Why do white sauces set as they cool?

White sauces, such as béchamel, velouté, and allemande, set as they cool due to a process called gelation. This process involves the formation of a three-dimensional network of molecules that trap water and other liquid components of the sauce, causing it to thicken and solidify.

The gelling agent in white sauces is starch, which is present in the flour used to make the roux. When flour is heated in butter or another fat to create a roux, the starch molecules absorb moisture and swell, forming a viscous mixture. As the sauce continues to cook, the starch molecules break down into smaller chains, which are able to align themselves into a more organized structure, forming a gel.

The cooling process further enhances the gelation of the sauce. As the temperature drops, the starch molecules become less soluble and more tightly packed, causing the sauce to thicken and set. The exact temperature at which a white sauce sets depends on the type of starch used and the concentration of starch in the sauce.

Other factors, such as the presence of acids, salts, and other ingredients, can also influence the gelation process and the final texture of the sauce. For example, acidic ingredients can weaken the gel structure, while salts can strengthen it.

By understanding the role of starch in gelation, chefs can control the texture and consistency of white sauces, ensuring that they have the desired thickness and structure for their intended use.