Cornstarch, arrowroot starch, tapioca starch and wheat flour are substitutes for potato starch. During Passover, ground Passover matzo can be used to replace potato starch.
Potato starch is a white powder that has a similar consistency to cornstarch. It has a light potato flavor in its natural form but cannot be detected in the final product. Its scientific name is solanum tuberosum. Potato cells have starch grains called leucoplasts. The starch is removed when the potatoes are crushed; the leucoplasts are released from the cells that are destroyed in the process. The starch is washed out and then dried to a powder.
Potato starch can be used in the same fashion as cornstarch to thicken gravies or soups. It is not a grain-based product and as such is an option for gluten-free diets and for use during Passover.
There are a number of other starches that can be used instead of potato starch. Arrowroot starch is similar to potato starch, in that it has a neutral flavor. However, it is not good for dairy-based sauces.
Tapioca starch can be used instead of potato starch, particularly for thickening sweet fillings. Tapioca starch dissolves completely, thickens at a lower temperature than some other starches, and remains stable when frozen where other starches separate or change consistency. However, tapioca starch gives a shine to the foods in which it is mixed which may not be a desired outcome for soups and sauces.