Roux ( /ˈruː/) is a cooking mixture of wheat flour and fat (traditionally butter). It is the thickening agent of three of the mother sauces of classical French cooking: béchamel sauce, velouté sauce and espagnole sauce. Clarified butter, vegetable oils, or lard are commonly used fats. It is used as a thickener for gravy, other sauces, soups and stews. It is typically made from equal parts of flour and fat by weight. When used in Italian food, roux is traditionally equal parts of butter and flour. In Cajun cuisine, roux is almost always made with oil instead of butter and dark brown in color, which lends much richness of flavor, albeit less thickening power. Hungarian cuisine uses lard (in its rendered form) or—more recently—vegetable oil instead of butter for the preparation of roux (which is called rántás in Hungarian).
The fat is heated in a pot or pan, melting it if necessary. Then the flour is added. The mixture is stirred until the flour is incorporated and then cooked until at least the point where a raw flour taste is no longer apparent and the desired color has been reached. The final color can range from nearly white to nearly black, depending on the length of time it is over the heat and its intended use. The end result is a thickening and flavoring agent.
Roux is most often made with butter as the fat base, but it may be made with any edible fat. In the case of meat gravies, fat rendered from meat is often used. In regional American cuisine, bacon is sometimes rendered to produce fat to use in the roux. If clarified butter is not available, vegetable oil is often used when producing dark roux, as it does not burn at high temperatures, as whole butter does.
When combining roux with water-based liquids, such as broth or milk, it is important that these liquids are not excessively hot. It is preferable to add room temperature, or warm, roux into a moderately hot or warm liquid, or vice versa. To ensure the desired viscosity, they should be added in small quantities while stirring, briefly bringing the temperature up to boiling. Otherwise, the mixture will contain lumps.
Watch 45 Sec Lesson on How to Make a Roux