Cornstarch is best known for its thickening power in sauces, gravies, soups, pies, custards and puddings. To store cornstarch, keep it in an airtight container and like other starches, cornstarch loses its thickening power when exposed to air over a long period of time.
To use cornstarch - dissolve 1 teaspoon cornstarch in 1 tablespoon cold liquid (water is fine). Stir or whisk this cold mixture (known as slurry) into 1 cup hot liquid at the end of the cooking time. Cook until thickened, about 30 seconds to 1 minute, stirring constantly. Cook 1 minute more to remove any “cornstarch” taste. These proportions will make about 1 cup of medium-thick sauce, soup or gravy. For thinner sauce, use 1/2 teaspoon cornstarch. For thicker sauce, use up to 2 teaspoons cornstarch.
Replacing flour and arrowroot as a thickener - use 1 1/2 teaspoons cornstarch for each tablespoon of flour called for in the recipe. As for arrowroot, use 2 teaspoons cornstarch in place of 1 tablespoon arrowroot.
To prevent cornstarch-thickened sauces from thinning - stir gently and cook for no more than 1 minute over medium heat after the sauce has thickened. Any longer and the starch cells are apt to rapture. Also avoid licking the spoon. Enzymes in your saliva can break down a mixture thickened with cornstarch. If you need to lick the spoon, do not put it back into the mixture. Finally, avoid freezing mixtures thickened with cornstarch.
If cornstarch mixture has thinned upon cooling - add a little more liquid. It seems counterintuitive, but sometimes, cornstarch does not fully swell and remain thickened if there is not enough liquid in the mixture. A high proportion of sugar than liquid or a particularly high proportion of fat in a mixture can also be the culprit. Again, try adding more liquid to solve the problem. Lemon juice and other acidic ingredients can also reduce the thickening power of cornstarch. If you suspect that this may be the cause, slightly increase the cornstarch. To avoid this problem, add acidic ingredients after cooking.