Think of a soup that seems watery versus one that has been thickened with starch or gum. The thickened soup tastes better even when the flavor of the soups is identical. One cooking technique used to thicken a recipe is called reduction. Heating a mixture provides enough energy to the water molecules to allow them to rotate, vibrate, and escape the intermolecular forces that hold the water molecules in place. As the water “boils off,” the remaining molecules of complex carbohydrates or proteins are forced to interact with each other, increasing the viscosity of the fluid. Unfortunately, this preparation that requires high heat or long cooking times often alters the flavor and nature of the fluid in unintended ways.
The starches found in flour can form pastes and gelatinize for use in gravies and stews. Thick flour pastes are difficult to form, as the starch solidifies in a low water environment. Cooks will use fats to coat and interact with the starch. Such a mixture, called a roux, is the base for pasta gravies and some thick stews. How do you decide what thickening agent to use—a protein, starch, or gum? It depends upon the flavor and final presentation of the desired food. Is the food to be served hot or cold? Proteins coagulate upon cooling (making a solid mess), while gums and starches are more effective thickeners when cold. Starch thickeners quickly clump when reheated.
If you want a clear thickened soup, some gums (like xanthan gum) can be used in a small amount to avoid clumping and cloudiness. Both starch and xanthan gums work well for more viscous preparations. pH can also impact the behavior of a thickening agent. Acidic foods cause some gums to be less effective; thus, cornstarch or arrowroot starch is a better choice. Clear Jel is a chemically modified cornstarch that is specially made for baking and freezing acidic foods; it is a good choice for acid‐containing fruit pies that need thickening.