Why is roux a sauce used for thickening?

When roux is heated in the presence of liquid, the starch molecules in the flour absorb the liquid and swell, creating a viscous mixture. This thickening effect is due to the gelatinization of the starch molecules, which occurs when the starch granules break down and form a gel.

The gelatinization temperature of starch varies depending on the type of starch, but it is typically between 60°C and 75°C (140°F and 167°F). When roux is heated to this temperature, the starch granules swell and burst, releasing their contents into the liquid. The starch molecules then interact with the water molecules to form a gel, which gives the sauce its characteristic thick and smooth texture.

Roux is a versatile thickening agent that can be used in a variety of sauces, soups, and stews. It is particularly well-suited for sauces that are made with dairy products, as the milk proteins help to stabilize the gel and prevent it from breaking down.

In addition to its thickening properties, roux also adds flavor and color to sauces. The flour in roux browns as it cooks, giving the sauce a golden color. The browning reaction also produces a variety of flavor compounds, which can enhance the taste of the sauce.