What is the role of a roux in sauce preparation?

The primary role of a roux in sauce preparation is to thicken the sauce. A roux is a mixture of equal parts flour and fat—typically butter—that is cooked together. When the roux is added to a liquid, such as stock or milk, the starches in the flour swell and absorb liquid as it heats, effectively increasing the viscosity of the sauce. This thickening process creates a smooth and stable texture that is characteristic of many traditional sauces, such as béchamel or Espagnole.

While a roux may impart a slight flavor and color to the sauce due to the cooking process of the flour, its main function is to act as a thickening agent. It's important to note that sauces can be flavored and colored through other means, such as the inclusion of herbs, spices, or other ingredients, but the roux itself is specifically designed to achieve thickness.