What is the name of process that thickens white sauce?

A roux is a mixture of equal parts fat (like butter or oil) and flour that is cooked together over low heat. The flour absorbs the fat and thickens the sauce as it cooks.

Here's how it works:

1. Fat is melted: The fat is heated in a saucepan until it's melted.

2. Flour is added: Flour is whisked into the melted fat, and the mixture is cooked, stirring constantly, until it turns a light golden brown color.

3. Liquid is added: The roux is then whisked into a hot liquid, like milk or cream, to create a smooth, thickened sauce.

The amount of time the roux is cooked determines its thickness and color:

* White roux: Cooked for a very short time, resulting in a light, airy sauce.

* Blonde roux: Cooked for a longer time, resulting in a thicker sauce with a slightly nutty flavor.

* Brown roux: Cooked for an extended period, resulting in a very dark, rich sauce with a deep, nutty flavor.

The type of roux used depends on the desired consistency and flavor of the white sauce.