Wet milling is used to process grain, typically corn, by separating various useful parts, and then using those parts in different ways. It differs from dry milling in a number of ways, but the the initial steps are the most different. The process steeps the grain in water, which may or may not be mixed with sulphur dioxide, and then continues on through the rest of milling steps. Wet milling creates more byproducts out of grain, but also uses more energy.
Once the corn arrives at the mill, it is first cleaned and then steeped in the water, which is where the wet milling process gets its name. The grain is allowed to sit in water for 20 to 30 hours at a temperature of approximately 122;° Fahrenheit (50;° Celsius). The steeping weakens the gluten bonds in the corn, which helps to separate the starch from the rest of the material.
Often, the mill adds sulphur dioxide to the wet milling process. Without sulphur dioxide, bacteria may begin to grow in the water, and thus contaminate the grain. Adding this ingredient helps to prevent the growth of bacteria, but is otherwise not essential to the wet milling process, and is avoided by organic producers.
After being steeped, the next step is for the grain to be ground coarsely. This is done to separate the germ from the starch, fiber, and gluten. Other types of processors may discard the germ and produce corn oil, but some mills may also be capable of producing corn oil as well.
When the mill has completed the coarse grinding, the part of the grain containing the starch, fiber, and gluten, known as the slurry, is ground finely. This step separates the fiber from the rest of the slurry. The suspension then passes over screens, which catch the fiber for use as animal feed. The starch and gluten, which are the only components left in the liquid suspension, are then sent on to the next step.
To separate the starch from the gluten, the mill puts the suspension through a centrifuge, which spins it at a high rate of speed. The gluten does not have as much density as the starch, and therefore is easier to spin out. The starch then gets washed to remove as many impurities as possible, resulting in a product that is 99.5 percent pure starch. It is sold as corn starch or made into other products, such as corn sweeteners. The gluten is dried and often sold as feed for animals.