Typically, sauces are classified by how they are thickened. Most sauces that are made at home are thickened one of these four ways.
Cornstarch
Cornstarch is typically used to thicken fruit or desert sauces. It gives them a shiny, clear appearance. To use cornstarch, combine it with cold liquid. Mix the combination with the hot sauce. The cornstarch will not allow the sauce to thicken until it is almost boiling. The sauce will thicken quickly when it is cooled.
Egg
Egg proteins are also used as a thickener, and most of this is concentrated in the yolk. Yolk is very temperamental so it has to be heated carefully over heat. Heating it too quickly will give you sauce with scrambled eggs in it.
These sauces are typically made in a double boiler after the egg yolk has been tempered. To temper egg yolks, simply add a little bit of hot base into the egg yolks. After that, add the egg mixture back into the base and heat gently.
Reduction
A reduction is a sauce thickened by boiling away liquid. As the liquid evaporates, the sauce’s flavor intensifies and becomes stronger. This is a great approach to making sauce from roast broth or chicken drippings.
Roux
This is the most step-heavy method of thickening a sauce. A roux is an equal mix of flour and fat. The flour is cooked to remove the gritty flavor and prevent lumps.
White sauce is the classic roux-thickened sauce and is the base for many other sauces.
Medium white sauces are used to make scalloped and cream dishes.
To make a roux, use a wooden spoon to cook and stir flour, fat and seasonings in a small, heavy sauce pan over low heat until they are evenly combined and free of lumps. Slowly add milk to the roux. Stir constantly with a whisk to evenly blend the milk and roux. Stir the sauce over medium heat until the mixture bubbles across the entire surface. Cook one minute more to completely cook the flour in the sauce.
Successful Sauces
~ If lumps form, whisk vigorously to beat them out.
~ Cook sauces over low to medium heat. Do not cook them longer than the specified time.
~ Do not allow skin to form on a sauce. Once the skin forms, the sauce will get lumpy.