Why it Works: Flour Power
Flour is a good sauce thickener because it contains starch. However, the thickening power of flour’s starch is released only when heated. Mixing flour with a little cold water results in a thick paste or dough. Mixing it with a lot of water and heating it to boiling gives you a smooth sauce. Why? When heated, starch (which usually looks like a little ball) breaks down into tiny pieces. These pieces now absorb great quantities of liquid and thicken your sauce.