When you make a soup, sauce, gravy, pie filling or pudding that turns out too thin or watery, your first inclination may be to reach for cornstarch. Mixed with cold water and stirred into hot liquid, cornstarch thickens any liquid as soon as the mixture returns to its original cooking temperature. However, if you have food sensitivities or don't want to use cornstarch, other thickening agents work just as well.
Arrowroot has a milder flavor than cornstarch, and it thickens at a lower temperature. It also freezes better since it doesn't get spongy as cornstarch sometimes does when you defrost it; however, arrowroot is not recommended for dairy dishes because it gives them a slimy consistency. Instant or granulated tapioca is a good thickener, but its tiny granules don't dissolve, which is unpleasant to some palates. You can sometimes find tapioca starch, which has no granules, or pulverize the other types to remove the granules. If you prefer a gluten-free thickener, choose potato starch or potato flour, but don't let the thickened mixture boil as it may seize up.
Dairy and Eggs
Heavy whipping cream gives sauces and gravies a rich texture as it reduces and thickens the mixture. For lower fat content, use half-and-half or evaporated milk. You can also use plain, unflavored yogurt or sour cream for thickening, but it may add a slightly acidic taste to recipes. Egg yolks are ideal thickeners, but you must temper them before adding the yolks to hot liquids to prevent curdling. To temper egg yolks, whisk a bit of the hot liquid into them, and then stir the mixture back into the food you want to thicken.
Chocolate and Cocoa
Bittersweet chocolate thickens sweet liquids since the cocoa solids in it are filled with starches. However, unsweetened chocolate has more starch, and it is likely to overly thicken liquids and make them thick and gooey. Cocoa powder, which has a significant amount of starch, is used to increase the viscosity of savory dishes without adding sweetness.
Gelatin and Pectin
Unflavored gelatin thickens substances as they cool. It has no odor, taste or color, and you must rehydrate it before you add it to food. It is a very powerful thickener, so use it sparingly to avoid over-thickening. Unflavored gelatin is available in both regular and vegetarian versions. Pectin is a natural carbohydrate found in the cell walls of assorted fruits. Sold in powdered form, it is commonly used to thicken jams or fruit pie fillings.
Agar agar is made from a combination of algae. Agar agar fronds are freeze-dried and dehydrated into thin sheets and then formed into bars. You can find the substance in flake and powder form; it is sometimes used in cooking as a substitute for gelatin or pectin.