Corn wet-milling is a process of breaking

corn kernel Corn kernels are the fruits of Maize, 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 (grain), germ, pericarp, and tip cap. ...

s into their component parts:

corn oil Corn oil (North American) or maize oil (British) is oil extracted from the germ of corn (maize). Its main use is in cooking, where its high smoke point makes refined corn oil a valuable frying oil. It is also a key ingredient in some margarin ...

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

corn starch Cornflour, cornstarch, maize starch, or corn starch (American English) is the starch derived from corn (maize) grain. The starch is obtained from the endosperm of the seed, kernel. Corn starch is a common food ingredient, often used to thick ...

, and

fiber Fiber (spelled fibre in British English; from ) 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 inco ...

. It uses water and a series of steps to separate the parts to be used for various products.

History

The

corn Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout Poaceae, grass that produces cereal grain. It was domesticated by indigenous peoples of Mexico, indigenous peoples in southern Mexico about 9,000 years ago ...

wet-milling industry has been a primary component of American manufacturing for more than 150 years. Corn refiners established the process of separating

s into their component parts to produce a variety of

value-added Value added is a term in economics for calculating the difference between market value of a product or service, and the sum value of its constituents. It is relatively expressed by the demand curve, supply-demand curve for specific units of sale. ...

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) and lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturate ...

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 diet ...

, and

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, also known as sodium chloride (NaCl), in wat ...

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

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 ...

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 A sieve (), fine mesh strainer, or sift is a tool used 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 m ...

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 Spectrum, spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomeno ...

. 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

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 Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

and

lactic acid Lactic acid is an organic acid. It has the molecular formula C3H6O3. 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 wel ...

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#Pla ...

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 (microbicide) or stops their growth (bacteriostatic agent). Antimicrobial medicines can be grouped according to the microorganisms they are used to treat. For example, antibiotics are used aga ...

. 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 An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

antibodies An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...

. 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 A slurry is a mixture of denser solids suspended in liquid, usually water. The most common use of slurry is as a means of transporting solids or separating minerals, the liquid being a carrier that is pumped on a device such as a centrifugal pu ...

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 (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

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 over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...

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, cereal grains. The term ''gluten'' usually refers to the elastic network of a wheat grain's proteins, gliadin and glutenin primarily, that forms readily with the addition of water ...

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 Animal feed is food given to domestic animals, especially livestock, in the course of animal husbandry. There are two basic types: fodder and forage. Used alone, the word ''feed'' more often refers to fodder. Animal feed is an important input ...

. 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.
description page
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. 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.

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 gal ...

corn syrup or starch to be modified (using chemicals or enzymes) but it is not important during

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

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

. 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 A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalysis, catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products ...

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 (; 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 polysaccharides: : Endo ...

and an enzyme to degrade

phytic acid Phytic acid is a six-fold dihydrogenphosphate ester of inositol (specifically, of the ''myo'' isomer), also called inositol hexaphosphate, inositol hexakisphosphate (IP6) or inositol polyphosphate. At physiological pH, the phosphates are partia ...

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

References

{{Corn Maize Food processing