What is the main ingredient in a traditional béchamel sauce?

The main ingredient in a traditional béchamel sauce is milk. Béchamel, one of the classic French mother sauces, is primarily made by combining milk with a roux—made from equal parts flour and butter. The purpose of the roux is to thicken the sauce, but it is the milk that forms the base and gives béchamel its characteristic creamy texture and flavor.

In addition to the basic ingredients of milk, butter, and flour, béchamel can be seasoned with nutmeg, salt, or white pepper, but it is the foundation of milk that defines this sauce.

While butter and flour are important for creating the roux that thickens the sauce, without the milk, there would be no béchamel; it would simply be a preparation of fat and starch. Cheese, while it can be added to create variations like a Mornay sauce, is not part of the original béchamel definition. Thus, milk is the critical component that underscores the sauce's identity.