By Harold McGee
Wheat flour is made by grinding wheat grains and sieving the bran and germ from the starch-rich endosperm. Wheat flour is only about 75% starch, and includes about 10% by weight of protein, mainly the insoluble gluten proteins. It’s therefore a less efficient thickener than pure cornstarch or potato starch; it takes more flour to obtain the same consistency. A common rule of thumb is to use 1.5 times as much flour as starch. Flour has a distinct wheat flavor that cooks often transform by precooking the flour before adding it to a sauce. The suspended particles of gluten protein make flour-based sauces especially opaque and give their surface a matte appearance, unless the sauce is cooked for hours and skimmed to remove the gluten.