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One of the most remarkable properties of xanthan gum is its capability of producing a large increase in the viscosity of a liquid by adding a very small quantity of gum of the order of one percent. In most foods it is used at 0.5% or as low as 0.05%. The viscosity of xanthan gum solutions decreases with higher shear rates, this is called pseudoplasticity. Foods need high viscosity a low shear rates to be stable. But when consumed they cannot seem to be thick and heavy in the mouth. Due to the pseudoplastic properties of xanthan gum it can seem thin in the mouth (fairly high rates of shear) but still have good stabilization properties. Unlike other gums it is very stable under a wide range of temperatures and pH, and is accepted as a safe food additive in USA and Europe, with E number E415.

http://en.wikipedia.org/wiki/Xanthan_gum