Flour mills crush, grind or pound whole grains into smaller, powdery pieces that are called flour. Many different kinds of mills exist that operate using different mechanisms and powered by different sources.
The oldest method for milling flour was crushing grain using stones, as in a mortar and pestle. During the 1800s in the United States, large flour mills were built, powered by water wheels. These mills employed millstones that rotated against one another to crush grains into flour. Steam power, and then electric or gas power, replaced water-wheel technology, while iron or steel rollers were developed as more modern alternatives to stones. Impact mills employed specialized metal hammers to pound grain into flour rather than crushing it as in grist-style mills.
Today, roller mills are perhaps the most common mills used in industrial flour production. They rely on a process of crushing grain between two rapidly rotating rollers, then separating the ground grain into individual components, including bran, germ and endosperm. Finally, the ground material is filtered depending on the desired qualities of the flour.
Smaller home flour mills operate in the same way that larger industrial mills do. They usually run on electric power or can be operated manually. Electric home mills are quite easy to use and the operator simply puts grain in, turns the mill on, and very soon has fresh-milled flour.