A roux is a mixture of flour and fat used to thicken sauces, soups, and stews. It is traditionally used to thicken the classical French Béchamel, Velouté, and Espagnole sauces and in Cajun and Creole cooking. The roux is made from equal parts wheat flour and either butter, vegetable oil, bacon drippings or lard which are cooked together with or without colouring. In Cajun cuisine, the roux is almost always made with oil or lard instead of butter, which would burn and is dark brown in colour, adding greatly to the flavour of the finished sauce. In regional American cuisine, bacon is sometimes rendered to produce the fat to use in a roux for extra flavour, in recipes such a sausage gravy. In the case of meat gravies, fat rendered from meat is often used to make a pan gravy.
Cooking the roux produces the correct chemical structure to thicken the sauce and starts to remove the floury taste from the finished sauce. A light roux of flour and butter, cooked for two or three minutes without colouring is used in the traditional béchamel sauce. Cooking until the mixture becomes golden in colour and sandy in texture produces a blonde roux the base of velouté sauces. Darker roux can range from light brown to chocolate in colour and add a distinct nutty flavour to a dish. The darker the roux, the less thickening power it has, a chocolate roux has about a quarter of the thickening power, by weight, of a white roux.
The golden rules when thickening a roux based sauce is to allow the roux to cool slightly after cooking and use a liquid that is just off the boil to make combining the ingredients much easier. The sauce will then need to very gently simmer for at least thirty minutes to cook out and eliminate any floury taste. The resulting sauce will be smooth, rich and creamy in texture.