Thickening sauces is a basic lesson in any cooking class, but there are many methods depending on what you’re working with and the result you’re hoping to achieve. There are many liquids that might require thickening, such as gravy and soup, custard and pudding, yogurt and ice cream, jam and preserves, or even dressing and sauce. You probably won’t use the same method to thicken a sweet dessert as you would for savory gravy, so it’s good to learn a few of the methods and thickening agents that can be used.

Making a Starch Slurry

Select your starch. Cornstarch is the most common to use for thickening, but you can also use potato starch, arrowroot flour, tapioca flour, or rice flour. When combined with liquids and heated, these starches swell and form a thickening gel. Flour isn’t recommended for this method because it has a strong flavor and doesn’t have the same thickening power as the other starches. Similarly, instant flour can be whisked directly into sauces without being mixed with water first, but it’s not recommended.
Starches are often used to thicken soups, gravies, fruit toppings, and savory or sweet sauces.

1. Measure your starch into a separate bowl. Use one tablespoon of starch for every cup of liquid you want to thicken.
2. Whisk in equal parts cold water. For every tablespoon of starch you added, add one tablespoon of cold water to the starch. Whisk until there are no lumps and the starch is fully incorporated.
3. Whisk the slurry into your sauce. Pour the starch slurry slowly into the sauce you want to thicken, whisking constantly to incorporate the slurry into the sauce.
4. Bring to a simmer. To release the starch molecules, you must heat the sauce to a simmer, otherwise the starch won’t thicken.
5. Season if necessary. Since you’ve diluted the sauce by adding some water and starch, taste it again after thickening to see if you need to adjust any of the herbs or spices.

Using Food Gums

Select your gum. A few of the most familiar food gums that are used as thickening agents are xanthan gum, agar, pectin, and guar gum. They are popular because they only require small quantities to thicken, and they don’t alter the color or taste of sauces.

Xanthan gum is a versatile thickener that can be used in most dressings and sauces, and will also act as a preservative.

Agar (sometimes referred to as agar agar) is often used commercially to thicken dairy-based products, and can be used as a gelatin substitute in fruit preserves and desserts. It comes powdered or in flakes.

Pectin is often used in jams and jellies, and fruit-based sweets, but it can also be used to thicken yogurt and dairy products.
Guar gum will thicken when it’s cold and can be added to baked goods to increase their fiber quantity. It’s most often used in salad dressings.

Combine guar gum or agar with liquids first. Both agar and guar gum need to be mixed with other liquids before being added to sauces or dressings. Agar gets mixed and heated with water first, and guar gum can be added to oils already called for in the dressing recipe.For agar flakes, use the ratio of one tablespoon of flakes per cup of liquid; for agar powder, use one teaspoon of powder per cup of liquid. Dissolve agar into four tablespoons of warm water in a saucepan. Bring to a boil on the stove, and boil for five to 10 minutes. Mix into sauce you want thickened.   To thicken dressings with guar gum, use only one-half teaspoon per two and a half cups of liquid. Whisk or blend guar gum with oil called for in your dressing recipe before incorporating other ingredients.

Add pectin or xanthan gum directly to sauces. In the last 15 minutes of cooking, pectin and xanthan gum can be added right into your sauce. Pectin must be brought to a boil for at least one minute to activate the gelling action. Xanthan gum will thicken without being boiled.  Add three-quarters of a tablespoon of pectin per one cup of savory sauce, or 2 tablespoons of pectin per cup of sugar used in a sweet sauce. Whisk vigorously as the pectin comes to a boil and while it’s boiling.
Using the weight of your liquids as a base, add between 0.1 and one percent xanthan gum depending on the desired consistency. Vigorously whisk or blend xanthan gum into your sauce.

Making Beurre Manié

1. Place equal parts flour and butter in a bowl. Beurre manié is French for kneaded butter, because you knead the butter and flour together. Using a fork or your fingers, knead the flour and butter together until a smooth paste or dough is formed.  You can use a food processor for larger quantities of beurre manié.  Beurre manié is an ideal way to thicken savory soups, gravies, and sauces.
2. Roll the dough into teaspoon-sized balls. These portions will be added one at a time to whatever you are thickening.
3. Add individual balls to simmering sauce.
4. Whisk one ball of butter dough into your sauce at a time. For every ball you add, allow the sauce to simmer for at least one minute, giving it time to thicken, before adding another ball.
5. Add balls of beurre manié like this until your sauce reaches the desired consistency.
6. Leftover balls of beurre manié can be stored in the freezer for future use. Be sure to thaw to room temperature before adding to a sauce.

Making a Roux

1. Choose a fat to use. A roux is another French word that describes the paste created by cooking fat with equal parts flour. The recommended fats are oil, butter, or drippings. A roux can be used to thicken gravies, savory sauces, or soups.
2. Place fat into a sauce pan over medium heat. Depending on how thick you want your sauce, use between one and three tablespoons of fat plus equal amounts of flour per cup of liquid. For a thinner sauce, use one tablespoon of fat and one tablespoon of flour per cup of liquid; for a medium-thick sauce, use two tablespoons each of fat and flour; for a thick sauce, use three tablespoons each of fat and flour.  Stir in equal parts flour to the sauce pan. Depending on how much fat you used, add the equivalent in flour to your butter or oil.
3. Stir while cooking. For a basic white roux for thickening, cook the flour and fat together for a few minutes, until they are fully incorporated and start to froth.
4. Remove from heat. When the roux is cooked, set it aside and let it cool for a few moments. A hot roux will separate if you add it to a sauce.
5. Whisk the cooled roux into your sauce. Bring the sauce to a simmer and cook for at least 20 minutes to cook off any remaining flour taste.
6. Adjust seasonings as needed. If any herb or spice tastes have been lost in the thickening process, add them now prior to serving.

Thickening With Egg Yolks

1. Crack an egg and separate the yolk from the white. Using eggs as a thickening agent works best with custards, puddings, and rich cream sauces.
2. Beat the egg yolk in a separate bowl.  As you whisk the egg, slowly ladle in small portions of your warm sauce (such as your Alfredo or pudding). This is called tempering the egg, which means that you slowly heat it so that you can add it to something hot without it immediately cooking and scrambling.
3. Add small amounts of liquid until you have a full cup. Once you’ve added enough liquid, continue whisking for a few seconds so that the egg is fully incorporated into the liquid.
4. Whisk the egg mixture back into your sauce. Bring the sauce to a boil and simmer until it has thickened.

Reducing Liquids to Thicken

1. Bring your sauce to a simmer. Don’t let it boil. This method works well with most sauces, because as a sauce heats up, the water will evaporate, leaving a thicker and more concentrated sauce behind.  Reducing a sauce will particularly concentrate sweet, sour, and salty flavors, but it may also boil off some of the herb and spice flavors, so taste the sauce as it reduces and be prepared to adjust the seasonings when it has fully reduced.
2. Stir occasionally to prevent burning. As the water evaporates and the sauce reduces in quantity, it will continue thickening. Depending on what you’re making, your recipe may tell you to reduce the sauce to half, a third, or even a quarter of its original volume.
3. Reduce until you achieve the desired consistency. If you aren’t working from a recipe, the rule of thumb is that a sauce is ready when it reaches nappé consistency, which basically means it will coat the back of a spoon without running off.

Adding Potato Flakes as a Thickener

1. Measure out a tablespoon of potato flakes for every cup of sauce. Potato flakes are pre-packaged dried mashed potatoes, and you can add them as a thickener to country-style sauces and rich gravies, stews, and soups. Avoid this method with clear sauces or sauces with delicate flavors.This is a quick-fix thickening method, so the potato flake proportions are more to taste than an exact measurement.
2. Gradually add potato flakes to your sauce. As your sauce simmers, add potato flakes a little at a time. Stir to incorporate, giving each addition time to thicken. Add more potato flakes as needed until the desired consistency is reached.  Adding foods like potatoes, pasta, or oats to a rich and savory sauce will also naturally thicken the sauce because of the starch present in the food.
3. Adjust the seasoning as necessary. Before serving, taste your sauce and adjust the herbs and spices if the potato flakes have altered the flavor of the sauce.

Source:  wikiHow