![Wet milling facility operated by A](https://upload.wikimedia.org/wikipedia/commons/e/eb/Wet_milling_facility_operated_by_A.E._Staley_Manufacturing_Company.jpg)
Corn wet-milling is a process of breaking
corn kernel
Corn kernels are the fruits of corn (called maize in many countries). 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.
One ear of corn contai ...
s into their component parts:
corn oil,
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
,
corn starch
Corn starch, maize starch, or cornflour (British English) 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 sou ...
, and
fiber
Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...
. It uses water and a series of steps to separate the parts to be used for various products.
History
The
corn
Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...
wet-milling Wet-milling is a process in which feed material is steeped in water, with or without sulfur dioxide, to soften the seed kernel
Kernel may refer to:
Computing
* Kernel (operating system), the central component of most operating systems
* Kernel ...
industry has been a primary component of American manufacturing for more than 150 years. Corn refiners established the process of separating
corn kernel
Corn kernels are the fruits of corn (called maize in many countries). 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.
One ear of corn contai ...
s into their component parts to produce a variety of
value-added
In business, total value added is calculated by tabulating the unit value added (measured by summing unit profit sale price and production cost">Price.html" ;"title="he difference between Price">sale price and production cost], unit depreciation ...
products. The four main component such as
oil
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
,
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
,
starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...
, and
fiber
Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...
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
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
solution, which is used in the
steeping
Steeping is the soaking of an organic solid, such as leaves, in a liquid (usually water) 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 a ...
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
The United States Department of Agriculture (USDA) is the federal executive department responsible for developing and executing federal laws related to farming, forestry, rural economic development, and food. It aims to meet the needs of comme ...
, Grade 5 corn is usually used for wet-milling.
Harvested corn has to be cleaned before it is milled.
Dockage tester with appropriate
sieve
A sieve, fine mesh strainer, or sift, is a device for separating wanted elements from unwanted material or for controlling the particle size distribution of a sample, using a screen such as a woven mesh or net or perforated sheet material. T ...
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 an NIR
spectrometer
A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...
. 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
Drinking is the act of ingesting water or other liquids into the body through the mouth, proboscis, or elsewhere. Humans drink by swallowing, completed by peristalsis in the esophagus. The physiological processes of drinking vary widely among o ...
in order to loosen
starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...
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
Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as natu ...
are also added to the water. Lactic acid breaks down the
endosperm
The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the embryo and ...
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
An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals a ...
. 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.
Use of concentrated steepwater: This extract is protein-rich and can be used as nutrient media for fermentation to produce
enzymes
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
or
antibodies
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
. It is also used in animal feed.
Germ recovery
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.
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
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
extraction can also be used alternatively. The solid particles remaining after oil extraction is called germ meal which is further dried.
Germ meal is a good source of
amino acids
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
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.
Fiber recovery
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
Gluten is a structural protein naturally found in certain cereal grains. Although "gluten" often only refers to wheat proteins, in medical literature it refers to the combination of prolamin and glutelin proteins naturally occurring in all grain ...
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.
Protein recovery
The slurry containing just the protein and starch is called millstarch. 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
hydrocyclone Hydrocyclones are a type of cyclonic separators that separate product phases mainly on basis of differences in gravity with aqueous solutions as the primary feed fluid.
As opposed to dry or dust cyclones, which separate solids from gasses, hydrocy ...
s with fresh water. The starch stream typically has 90% starch and the gluten stream consists of 60% protein.
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
Zein is a class of prolamine protein found in maize (corn). It is usually manufactured as a powder from corn gluten meal. Zein is one of the best understood plant proteins.Momany, Frank A.; Sessa, David J.; Lawton, John C.; Selling, Gordon W.; H ...
products.
Starch processing
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
Fructose, or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galacto ...
corn syrup or starch to be modified (using chemicals or enzymes) but it is not important during
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
production. After
centrifugation
Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate ...
and washing, the starch is dried.
Co-product manufacture
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.
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
Gluten is a structural protein naturally found in certain cereal grains. Although "gluten" often only refers to wheat proteins, in medical literature it refers to the combination of prolamin and glutelin proteins naturally occurring in all grain ...
. However, the co-product from this process will produce corn oil,
corn gluten meal
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 tr ...
, 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
A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
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
Cellulase (EC 3.2.1.4; systematic name 4-β-D-glucan 4-glucanohydrolase) is any of several enzymes produced chiefly by fungi, bacteria, and protozoans that catalyze cellulolysis, the decomposition of cellulose and of some related polysacchar ...
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.
[Haros, Monica, Suarez, Costantino. 1997. Effect of drying, initial moisture and variety in corn wet milling. Journal of Food Engineering 34(4):473-481]
See also
*
Maize
Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...
References
{{Corn
Maize
Food processing