Xanthan gum  (E 415) is a polysaccharide secreted by the bacterium Xanthomonas campestris, used as a food additive and rheology modifier, commonly used as a food thickening agent (in salad dressings, for example) and a ster (in cosmetic products, for example, to prevent ingredients from separating). It is composed of pentasaccharide repeat units, comprising glucose, mannose, and glucuronic acid in the molar ratio 2:2:1. It is produced by the fermentation of glucose, sucrose, or lactose.

One of the most remarkable properties of xanthan gum is its ability to produce a large increase in the viscosity of a liquid by adding a very small quantity of gum, on the order of one percent. In most foods, it is used at 0.5%, and can be used in lower concentrations. The viscosity of xanthan gum solutions decreases with higher shear rates; this is called shear thinning or pseudoplasticity. This means that a product subjected to shear, whether from mixing, shaking or even chewing, will thin out, but, once the shear forces are removed, the food will thicken back up.

In foods, xanthan gum is most often found in salad dressings and sauces. It helps to prevent oil separation by stabilizing the emulsion, although it is not an emulsifier. Xanthan gum also helps suspend solid particles, such as spices. Also used in frozen foods and beverages, xanthan gum helps create the pleasant texture in many ice creams, along with guar gum and locust bean gum. Xanthan gum also helps thicken commercial egg substitutes made from egg whites, to replace the fat and emulsifiers found in yolks. It is also a preferred method of thickening liquids for those with swallowing disorders, since it does not change the color or flavor of foods or beverages at typical use levels.

Xanthan gum is also used in gluten-free baking. Since the gluten found in wheat must be omitted, xanthan gum is used to give the dough or batter a "stickiness" that would otherwise be achieved with the gluten.

Amounts used

The greater the ratio of xanthan gum added to a liquid, the thicker the liquid will become. An emulsion can be formed with as little as 0.1% (by weight). Increasing the amount of gum gives a thicker, more stable emulsion up to 1% xanthan gum. A teaspoon of xanthan gum weighs about 2.5 grams and brings one cup (250 ml) of water to a 1% concentration.

To make a foam, 0.2–0.8% xanthan gum is typically used. Larger amounts result in larger bubbles and denser foam. Egg white powder (0.2–2.0%) with 0.1–0.4% xanthan gum yields bubbles similar to soap bubbles.

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