What is roux?

Mixing the starch with fat is the principle of roux. It is a cooked mixture of equal parts by weight of fat and flour. Although exact amounts of fat and flour are called for in recipes it is solely the amount of flour that determines the thickness of the sauce.

Fat lubricates and smooth the flour so it does not form lumps with combined with stock or other liquid. The roux is used to thicken soups, stews, gumbo, white sauce or gravy. The fat should be a refined vegetable oil suitable for high temperature cooking, such as canola oil. Any grain or legume flour can be used to make roux.

A roux must be cooked so the finished sauce does not have the raw, starchy taste of flour. A roux is started by melting butter or other fat, adding flour and cooking the two together over low heat, whisking or stirring constantly to prevent scorching.

During this process, which takes only a few minutes, the starch is the flour expands as it blends with the fat. A good roux is stiff, not runny or pourable. A roux with too much fat is called a slack roux.
What is roux?