What is the purpose of a roux in sauce preparation?

A roux plays a crucial role in sauce preparation primarily by acting as a thickening agent. It is created by cooking equal parts of fat, usually butter, and flour together over heat. This combination forms a paste that, when added to liquid ingredients such as stock or milk, helps to thicken the mixture. The starchy flour in the roux absorbs moisture from the liquids, resulting in a smooth, cohesive sauce or soup.

The process of cooking the roux also allows it to develop flavor, depending on how long it is cooked. A roux can be light, medium, or dark, with each stage offering a different flavor profile. However, its main functional purpose remains its thickening capability, which is essential for achieving the desired texture in various sauces.