What is the difference between cornstarch and flour in regards to thickening gravy?!

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You'll need two times the amount of cornstarch as flour to get the same thickness. Also, flour will sometimes leave an aftertaste that you won't get with cornstarch.

Cornstarch gives a much smoother gravy due to its fine granulation.

Besides giving a slightly different texture, there are different ways to add cornstarch and flour:

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.

Starches cannot just be directly added to a sauce. Doing so would cause dreadful lumps that will never incorporate into the surrounding liquid. They must be prepped first. Flour is almost always mixed with butter, (or some other form of fat), before being introduced into a sauce. Take an equal amount of flour and softened butter and knead them together until a pliable, but not melted paste is achieved and you have what the French call beurre manié. To employ beurre manié as a thickener, bring the sauce to a simmer, and whisk it in one piece at a time, waiting for the previous one to melt before adding the next. When all of the beurre manié is incorporated, bring the sauce to a near boil and simmer for three minutes.

If you were to cook the flour and butter together first, then you would have a roux. Melt the butter in a skillet or saucepan over low to medium heat; add an equal amount of flour and cook, stirring constantly until the desired degree of doneness. The longer you cook roux the darker it will become. If you do not wish to darken your sauce but merely want to cook out the raw, floury taste of the roux, cook for just a few minutes. You can add the roux to the sauce or vice versa but they must be at different starting temperatures, (at the very least, one room temperature and one hot). You must also gradually add one to the other and whisk incessantly. All of these steps help prevent lumping.

Employing a roux is the premier method for making gravy for your chicken or turkey. After the bird has roasted, add flour to the drippings in the roasting pan, cook for a few minutes, and then gradually whisk in chicken stock. Bring to a near boil, simmer to the desired consistency, and season with salt and pepper.

Cornstarch and arrowroot will thicken more efficiently than flour since they contain no protein. They have 50 to 100% more thickening power than flour and thus, less of them is needed. They also thicken at a somewhat lower temperature and do not need to be pre-cooked, like roux. However, they do need to be dissolved in fluid first. Mix the cornstarch or arrowroot in just enough cold water to form the consistency of heavy cream. This is called a slurry. Add it to your simmering sauce, bring to a near boil and then simmer for only a minute or two. Extended cooking will reverse the thickening process.

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