Corn wet-milling is a process of breaking corn kernels into their component parts: corn oil, protein, corn starch, and fiber. It uses water and a series of steps to separate the parts to be used for various products. [1]
Corn wet-milling is a process of breaking corn kernels into their component parts: corn oil, protein, corn starch, and fiber. It uses water and a series of steps to separate the parts to be used for various products. [1]
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. [2]
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. [3] 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.[ citation needed ]
As per the standards of the U.S. Department of Agriculture, Grade 5[ further explanation needed ] corn is usually used for wet-milling. [4] 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. [5] The cleaned corn is then analyzed for its composition using an NIR spectrometer. 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.[ citation needed ]
| Moisture (in %) | Protein (in %, dry basis) | Oil (in %, db) | Starch (in %, db) | Density (in g/cc) | Test Weight (in lb/bu) |
|---|---|---|---|---|---|
| 13.8 | 8.93 | 4.29 | 70.4 | 1.282 | 65.6 |
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. [4] Use of concentrated steepwater: This extract is protein-rich and can be used as nutrient media for fermentation to produce enzymes or antibodies. It is also used in animal feed.[ citation needed ]
As the process step suggests, in this step the germ is separated from the other parts of the corn. Recovering the germ as intact as possible is necessary to prevent any oil contamination in the final products. Attrition mills such as disk mills are used to coarsely grind the softened corn kernels. The grinding is slow and the elements used to grind are blunt to ensure intact removal of germ. Water is added to the ground material to make a thick slurry of macerated kernels and whole germ. [4] The 40-50% crude oil content of germ makes it less dense than other particles and as a result germ floats in the mixture. The mixture is then passed through germ hydrocyclones with an over- and underflow. Overflow will be composed majorly of germ and water and underflow will have fiber, starch, protein and water. The overflow is passed through the hydrocyclone multiple times since 100% separation cannot be achieved in a single pass. The separated germ is cleaned, dried and passed through a germ press to extract oil from it. Solvent extraction can also be used alternatively. The solid particles remaining after oil extraction is called germ meal which is further dried.[ citation needed ]
Germ meal is a good source of amino acids and is a carrier of micro-ingredients in animal food formulations. The refined corn oil can be used as salad oil and cooking oil. It is also used to prepare corn oil margarines.[ citation needed ]
The underflow from the hydrocyclone consisting of fiber, protein and starch is finely ground and screened using multiple grind mills and pressure-fed screens. Screens are used to separate the fiber from the mixture. Various screen sizes are used to remove coarse and fine fibers. A wedge bar or profile bar screen is used. Starch and protein passes through the screen and collected whereas the fibers remain on the screen and it is called corn gluten feed. The principle of separation is difference in size. The corn gluten feed has approximately 21% protein, 1% fat and 10% fiber and 15% starch. Since it is high in water-soluble nutrients, it is used as one of the main ingredients in animal feed. It can also be used to produce refined corn fiber to be used for human consumption. [4]
The slurry containing just the protein and starch is called millstarch. [6] 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. [4] The lighter gluten, separated out from the top, is thickened and the heavy gluten is further sent for dewatering into vacuum rotary filter. This corn gluten meal consist of approximately 60% protein, 1% fat and 3% fiber. The process water from both the processes are either added to steepwater or removed. Since it has around 60% protein, it is used as animal feed and zein products.[ citation needed ]
Starch goes through multiple-stage washing using hydrocyclones. The supernatants are separated at each washing stage. The water from each stage is recycled to the previous hydrocyclones to ensure the maximum amount of starch is separated. A very high purity of starch (>99.5% db) can be recovered by wet-milling. Purity is important when the end product is high-fructose corn syrup or starch to be modified (using chemicals or enzymes) but it is not important during ethanol production. After centrifugation and washing, the starch is dried. [4]
Co-products account for 34% of wet-milled yield. In fact, 23% of corn that is processed has very low or no value. The fiber, concentrated steepwater and germ meal are mixed to produce corn gluten feed. As mentioned before, corn gluten meal is also used as animal feed. Although both have ‘gluten’ in the name, no gluten protein is present in them – there is none in corn on whole. [7]
A typical solid yield (on db) data for yellow dent corn is shown in the table below. [8]
| Fraction | Yield on dry basis (in %) |
|---|---|
| Starch | 58-68 |
| Gluten Meal | 5.8-15.4 |
| Fiber (coarse+fine) | 8.8-19.2 |
| Germ | 5.2-10.5 |
| Steepwater solubles | 5.1-7.5 |
| Total solids recovery | 97.3-99.9 |
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. [9] [10]
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. [11] 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. [12] 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. [13]
Hominy is a food produced from dried maize (corn) kernels that have been treated with an alkali, in a process called nixtamalization. "Lye hominy" is a type of hominy made with lye.
The oat, sometimes called the common oat, is a species of cereal grain grown for its seed, which is known by the same name. While oats are suitable for human consumption as oatmeal and rolled oats, one of the most common uses is as livestock feed. Oats are a nutrient-rich food associated with lower blood cholesterol when consumed regularly.
Flour is a powder made by grinding raw grains, roots, beans, nuts, or seeds. Flours are used to make many different foods. Cereal flour, particularly wheat flour, is the main ingredient of bread, which is a staple food for many cultures. Corn flour has been important in Mesoamerican cuisine since ancient times and remains a staple in the Americas. Rye flour is a constituent of bread in central and northern Europe.
Field corn, also known as cow corn, is a North American term for maize grown for livestock fodder (silage), ethanol, cereal, and processed food products. The principal field corn varieties are dent corn, flint corn, flour corn which includes blue corn, and waxy corn.
Corn starch, maize starch, or cornflour is the starch derived from corn (maize) grain. The starch is obtained from the endosperm of the kernel. Corn starch is a common food ingredient, often used to thicken sauces or soups, and to make corn syrup and other sugars. Corn starch is versatile, easily modified, and finds many uses in industry such as adhesives, in paper products, as an anti-sticking agent, and textile manufacturing. It has medical uses as well, such as to supply glucose for people with glycogen storage disease.
Bran, also known as miller's bran, is the hard layers of cereal grain surrounding the endosperm. It consists of the combined aleurone and pericarp. Corn (maize) bran also includes the pedicel. Along with the germ, it is an integral part of whole grains, and is often produced as a byproduct of milling in the production of refined grains.
Wheat flour is a powder made from the grinding of wheat used for human consumption. Wheat varieties are called "soft" or "weak" if gluten content is low, and are called "hard" or "strong" if they have high gluten content. Hard flour, or bread flour, is high in gluten, with 12% to 14% gluten content, and its dough has elastic toughness that holds its shape well once baked. Soft flour is comparatively low in gluten and thus results in a loaf with a finer, crumbly texture. Soft flour is usually divided into cake flour, which is the lowest in gluten, and pastry flour, which has slightly more gluten than cake flour.
Steeping is the soaking of an organic solid, such as leaves, in a liquid to extract flavours or to soften it. The specific process of teas being prepared for drinking by leaving the leaves in heated water to release the flavour and nutrients is known as steeping. Herbal teas may be prepared by decoction, infusion, or maceration. Some solids are soaked to remove an ingredient, such as salt, where the solute is not the desired product.
Nixtamalization is a process for the preparation of corn, or other grain, in which the grain is soaked and cooked in an alkaline solution, usually limewater, washed, and then hulled. The term can also refer to the removal via an alkali process of the pericarp from other grains such as sorghum.
Distillers grains are a cereal byproduct of the distillation process. Brewer's spent grain usually refers to barley produced as a byproduct of brewing, while distillers grains are a mix of corn, rice and other grains.
Corn kernels are the fruits of corn. Maize is a grain, and the kernels are used in cooking as a vegetable or a source of starch. The kernel comprise endosperm, germ, pericarp, and tip cap.
Cottonseed is the seed of the cotton plant.
Corn gluten meal is the principal protein of corn (maize) endosperm consisting mainly of zein and glutelin. It is a byproduct of corn processing that has historically been used as an animal feed. Despite the name, corn gluten does not contain true gluten, which is formed by the interaction of gliadin and glutenin proteins.
Corn ethanol is ethanol produced from corn biomass and is the main source of ethanol fuel in the United States, mandated to be blended with gasoline in the Renewable Fuel Standard. Corn ethanol is produced by ethanol fermentation and distillation. It is debatable whether the production and use of corn ethanol results in lower greenhouse gas emissions than gasoline. Approximately 45% of U.S. corn croplands are used for ethanol production.
Wet-milling is a process in which feed material is steeped in water, with or without sulfur dioxide, to soften the seed kernel in order to help separate the kernel’s various components. For example, wet-milling plants can separate a 56-pound bushel of corn into more than 31 pounds of cornstarch, 15 pounds of corn gluten meal for use in animal feed, and nearly 2 pounds of corn oil.
Starch production is an isolation of starch from plant sources. It takes place in starch plants. Starch industry is a part of food processing which is using starch as a starting material for production of starch derivatives, hydrolysates, dextrins.
In agriculture, grain quality depends on the use of the grain. In ethanol production, the chemical composition of grain such as starch content is important, in food and feed manufacturing, properties such as protein, oil and sugar are significant, in the milling industry, soundness is the most important factor to consider when it comes to the quality of grain. For grain farmers, high germination percentage and seed dormancy are the main features to consider. For consumers, properties such as color and flavor are most important.
Feed manufacturing refers to the process of producing animal feed from raw agricultural products. Fodder produced by manufacturing is formulated to meet specific animal nutrition requirements for different species of animals at different life stages. According to the American Feed Industry Association (AFIA), there are four basic steps:
Dry milling of grain is mainly utilized to manufacture feedstock into consumer and industrial based products. This process is widely associated with the development of new bio-based associated by-products. The milling process separates the grain into four distinct physical components: the germ, flour, fine grits, and coarse grits. The separated materials are then reduced into food products utilized for human and animal consumption.
The Roller Mill was created by Hungarian bakers in the late 1860s and its popularity spread worldwide throughout the 1900s. Roller mills now produce almost all non-whole grain flour. Enriched flour is flour that meets an FDA standard in the United States. Roller milled white enriched flour makes up over 90% of the flour that comes out of the United States.
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