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Thickening agents, or thickeners, are substances which, when added to a mixture, increase its viscosity without substantially modifying its other properties, such as taste. They provide body, increase stability, and improve suspending action. Thickening agents are often food additives.

Food thickeners are frequently based on polysaccharides (starches or vegetable gums) or proteins (egg yolks, demi-glaces, or collagen). Common examples are agar, alginin, arrowroot, carageenan, collagen, cornstarch, fecula, furcellaran, gelatin, guar gum, katakuri, locust bean gum, pectin, rehan, roux, tapioca, and xanthan gum.

Flour is often used for thickening gravies, gumbos, and stews. It must be cooked in thoroughly to avoid the taint of uncooked flour. Roux, a mixture of flour and fat, made into a paste before the liquid is added, is used for gravies, sauces and stews. Cereal grains (oatmeal, couscous, farina, etc.) are used to thicken soups. Yogurt is popular in Eastern Europe and Middle East for thickening soups. Soups can also be thickened by adding grated starchy vegetables before cooking, though these will add their own flavour. Tomato puree also adds thickness as well as flavour. Egg yolks have rich flavor and offer a velvety smooth texture but can prove to be difficult to use. Pectin is used as a gelling agent for jams and jellies. Other thickeners used by cooks are nuts or glaces made of meat or fish.

For acidic foods, arrowroot is a better choice than cornstarch, which loses thickening potency when mixed with acids. If the food is to be frozen, tapioca or arrowroot are preferable over cornstarch, which becomes spongy when frozen.

When using a thickening agent, care must be taken not to overcook the food. Some starches lose their thickening quality when cooked for too long or at too high a temperature, and thickened food may burn more easily during cooking. As an alternative to adding more thickener, recipes may call for reduction of the food's water content by lengthy simmering. When cooking, it is generally better to add thickener cautiously; if over-thickened, more water may be added but loss of flavour and texture may result.

Many fuels used in incendiary weapons require thickening for increased performance. Aluminium salts of fatty acids are frequently used. Some formulations (e.g. Napalm-B) use polymeric thickeners, namely polystyrene. Thickened pyrophoric agent, a pyrophoric variant to napalm, is a triethylaluminium thickened with polyisobutylene.

• Cook's Thesaurus: Thickeners