“There is something comforting about the fact that if you melt butter and add flour, then hot stock, it will get thick!”– Nora Ephron
One the most basic skills in the kitchen is knowing how to thicken something, whether that be soup, stew, or sauce. Luckily there are several different ways that can accomplish this feat, each with its own methodology and preferred application.
One of the most traditional ways to thicken things up is to make a roux. A roux is simply equal parts flour and fat, by weight, that have been mixed together and cooked to form a paste. The cooking helps the fat coat the starch granules from the flour and prevents them from lumping together. Cooking the roux for different amounts of time produces different shades of roux. A white roux is cooked only long enough for the starch and fat to come together. It is ideal for white sauces and soups. A blonde roux is allowed to cook for a slightly longer time and will develop a darker color. This roux is used with ivory colored foods or where a richer flavor is desired. Lastly a brown/dark roux is allowed to cook until the roux becomes a dark, almost brick color, shade of red. This roux will impart the most flavor but also has the least thickening power. Making a dark roux it requires a lot of attention as it can go from almost done to burnt very easily. When your roux done you can either add hot roux to a cold liquid or cold roux into a hot liquid. When adding the roux it is important to keep stirring and allow the liquid to almost come to a boil. Stirring avoids lumps as well as incorporating the roux while bringing it up to a near boil cooks out any starchy flavor and allows the roux to work. Flour and butter can also be mixed and kept raw and added at the end as a finishing technique or quick thickener. This is called a beurre manie.
Another commonly used method to thicken things is with the use of cornstarch. All you need is to mix the cornstarch with water until the two mix, then just slowly stir it into hot (simmering) liquid. Unlike roux, cornstarch will have a more immediate effect. It gives liquids a glossy sheen that may or may not be desired. Cornstarch also has the benefit of having twice the thickening power of flour. However, liquids thickened with cornstarch are less stable as it can lose its thickening effects over prolonged heating. Also, products that have been thickened with cornstarch should not be reheated.
An alternative to cornstarch and flour as a thickener is arrowroot. Arrowroot is derived from the roots of several tropical plants and is similar in texture and appearance to cornstarch and is used it the very same way. Arrowroot has slightly more thickening power than cornstarch, is not affected by acidic ingredients or freezing, thickens at a lower temperature than flour or cornstarch, and has a more neutral flavor. Arrowroot however, does not work well with dairy products, producing a slimy texture when it does. Arrowroot also has a tendency to break down when overheated and is more expensive.
One final note about thickening. When thickening keep in mind that if something gets too thick you can always add more liquid and seasoning to adjust. You may end up with slightly more product that you had originally intended, but that is better than serving a gravy when you just wanted a light sauce.