This question used to plague me often in my early cooking days. Sometimes I still have to refresh my memory when I’m making something new that needs a thickening agent and so I go back through my notes and cookbooks to learn what I forgot about corn starch and flour. It’s really good information to know and I thought you might just find it as useful as I do.
When cornstarch is added to cooking foods, the heat causes the starch to bind the water molecules and the individual starch granules absorb liquid and swell. By the time the mixture nears boiling, the starch granules will have grown to about 10 times their size at room temperature. At temperatures above 205°F, however, the large starch granules start to shrink and as these swollen granules deflate, the sauce becomes thinner, so do not let cornstarch thickened sauces boil.
Although flour is the traditional thickening agent in most cooking, cornstarch, also known as corn flour, is a fine, powdery flour ground from the endosperm, or white heart, of the corn kernel. Both are starches, but cornstarch is pure starch, while flour contains gluten. The gluten reduces the thickening power of flour, so lacking gluten, cornstarch has twice the thickening power of flour. Sauces thickened with cornstarch will be clear, rather than opaque, as with flour-based sauces, and it doesn’t cause lumps like flour.
Cornstarch has twice the “thickening power” of flour, so it’s necessary to use only half as much. Example: If recipe calls for 1/4 cup (there are 4 tablespoons per ¼ cup) of flour, use just 2 tablespoons cornstarch. Cornstarch thickens with a satiny smoothness and glossy appearance. It adds no taste of its own to mask the flavor of foods.
Recipes thickened with cornstarch have a brighter, more translucent appearance than those thickened with flour. Cornstarch also blends more easily with liquids than flour because it doesn’t absorb liquid until it’s cooked. Cooking with cornstarch is easy when you follow a few simple guidelines. Gradually stir cold liquids into cornstarch until completely smooth, cook over medium-low heat. Cooking over high heat can cause lumping, but avoid stirring too vigorously because it may break down and thin out. Stir gently while it thickens, the starch granules will have swelled to their full capacity in about 1 minute.
Use 1 tablespoon of cornstarch to thicken every 2 cups of liquid to a medium consistency. Cornstarch mixed with a little cold liquid, is stirred into the hot food during the final stage of cooking, and it must be cooked to 203°F (95°C) before thickening begins. At that point, it usually thickens fairly quickly and the sauce turns from opaque to transparrent. Always mix a slurry of cornstarch and a small amount (1/4 cup) of cold liquid (water, stock, wine, etc.) until smooth, then add this mixture to the food that you want thickened. Do not mix with liquids that are acid such as citrus juice or apple juice or it’s thickening power is cut in half. Remember not to boil a cornstarch thickened sauce or it will thin out.
If there is not enough liquid (water, milk, juice) in the mixture, the corn starch granules will not fully swell and remain thickened when the mixture cools. Adding a little more liquid (not more corn starch) is likely to solve the problem.
A higher proportion of sugar than liquid (water, milk, juice) in a mixture can interfere with the swelling of the corn starch granules and prevent thickening during cooking and/or cause thinning during cooling. Adding more liquid (not more corn starch) will often solve the problem.
An excessively high proportion of fat or egg yolks in a mixture can interfere with the swelling of the corn starch granules and prevent thickening during cooking and/or cause thinning during cooling. Adding more liquid (not more corn starch) will usually solve the problem.
Acid ingredients such as lemon juice, lime juice or vinegar will reduce the thickening ability of the starch or prevent the mixture from thickening. Increase the starch level slightly or stir acid ingredients in after cooking.
Excessive or rough stirring with a wire whisk or even a spoon may break the starch cells and cause the mixture to thin out.
Simmering or boiling a corn starch thickened mixture for an extended period of time may cause the starch cells to rupture and the mixture to thin.
Freezing corn-starch thickened mixtures will rupture the starch cells and cause the mixture to thin out.