If I have a sauce recipe that calls for wheat flour as a thickener and I want to use cornstarch instead, how much should I use?--Patti
Generally, the substitution would be to use one half to two thirds as much cornstarch as the amount of flour called for in the recipe. If possible, start with the lesser amount and add a bit more if necessary. This means that to thicken a cup of liquid to a sauce of medium consistency, where you would use 2 tablespoons of flour, you would start with one tablespoon of cornstarch, and if needed add as much as a teaspoon more.
The easiest way to do this in most cases would be to make a slurry by mixing the cornstarch with some liquid and adding it to the sauce. You can either use some of the cooking liquid, in which case you want to let it cool down until it is lukewarm before adding the cornstarch, or you can mix it with some water. The slurry can then be mixed back into the hot liquid and brought back to a near boil until the sauce thickens.
To avoid getting a sauce that is too thick, mix some of the cornstarch slurry in, cook the sauce until it thickens, and if you want add more.
Putting this all together, to replace 2 tablespoons, say, of flour, mix a slurry of 4 teaspoons of cornstarch with about a quarter to half cup of cold water. With cornstarch, I find it helps to mix it beforehand and let it sit until it is ready to use. When the time comes, stir about 3/4 of this mixture into the sauce. That would mean that you have added 3 teaspoons, or 1 tablespoon, of cornstarch. Cook the sauce until it thickens and if you want it thicker, stir in the last of the cornstarch mixture.
Just remember that once the sauce is off heat and starts to cool, it will become thicker so you always want to have it a bit too thin to start with.