What ingredient is traditionally thickened in a Béchamel sauce?

Béchamel sauce, one of the classic mother sauces in French cuisine, is traditionally made by thickening milk with a white roux, which is a mixture of equal parts flour and fat (typically butter). In making a Béchamel, the roux is cooked gently until it takes on a light color and then is combined with warm milk. This heating process allows the flour to absorb the liquid, resulting in a smooth and creamy sauce.

While cream can also be used to make variations of creamy sauces, the foundational element for Béchamel is milk, which provides the primary liquid component that is thickened. Vegetable stock is not used in traditional Béchamel, as the sauce aims for a neutral flavor that comes primarily from milk. Butter is a key ingredient in creating the roux but is not the liquid that is thickened in this sauce. Thus, milk is the correct choice as it is the main ingredient being thickened in a Béchamel sauce.