Skilled cooks and bakers can draw on a lot of techniques for thickening liquids, ranging from custards thickened with egg yolks to broths made rich and satisfying with natural gelatin. Some of the most useful and versatile of all thickeners are starches. They're appropriate in everything from your mom's pie to industrial processed foods, and two of the most useful -- cornstarch and flour -- are probably already in your cupboard.
How They Work
When they're left alone, the starches in most foods take the form of microscopic nuggets, or granules. When they're moistened and heated, those granules begin to absorb water and swell up, like a child's toy. This puts great strain on the chemical bonds holding them together, and as the heat increases, those granules rupture. The starch molecules, instead of binding together in a tight ball, form a three-dimensional web of elongated strands that trap water into a soft gel.
Cornstarch is a refined starch, so it's a very powerful thickener. To use it, you'll usually mix a small amount of dry cornstarch into cold water or other liquid. When you stir this slurry into your hot sauce or soup, the starch granules disperse through the hot liquid before they can become fully heated. Then, as their temperature rises above 144 degrees Fahrenheit, the granules quickly gel and thicken the liquid.
Flour is a less-effective thickener than cornstarch, because it's not refined. It contains a relatively high level of protein and a number of other substances which are necessary for baking and other purposes, but don't contribute to thickening. Flour takes longer to fully gel than cornstarch, often 20 minutes or more, and doesn't reach its full thickening power until it reaches 180 F to 185 F. Specialized quick-mixing flour, or gravy flour, is made by heating ordinary flour until it reaches gelling temperature and then drying it in that form. Because it's already gelled, it will thicken sauces almost instantly.
Cornstarch and flour are both versatile thickeners, but they have definite differences. Cornstarch thickens more effectively at low temperatures, so it's better suited for pies and other uses where preserving a fresh flavor is important. It also makes a clear and beautiful sauce, which is another reason it's used with fruit. Unfortunately, cornstarch breaks down quickly under prolonged heating. Flour -- except the quick-mixing variety -- takes longer to gel, but holds up better under prolonged heating. It makes an opaque, heartier sauce, well suited for stews and meat sauces.