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One of the most frequent tasks in home cooking is thickening a sauce. There are two common ways to do it: reducing the sauce, or adding a thickener.

Reducing a sauce is a technique true to its name: it involves boiling (usually a simmer or low boil) the liquid until about half of the liquid has evaporated off. What remains, if the liquid contains anything other than water, will be thickener than it started. It will also have more concentrated flavors. Tomato sauces are commonly reduced; the “meat” of the tomato does a good job thickening up the liquid. But because you are applying heat, the other ingredients are also cooking while it’s reducing. That will tend to break down the texture of the other sauce ingredients, which may or may not be a goal. Cooking also changes the flavor of the ingredients — also sometimes desirable, but sometimes not.

The alternative to reducing a sauce is adding a thickening agent. The commercial food industry uses a variety of chemicals to thicken liquids, but at home we tend to rely on starches to do the job for us. Starches are carbohydrates that swell up when heated and then latch on to neighboring water molecules; then when the liquid cools it appears to have thickened.

There are a couple of commonly-used thickeners in home kitchens, and a couple of less-common ones that are worth knowing about and keeping handy.

Corn starch is perhaps the most common thickener used at home: it’s easy and inexpensive to buy and very reliable. About one tablespoon of corn starch can thicken one cup of liquid to a medium-level thickness; add a little more to make it thicker, a little less for thinner. There are a couple of tricks with using corn starch though: if you add it directly to a hot liquid it will clump up rather than dissolve, and then it won’t work its thickening magic. The best practice for adding corn starch is to dissolve it in a small amount of cold water first to create a “slurry,” then pour the slurry into the hot liquid. Also, once you’ve added the corn starch and brought the liquid up to near-boiling, don’t cook it for more than five minutes; after that point, the corn starch molecules will begin to break down and lose their thickening power. Because of this, corn starch is usually added near the end of a recipe.

Potato starch (sometimes called “potato flour”) is common in recipes in Europe and sometimes Asia, but is less common in the United States. It works similarly to corn starch, though it thickens at a lower temperature (about 175 degrees F) and it thickens to a clear gel (corn starch can remain a bit milky). You can find potato starch online, at Asian grocery stores, and at some high-end stores like Whole Foods — though not reliably at neighborhood supermarkets.

Arrowroot is another less-common thickener here, and an under-rated one. It’s about twice as strong as corn starch, but can be cooked for much longer without breaking down. Used in excess, however, it thickens a liquid up into a somewhat “filmy” substance. Arrowroot can be bought online, at high-end grocery stores, and at some supermarkets.

Some wheat-based flours can be used as thickeners; though the lower the protein content the better. Cake flour works well; all-purpose flour is ok; bread flour and whole-wheat flour should never be used. The most common use of wheat-based flour as a thickener is to make a “roux,” which is a mixture of equal parts flour and butter. A roux is made by melting the butter in a saucepan, stirring in the flour, and then cooking it for a short amount of time before adding in the liquid. Cooked for a short amount of time, it’s called a “white roux”; cooked for a long time, a “brown roux.” Cooking reduces the starchy flavor and replaces it with better flavors by browning the proteins in the flour and the milk. The longer you cook it, the better the flavor — but the less thickening power it has; in other words, a white roux thickens better than a brown roux. Even in the case of a brown roux, the cooking time is pretty short — just a few minutes.

Wondra is a special product that you can buy online and in some grocery stores (it comes in a telltale blue canister). It is low-protein flour that has been pre-cooked, cooled, and then ground into a fine powder. Because it has been pre-processed this way, it can be used as a last-minute, quick fix thickener for sauces that you can just sprinkle directly into a hot liquid (you don’t need to dissolve it in cold water first like corn starch). Wondra is about twice as strong a thickening agent as corn starch: use 1/2 a tablespoon for every cup of liquid to achieve a medium thickness.

There are plenty of other thickeners out there, including powdered gelatin and tapioca starch. They are worth picking up only if you have a recipe that calls for them specifically. Also: boiling pasta causes it to shed some of its starch, which ends up in the water. Some recipes call for reserving the water that you boiled the pasta in because it can be used to thicken a pasta sauce too.

These starchy powders are also often used as undercoating for breaded food, particularly meat, because they absorb liquid and nicely dry out the outside to assist with browning.

Thickeners are also used sometimes with soups.

The go-to, workhorse thickeners that you should definitely stock in your kitchen are corn starch and all-purpose flour. They are by far the most commonly used ones and will give you what you need almost all of the time. Arrowroot is a nice to have, as is Wondra.

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