Finely ground cereal grain or other starchy portions of plants. Flour, especially wheat flour, is a basic ingredient of baked goods (seebaking). In the production of refined flour, the starchy endosperm is separated from the other parts of the kernel by milling through rollers. In the production of whole-wheat flour, all parts of the kernel are used. Following milling, the particles of endosperm (called semolina) are ground to flour and often bleached to imitate natural aging. Flour grades are based on the residual amount of branny particles. When flour is mixed with water to make dough, its protein content is converted to gluten, an elastic substance that forms a continuous network throughout the dough and is capable of retaining gas, thus causing the baked product to expand, or rise.