Starch is a complex carbohydrate found mainly in the fruit, seeds, rhizomes, and tubers of plants, and is the major source of energy in these foods. Chemically, it can be thought of as a "polymer of sugars" and can therefore be broken down by enzymes in the body so that these sugars can provide energy. (This is why chewing a piece of bread for a while can make the taste in your mouth change from neutral to sweet - the amylase in saliva breaks down the long starch chains into substances such as glucose and fructose.)
The major sources for the commercial production of starch, and for its general consumption by humans and animals worldwide, are rice, wheat, corn, and potatoes. Incidentally, fresh chestnuts have twice as much starch weight for weight as potato. As the chestnut ripens, some of its starch is gradually converted into sugars. Cooked foods containing starches include boiled rice, various forms of bread and noodles (including pasta).
In a commercial form it is a white powder and, depending on the source, may be tasteless and odourless. Microscopically, starch grains are fine crystals or lumps; the precise form of these grains varies within the plant kingdom. Today, commercial starches include cornflour, arrowroot, potato starch, sago and tapioca. Historically, they also included Florida arrowroot. Starch in its basic refined form is used in cooking to thicken foods such as sauces. It is also used in the form of flakes, sticks, and pearls (tapioca and sago).
As an additive for food processing, arrowroot and tapioca are commonly used. Commonly used starches around the world are: arracacha, buckwheat, banana, barley, cassava, kudzu, oca, sago, sorghum, regular household potatoes, sweet potato, taro and yams. Edible beans, such as lentils and peas, are also rich in starch.
When a starch is pre-cooked, it can then be used to thicken cold foods. This is referred to as a pregelatinized starch. Otherwise, starch requires heat to thicken, or "gelatinize". The actual temperature needed depends on the type of starch.