What are the substitutions for using cornstarch or some other thickener in place of flour when making sauces?-- Sally
Talk about a question that will lead you to a whole bunch of confusing answers!
Here's the science -- what we call starch is actually two different molecules. One, amylose, is a long straight chain of glucose molecules. The other, amylopectin, is more branching, like a tree with chains coming off the sides of other chains. Flour, cornstarch and other thickeners vary in their proportions of these molecules. Amylose and amylopectin act differently when used as thickeners. Also, the number of glucose molecules in a single starch molecule can be anywhere from hundreds to thousands. Then, just to make things even worse, starch actually comes not as single molecules but as granules containing several molecules of starch each.
In practice, all of these differences affect how a starch reacts, both when making the sauce and as it cools down when served. And, because of this, references vary somewhat in their substitutions.
Generally, the suggestion is to replace 2 tablespoons of wheat flour with 1 tablespoon of cornstarch, 2 1/2 teaspoons of arrowroot starch or 2 teaspoons of potato or rice starch. Depending on the dish, the actual amount may need to be a bit more, but it is always easier to add more than to take some out.
Starches from the roots of plants, like potato starch and arrowroot, have longer starch chains, which may make the sauce seem stringy and will make it thicker when it cools down.
While flour may be cooked in a roux or worked into a flour and butter paste called beurre manié (pronounced Burr mahn-ee-AY), the other starches are usually mixed with around twice as much cold water to form a slurry which is stirred into to the hot liquid being thickened.