Soups and sauces can be thickened in a variety of ways. A sauce must the thick enough to cling to the food, but not so thick it stands up on its own. Starches are by far the most common thickening agent. Cornstarch, arrowroot, waxy maize and the ever popular, roux (roo). But what is a roux and how does it work?
Roux is a cooked mixture of equal parts by weight of fat and flour. If you mix a starch with water, such as cornstarch it is called a slurry
How does it work?
Starches thicken by absorbing water and swelling to many times their original size. This process is called gelatinization. In order for the starch to function at its maximum, each granule of starch must be separated before heating in order to avoid lumps. If granules are not separated the starch on the outside of a lump quickly gelatinizes into a coating that prevents the liquid from reach the rest of the starch inside. This is accomplished in two ways.
1. By mixing the starch with cold water - This is used with starches such as arrowroot and cornstarch. This method is not recommended for flour because it lacks flavor and has an undesirable texture.
2. By mixing the starch with fat - This is the principle of the roux. A roux must be cooked for a short period of time so the finished sauce or soup does not have the starchy taste of flour. If cooked for just a short period of time, it is called a blond roux. If cooked longer until it takes on a light brown color, it is called a brown roux.
The most preferred roux in cooking is made by mixing melted butter and flour. Many cooks clarify the butter first because the liquid in whole butter tends to gelatinize some of the starch and make the roux hard to work with. A roux made with butter gives a nice rich flavor to sauces and is easy to work with.
Margarine and oils can be used to make a roux as well, but because of there lack of flavor they are very seldom the top choice.
Fat drippings from animals such as chicken and beef can make superior sauces. Animal fats enhance the flavor of sauce, but again must be clarified to eliminate any liquid that might cause lumping.
Mixing it all together
A roux can be added to the liquid or the liquid may be added to the roux. The general rules are: The liquid can be hot or cool, but not cold. A very cold liquid will solidify the fat in the roux. The roux in the same way can be warm or cold, but not hot. A hot roux could cause spattering and possibly lumps. For medium sauces and soups I use 8 ounces butter and 8 ounces flour per gallon of liquid. For home it comes out to about 1 tablespoon each per cup of liquid. Use less or more depending on how thick you like your sauce. By follow these simple steps you'll have lump free soups and sauces for the rest of your life.