Cornstarch usually is refined by a process called wet-milling. This refining process breaks down the corn's kernel then the endosperm to result in pure starch. It's a complex process that uses specialized equipment to extract and dry the starch. This product isn't a good candidate for a homemade version. For cooking, cornstarch has advantages over flour as a thickener; it becomes smoother and colorless. If you prefer to avoid genetically modified corn, look for cornstarch labeled non-GMO or substitute arrowroot.
Dent corn, also called "field corn," contains hard and soft starch. This is the corn variety usually used for making cornstarch. Although cornstarch is called "corn flour" in the U.K, Australia and New Zealand, in the U.S., corn flour is a separate product from cornstarch. Cornstarch is used extensively in processed foods and in cooking and baking. Cornstarch also has many utilitarian and industrial applications. On the green front, cornstarch is used to create biodegradable eating utensils.
When the corn arrives at the processing plant, processors inspect it, husk it, separate the kernels from the cobs and wash them. Processors may store the kernels and clean them again. The kernels are the seeds of the corn plant. They're made up of the hull, the germ and the endosperm. The hull is the outer part. The germ contains the oil often refined into corn oil or used in livestock feed. The endosperm contains protein and starch to provide energy for the seed to germinate.
Beginning of Refinement
The corn kernels soak in huge tanks of hot water with acid and sulfur dioxide as preservatives for 20 to 48 hours. The steeping causes fermentation and softens the kernels. Milling crushes the grain, turning it into a paste. The hulls and endosperm are heavier than the the corn germ, so the germ gets skimmed off the top. The wet mass flows through a screen that removes the corn hulls. All that remains is the endosperm in the form of a slurry. High-speed centrifuges or hydrocyclones and washing remove the lighter protein from the starch, resulting in pure starch.
Box and Dry
Cornstarch manufacturers wash the resulting pure starch then dry it. The drying process generally involves removing moisture in a peeler centrifuge followed by flash drying, which quickly removes all the moisture. A peeler centrifuge uses centrifugal force, rotating filtration baskets to force out liquid. Machines grind the dry starch into a super-fine powder or to the preferred specifications of the buyer. In some cases, cornstarch is modified at this point, depending on its end use. For food use, manufacturers package cornstarch in packaging that helps protect it from moisture.