Corn wet-milling

The corn wet-milling industry has been a primary component of American manufacturing for more than 150 years. Corn refiners established the process of separating corn kernels into their component parts to produce a variety of value-added products. The four main component such as oil, protein, starch, and fiber are the primary product from the corn wet-milling process. The Associated Manufacturers of Products from Corn was formed in 1913 when the group of corn refining companies’ industry successfully grew. ==Description==

Corn wet-milling is a process where components of corn kernels are extracted to produce a highly purified product. Most of the products from this process are valuable and mainly required by the food industry. Through this process, every part of the corn is useful to produce the quality ingredients. The characteristics of this process are based on physical separation of components, mostly by weight and size. Water is needed as it is a wet process and it works as separation/carrier agents in washing steps. Therefore, this process can be considered as having high capital cost. The only chemical use in this process is aqueous sulfur dioxide solution, which is used in the steeping process. The corn is soaked in this solution to soften the kernel so that the oil in the germ will not contaminate other products and is easy to separate. == Process steps ==

Cleaning As per the standards of the U.S. Department of Agriculture, Grade 5 corn is usually used for wet-milling. Harvested corn has to be cleaned before it is milled. Dockage tester with appropriate sieve number is used to removes particles other than the required grain like cob pieces, foreign seeds, metal pieces, leaves, dirt and the percentage of dockage contained can be calculated. The cleaned corn is then analyzed for its composition using a near-infrared spectrometer (NIRS). The compositional analysis of yellow dent corn carried out at the Center for Crops Utilization Research, Iowa State University, is recorded in the table below. Steeping In this process, the corn is hydrated in order to loosen starch granules from the protein matrix while leaving the germ resilient to milling. This process reduces the germ density and softens the kernel, making the milling easy. Chemicals like sulphur dioxide and lactic acid are also added to the water. Lactic acid breaks down the endosperm protein matrix and helps in better separation of starch. It also lowers the pH, preventing growth of microbes. SO2 reacts with the disulphide bonds and weakens the matrix, allowing starch granules to separate out cleanly. It also serves as an anti-microbial. At the end of steeping, the protein matrix is weakened, endosperm proteins are solubilized and some soluble solids diffuse out into the steepwater. The clean corn is steeped in large tanks with water at 125-130˚F containing lactic acid and sulphur dioxide for nearly 40 hours. The steepwater is then drained using appropriately sized mesh screens and concentrated using multiple effect evaporators. Water is removed from the millstarch in a thickener before moving it into a separator. Centrifugal forces are applied to separate starch and gluten which differ in density. The heavier starch slurry is then washed multiple times in hydrocyclones with fresh water. The starch stream typically has 90% starch and the gluten stream consists of 60% protein. A typical solid yield (on db) data for yellow dent corn is shown in the table below. == Primary products ==

The wet-milling process will have five major products: steep water solids, germ, fiber, starch, and gluten. However, the co-product from this process will produce corn oil, corn gluten meal, corn germ meal, corn gluten and feed steep water. The average of one bushel of corn generally will have about 32 lb of starch or 33 lb sweeteners or 2.5 gallons of fuel ethanol and 11.4 lb gluten feed and 3 lb gluten meal and 1.6 lb corn oil. == Research ==

Though corn wet-milling has been used for years to produce food, animal feed, and fuel, researchers continue seeking efficiencies. For example, one study showed that steeping time can be decreased from 40 hours to 6–8 hours, if enzymes like protease are added and if milling is done in two stages. This method eliminates sulphur dioxide and yields the same amount as the conventional process. Another study, showed that adding cellulase and an enzyme to degrade phytic acid reduces steeping time. In the conventional process, the corn's phytic acid largely ends up in the corn steep liquor. Yet another study tried decreasing the initial moisture content and increasing the drying air temperature but obtained reduced yields because low water content impeded separating protein from starch. == See also ==