Root startches from potatoes are used to thicken soups, gravies and sauces.
Chemical and physical changes occur in starches during heating in liquids. Root starches from potato, tapioca and arrowroot provide consistency and texture to foods and are used to thicken gravies, soups, sauces, custards, pie fillings and puddings. Starches are composed of starch granules, and the granule size, the shape and the relative amounts of the two main starch granule carbohydrates -- amylose and amylopectin -- have functional roles in starch thickening properties.
Potato starch is composed of potato starch granules, whereas potato flour is made from the whole potato. Potato starch granules are the largest of the food starch granules, and potato starch begins to gelatinize at a lower temperature than starches with a finer granular structure. Because of the large size of the amylose molecules in potato starch, the molecules cannot associate with each other to form a rigid gel during cooling. Therefore, potato starch is used in soups, gravies and sauces in which some thickening is desirable but gel formation is undesirable as the product cools.
Tapioca starch is made from the roots of the cassava plant, or Manihot esculenta, which is native to Brazil. Tapioca starch granules, similar to potato starch granules, gelatinize at a relatively low temperature and do not form rigid gels during cooling. Tapioca typically is used to thicken custards, puddings and fruit pie fillings. In addition to a powder form, tapioca is available as a quick-cooking, pearl variety, often used in two-crust fruit pies. The pearl variety of tapioca does not always completely dissolve during cooking and may be seen in the pie filling as small granular dots.