blanc is the classic French butter sauce made of a reduction of wine, vinegar and shallots into which chunks of cold butter are whisked to form a thick, smooth sauce. It is traditionally served with fish, poultry, eggs and vegetables. This sauce cannot be reheated because it will separate. Stir any leftover sauce into hot cooked vegetables, pasta or rice.

Cornstarch-thickened sauces can be savory or sweet. Cornstarch gives sauces a clear, glossy appearance. Most common are Asian stir-fry sauces and dessert sauces, such as lemon and orange sauces. Cornstarch must be dissolved in cold liquid before cooking.

Dessert sauces include butterscotch, caramel and chocolate. These are generally lightly cooked sauces that can be made at home or purchased.

Fruit purées are simple sauces that contain no thickeners. Fruit may be raw (raspberries) or cooked (persimmons). Some cooked vegetables, such as red bell peppers, can also be puréed and used as sauces.

Reductions are liquids that are simmered in order to evaporate liquid and concentrate flavor. The result is a sauce that contains no thickener. Flourless cream sauces and reduced meat stocks are examples of reductions.

Tomato sauces can be made from either fresh or canned tomatoes. When using fresh tomatoes, the quality of the sauce is dependent on the ripeness of the tomatoes. If the tomatoes are commercially grown, they may be pale and lacking in flavor. For more flavor, add a tablespoon or two of tomato paste or a tablespoon of chopped sun-dried tomatoes. Cooking time affects the thickness and the flavor of tomato sauces. Longer cooking results in a more full-bodied flavor.