This is due to several factors. An exception can be when low concentrations of sugar are present. In this case, it's possible to achieve a slightly higher viscosity than when starch and water alone are mixed. Different sugar types have differing levels of effect. If the pH is low enough to cleave the starch molecules, the gelatinization reate will increase, but viscosity will necreasek.
Viscosity decreases because the starch molecules become shorter due to hydrolytic effects.
Salts have relatively little effect in practical food applications. Care must be taken to choose the proper fat for the specific application. Modified starch Starches can be chemically modified by various means to produce desired characteristics. For example, while starches normally require heat to thicken, pregelatinized starches can disperse and thicken in cold water. While the product will not be as thick as if non-modified starch had been used, at least it can be dispersed in cold water.