The Science of Thickening Agents

The goal of thickening a soup or sauce is to add viscosity, texture, and mouth-feel, helping the food to linger on the tongue and taste buds. Thin sauces have a more intense and direct flavor impact but will not have as lasting an effect on the palate. Adding a starch thickener helps the sauce cling to the tongue but is can block flavors so they may require more seasoning or flavoring.

There are numerous ways to thicken and enrich sauces, soups, stews, and other dishes. They are as simple and crude as a whitewash made with flour and water, as light as a slurry made of arrowroot, or as complex as a dark brown roux. Emulsions of fats and liquids are used in hollandaise and mayonnaise sauces. Contemporary trends have brought the use of refined starches and even foams into the mainstream of culinary arts. Thickening agents fall into one of the following areas.

Suspension – Pureed foods especially vegetables, fruits, and herbs.

Dispersion – Starches, pectin, and gelatin work by swelling and absorbing liquids to create viscosity in sauces, jams, and aspic.

Emulsion – Two or more normally unmixable liquids including fats and oils, combined with emulsifiers of cream, egg yolk (which contains lecithin), ground herbs, and spices. Emulsion sauces include hollandaise, béarnaise, mayonnaise, and beurre blanc.

Foam – Incorporating air into cream or other products, sometimes with the aid of leavening agents including yeast (beer, sparkling wine), creates thick textures. Foams tend to be temporary unless stabilizers are incorporated into them, or as in the case of bread or cakes, when then item is baked.

Thickeners, Liaisons, and Emulsifiers

Reduction – The simplest type of a sauce is derived from pan juices of roasted or sautéed proteins including meat, poultry or fish, along with the addition of a prepared stock, wine, and other suitable liquids that are reduced to concentrate and enhance flavors. Many of the classic European sauces involve various types of stocks (veal, chicken, lamb, poultry, game, fish, and seafood), and wines. Because meat contains gelatin, a natural thickening agent, it has a savory richness that lingers on the tongue. A reduction of a stock, called a glace, is prepared by cooking down and concentrating its flavor which also increases the viscosity. Sauces prepared in this manner will have an intense flavor unobstructed by thickening agents. They are also prohibitively expensive and are therefore not practical in a casual dining operation. A glace though can be useful as a natural enhancement to enrich weak sauces.   

Flour as a Thickener

Wheat Flour- The most common thickening agent used in kitchens for hundreds of years, flour is added in various ways but is best when combined with fat to prevent lumping in the sauce. Some refined flours, including Wondra, can be added directly to a liquid without causing this problem. It is also used as a thickener in a stew through dredging the meat in flour before the searing process. Flour serves two purposes in this situation: 1) the meat will retain more juices and therefore have a better mouth feel, 2) when liquid is added, the flour granules will swell creating a viscous sauce.

Whitewash – The most basic type of thickening agent, whitewash is flour blended with water to make a paste. The flavor can be quite crude and raw in taste. This is rarely done in professional kitchens today.

Roux – Equal parts by weight of oil (vegetable, peanut, soy, etc.) or fat (butter, lard, bacon fat, meat or poultry fat) and flour, a roux is the most common type of thickening agent found in professional kitchens. Depending how long it is cooked and its intended use, roux can be white, blond, brown, or even a chocolate color.

Lighter roux, like white and blond, is used in white sauces, Béchamel and Velouté respectively; and darker roux is used in brown sauces and ethnic cuisines including Creole and Cajun cooking. Remember that prolonged heat will cause flour grains to close up and therefore not swell as much when dispersed in liquid, so the more a roux is cooked the less thickening power it possesses.

Brown flour, also called a dry roux, is used in Cajun and Caribbean cooking as a thickening agent. It is prepared either in a cast iron skillet on the stove with continuous stirring or placed on a sheet pan in a 350°F/175˚C oven for 1 hour and stirred about every 15 minutes. The flour is added directly to the liquid or combined with fat to create a roux. It is shelf stable and can be combined with fats to create an instant brown roux.

Beurre manié – Also called kneaded butter, beurre manié is equal parts softened butter and flour that are worked together into a smooth paste. It is used at the last minute to adjust the consistency of sauces, soups, and stews. Add a little at a time and allow to cook so the flour can swell and absorb liquid. Since the flour is raw it is a good idea to simmer it a few minutes to cook out the taste.

Bread Crumbs – Cooked wheat products including bread, crackers, and cookies, crumbled or pulverized, are economical ways thicken soups, sauces and stews. This practice has been documented since ancient Roman times and is a useful purpose for leftover stale bread.

Preparing a Roux

Types of Roux

White or Pale Roux
Uses: Cream soup, Béchamel sauce
Combine the flour in a sauce pan with the fat and stir while cooking for about 5 minutes being careful not to let the roux color.

Blond Roux
Uses: Velouté sauce, soups, white stews
Combine the flour in a sauce pan with the fat and stir while cooking for about 10 minutes to give it a gold color.

Brown Roux
Uses: Espagnole sauce, brown stews and braises
Combine the flour in a sauce pan with the fat and stir while cooking for at least 30 minutes to give it a rich brown color.

Alternately begin preparation on the stove, cover it with a lid and stransfer to a 350˚F/175˚C oven. Stir occasionally until a rich brown color is achieved.
Brown roux will lose about ½ to ⅔ of its thickening power compared to a white or blond roux

Black Roux
Uses: Gumbos and Cajun stews
Combine the flour in a sauce pan with the fat and stir while cooking for at least 45 minutes to give it a deep black color. Use oil because it has a higher smoke point than butter and will prevent scorching.

How Much Roux Do I Need?

Although there are basic ratios for the amount of liquid to roux is needed to thicken a liquid, be mindful of variables that produce different results.  Some of these include:
Flour varies in starch, gluten, and moisture content based on its region, the growing season, and harvesting climate.  More starch and less protein are desirable for a roux so avoid high protein bread flour. The age of the flour affects the flours ability to absorb and swell.

Heat and the amount of cooking time a roux is cooked affects its thickening power. Brown roux will have about half the thickening power of a blond roux because the grains of flour have seized up and become closed during prolonged cooking. Add more flour to the roux as it is cooking to counteract this you affect. Other factors that affect the power of thickening agents include salts, sugars, and acids

Adding Roux to a Sauce

Extremely high temperatures of the liquid or the roux results in the sauce lumping. To avoid this problem have one ingredient cooler than the other one. Calculate the ratio of roux accurately; to avoid adding too much roux and creating an overly thick sauce. Allow the roux to cook for about 20 minutes to achieve its full thickening potential. Be patient and allow time for the starch to swell and absorb the liquid.
Cook properly to remove the raw flour taste. For a lighter sauce, use less roux and allow it to thicken by reduction.



Refined Vegetable Starches

Refined vegetable starches are simple thickeners that require little time to prepare and are neutral in taste. They are combined with water or other liquids to create a slurry before adding to hot liquids. This prevents the starches from clumping. Prolonged cooking can weaken the power of these starches so they are usually added at the end of the cooking process.

Arrowroot - The root of a perennial plant grown in rainforest climates, arrowroot has a neutral taste that is preferred by chefs over flour or cornstarch, and thickens at a lower temperature than either of the others. It is not affected by acidic ingredients or by freezing. But it is not recommended in combination with dairy products because it produces a slimy texture. Overheating tends to break down arrowroot's thickening property. Substitute two teaspoons of arrowroot for one tablespoon of cornstarch, or one teaspoon of arrowroot for one tablespoon of wheat flour.

Cornstarch – Refined from the endosperm of corn, cornstarch produces a clear mixture used for sauces, glazes, pie fillings, and puddings. It tends to weep and clump up when held in a steam table and should not be allowed to freeze. 1 tablespoon of cornstarch should be substituted for 2 tablespoons of flour.

Kuzu (Kudzu)Root - A Japanese plant now found in the American south, the kuzu root is refined and used similar to other refined starches. About 1 3/4 Tb replaces 1 Tb. of cornstarch.

Potato Starch – A starch that acts similar to arrowroot and results in a translucent appearance.

Rice Flour - A gluten-free alternative, rice flour is a more expensive thickener and possesses slightly weaker thickening power than arrowroot. It is used similar to other refined starches.

Tapioca – From a plant known by several names as manioc, cassava, or yuca, tapioca is native to South America but now grown all over the world. It provides a translucent viscosity.

Preparing a Slurry

  1. Combine equal parts by volume of refined starch and cool water
  2. Make sure the liquid to be thickened is at a simmer. To prevent lumps, stir continuously while whisking in the slurry
  3. Return to a simmer and cook for a few minutes until the mixture begins to thicken
  4. Adjust consistency with more starch if necessary

Dairy Products

Butter – Butter is an emulsion in itself and is swirled into sauces at the end of the cooking process; a term known as Monter au Beurre.

Cream – Added to enrich a sauce, cream as a thickener must be reduced to provide viscosity to liquids. It works best when reduced by about one-third of its volume to concentrate fat globules to about 55% of the total volume.

Egg Yolks- Both eggs and egg yolks thicken a variety of liquids to produce custards and quiches. Lecithin, found in egg yolks, helps to thicken and emulsify sauces. Egg yolks are sensitive to heat and must not be boiled otherwise they will curdle and thin out the liquid.

Liaisons of Egg Yolks and Cream – A liaison is used in certain classic soups, sauces, and stews as a thickening agent. Because the yolks congeal at a low temperature, care must be taken never to boil the mixture, or it will curdle and take on a grainy texture and appearance.

  • Protein
  • Blood or Liver (Foie Gras)
  • Vegetables and Grains
  • Cooked Legumes  
  • Cooked Rice  
  • Pureed Potatoes
  • Ground Nuts
  • Pureed Fruits
  • Pureed Vegetables
  • Seeds