Corn starch is a major use for corn grown in America. It has dozens of applications, ranging from paper and textile production to a thickening agent in cooking and the manufacture of adhesives. Its versatility derives from its chemical structure because although corn starch may look simple at first glance, that simplicity conceals some fascinating chemistry.
Starch is a polymer of glucose sugar molecules strung together into a long chain. If the chain is branched, the starch molecule is called an amylopectin, whereas if it's straight it's called amylose. Each glucose molecule has hydroxide groups that can form weak bonds with water or other starch molecules. That's how starch polymers associate to form small granules that dissolve neither in water nor most other solvents. If you heat water with starch granules, however, the granules gradually fall apart, and the starch polymers mix with the water to form a thick paste.
Corn starch is generally about 27 percent amylose polymer with the remainder being amylopectin. This ratio is genetically determined for the corn plant, so it varies little from one batch to the next. Starches from other species like potatoes and tapioca, by contrast, contain the same polymers but typically feature a different ratio of amylose to amylopectin. Plant geneticists have bred corn plants that produce starch with a higher or lower amylose-to-amylopectin ratio, however, and these starches find some niche uses for specific applications.
Corn kernels contain many other kinds of molecules besides starch, and although the milling process is designed to isolate the starch, traces of these may linger on as impurities in the final product. The other chemicals found in a corn kernel include fiber, gluten proteins and oils and fats. Usually, the oil and gluten protein removed during processing is sold separately as cooking oil and gluten meal.
Millers begin by cleaning the corn to remove any debris like seeds or pieces of cob. Next they mix it with warm water and a low concentration of sulfur dioxide, which reacts with water to form weak sulfurous acid, preventing fermentation and extracting water-soluble components like proteins from the corn kernels. The softened corn kernels are agitated in water to break them open, then they are centrifuged to remove the germ or plant embryo inside the kernel, leaving behind a mixture of kernel particles, proteins and starch. The kernel particles are removed through a filter, while spinning the mixture at high speeds in a centrifuge removes the proteins. Finally, the starch slurry is washed again to remove any remaining water-soluble components then dried and sold as pure starch.