From The Cook's Bible:
How to Thicken a Sauce

by Christopher Kimball

Continued from the previous page...

The Science of Cooking
Why do flour and cornstarch thicken sauces?

You've probably noticed that when stirred into water at room temperature, neither flour nor cornstarch thicken. They just turn into a loose paste. What is missing is the application of heat, which encourages the bonding of starch and water molecules. (Both flour and cornstarch are mostly starch, although flour contains many other ingredients such as proteins.) The starch granules then start to enlarge (think of blowing up a beach ball), trapping water as they grow. Finally, at temperatures over 150 degrees and up to a point just below boiling, the rigid structure of the granules breaks up, creating a spidery web of bonded starch and water molecules. This mesh prevents the free movement of water molecules and results in a thick sauce. You probably have also noticed that at this point, the sauce starts to become clearer. That is because the starch molecules are no longer packed tightly together--they are in a looser meshwork after heating--and therefore light is less likely to be deflected. At temperatures above 200 degrees, however, the large starch granules start to shrink in size, leaking starch molecules into the sauce. As these swollen granules deflate, the sauce becomes thinner.

Although flour is the traditional thickening agent in French cooking, cornstarch is a more powerful thickener because it is a purer form of starch. It will also create a clearer, shinier sauce. The French clarified their sauces through hours of slow cooking, skimming off the protein as the sauce simmered, and turning them into perfectly clear, shiny liquids, which keep their shine even with the addition of flour. Given the realities of home cooking, this makes no sense and, therefore, cornstarch is preferable.

The White Sauce

The most basic sauce is based on a roux, which is equal volumes of butter and flour whisked together in a saucepan over heat in order to ameliorate the flavor of the flour and to remove lumps. Click to continue...