Corn IntroductionOrigin, History, and Uses of CornHistory and OriginFor western civilization, the story of corn began in 1492 when Columbus's men discovered this new grain in Cuba. An American native, it was exported to Europe rather than being imported, as were other major grains. Like most early history, there is some uncertainty as to when corn first went to Europe. Some say it went back with Columbus to Spain, while others report that it was not returned to Spain until the second visit of Columbus.The word "corn" has many different meanings depending on what country you are in. Corn in the United States is also called maize or Indian corn. In some countries, corn means the leading crop grown in a certain district. Corn in England means wheat; in Scotland and Ireland, it refers to oats. Corn mentioned in the Bible probably refers to wheat or barley. At first, corn was only a garden curiosity in Europe, but it soon began to be recognized as a valuable food crop. Within a few years, it spread throughout France, Italy, and all of southeastern Europe and northern Africa. By 1575, it was making its way into western China, and had become important in the Philippines and the East Indies. Although corn is indigenous to the western hemisphere, its exact birthplace is far less certain. Archeological evidence of corn's early presence in the western hemisphere was identified from corn pollen grain considered to be 80,000 years old obtained from drill cores 200 feet below Mexico City. Another archeological study of the bat caves in New Mexico revealed corncobs that were 5,600 years old by radiocarbon determination. Most historians believe corn was domesticated in the Tehuacan Valley of Mexico. The original wild form has long been extinct.Evidence suggests that cultivated corn arose through natural crossings, perhaps first with gamagrass to yield teosinte and then possibly with backcrossing of teosinte to primitive maize to produce modern races. There are numerous theories as to the ancestors of modern corn and many scientific articles and books have been written on the subject. Corn is perhaps the most completely domesticated of all field crops. Its perpetuation for centuries has depended wholly on the care of man. It could not have existed as a wild plant in its present form. Corn is often classified as dent corn, flint corn, flour corn, popcorn, sweet corn, waxy corn, and pod corn. The remainder of this discussion will be concerned only with dent corn, which is the major type cultivated in the United States.Corn was the most important cultivated plant in ancient times in America. Early North American expeditions show that the corngrowing area extended from southern North Dakota and both sides of the lower St. Lawrence Valley southward to northern Argentina and Chile. It extended westward to the middle of Kansas and Nebraska, and an important lobe of the Mexican area extended northward to Arizona, New Mexico and southern Colorado. It was also an important crop in the high valleys of the Andes in South America.The great variability of the corn plant led to the selection of numerous widely adapted varieties which hardly resembled one another. The plant may have ranged from no more than a couple of feet tall to over 20 feet. It was not like the uniform sized plant that most people know today. For the Aztecs, Mayas, Incas and various Pueblo dwellers of the southwestern United States, corn growing took precedence over all other activities.The principal role of the corn plant during the 19th century was closely tied to the development of the Midwest. In the movement westward, corn found its major home in the woodland clearings and grasslands of Ohio, Indiana, Illinois, Iowa, and adjacent states. These were places where it had not been grown widely in prehistoric times. As early as 1880, the United States grew over 62 million acres of corn. By 1900, this figure had reached approximately 95 million acres; by 1910, it was over 100 million acres. The highest acreage ever recorded in the United States was 111 million acres in 1917.From the beginning of records in the 1880s, through the mid-1930s, there was no significant increase in the national average corn yield. Yields during the 1920s and 1930s were no higher than those produced as a national average in the late 1800S.It was not until the vast technological advances in the early 1940s that corn yields started to show significant yield increases. Prior to this time, the highest U.S. average yield was recorded in 1906 at 31.7 bushels per acre. Following moderate yield increases in the 1940s and 1950s, yields shot up in the 1960s and early 1970s to a national average of 109.5 bushels per acre in 1979. In 2000, US farmers planted over 79 million acres of corn. More than 40% of the world's corn is produced in the United States. Total acreage is now less than in earlier years, but planting has increased in the more favorable areas of the Corn Belt. Iowa is normally the leading corn producing state, followed closely by Illinois. As early as 1910, Iowa had 8.5 million acres of corn, which averaged nearly 40 bushels per acre. In 1935, Iowa had 9.7 million acres of corn, averaging 39 bushels per acre. In 1960, Iowa averaged 62 bushels per acre on nearly 12.5 million acres. In 2000, Iowa farmers averaged 145 bushels per acre on more than 12 million acres. The highest all time record corn acreage in Iowa was 14.4 million acres in 1980.Corn and soybeans form a major base of the Iowa economy. The combination of favorable soils, weather, and management know-how for the production of these two crops is rivaled by few other places in the world. Although few people are directly involved in the production of these major crops, many jobs are associated with this industry. Industries involved in crop processing, marketing, production of farm machinery and other farm inputs exist because of our ability to grow crops in Iowa. Massive livestock industries also depend on feed produced from Iowa soils.Uses of CornDuring the mid-1960s, about 75 percent of the corn was fed to livestock, 13 percent was exported, and the remainder went into human food and industrial products. By 2000, the relative amount of corn fed to livestock had decreased to 60 percent, 22 percent was exported, 6 percent was used for High-Fructose Corn Sweetener, 6 percent was processed for ethanol, and 6 percent went into other products. Between 90 and 95 percent of the crop is harvested for grain; the remaining 5 to 10 percent is grown for silage. Of the corn fed to livestock in 1960, about 40 percent went to hogs, 20 percent to poultry, 30 percent to cattle on feed and milk cows, and 10 percent to other types of livestock. By 2000, these amounts had shifted to 29 percent to cattle on feed, 29 percent to poultry, 24 percent to hogs, 16 percent to dairy cattle, and 2 percent to other types of livestock. One reference lists over 500 different uses for corn. Corn is a component of canned corn, baby food, hominy, mush, puddings, tamales, and many more human foods. Some industrial uses of corn include filler for plastics, packing materials, insulating materials, adhesives, chemicals, explosives, paint, paste, abrasives, dyes, insecticides, pharmaceuticals, organic acids, solvents, rayon, antifreeze, soaps, and many more. Corn also is used as the major study plant for many academic disciplines such as genetics, physiology, soil fertility and biochemistry. It is doubtful that any other plant has been studied as extensively as has the corn plant.A bushel of shelled corn weighs 56 pounds.
Lance Gibson and Garren Benson, Iowa State University, Department of AgronomyRevised January 2002.Corn ProcessINSPECTION & CLEANINGCorn refiners use yellow dent corn, which is removed from the cob during harvesting. An average bushel of yellow dent corn weighs 56 pounds. Approximately 70 percent of the kernel is starch (from the endosperm), about 10 percent is protein (predominantly gluten), four percent is oil (extracted from the germ), and two percent is fiber (from the hull). It is the goal of the corn refining process to separate each component and then further refine it into specific products. Corn arrives at the refining facility by truck, barge or railcar. Refinery staff inspect arriving corn shipments and clean them twice to remove pieces of cob, dust, chaff, and foreign materials. The corn is then conveyed to storage silos, holding up to 350,000 bushels, until ready to go to the refinery for steeping, the first processing step.
Corn Wet Milling Process DescriptionThe wet mill of a corn plant refers to area where the corn is separated into its individual components of starch, gluten, fiber, and germ. The separations in the wet mill are mostly physical through grind mills, screens, cyclones, centrifuges, presses, and filters. The main product of the wet mill is a relatively pure starch stream, either dried or in a slurry form. The byproducts of the wet mill include the germ, fiber, and gluten, which are further processed or marketed as feed products.STEEPINGThe corn after arriving to the plant, must be cleaned to meet the standards of the U.S. Yellow Dent #2 corn. The cleaned corn is conveyed and metered into the steep tanks. Steeping the corn prior to milling is done by soaking the corn in a solution of sulfur dioxide and water at controlled temperature for a length of time between 30 and 45 hours. The purpose of steeping is to soften the kernel, allowing for separation of the germ without cracking during milling, to partially breakdown the protein matrix in which the starch is embedded allowing for separation in subsequent milling stages, and to remove the soluble impurities contained within the corn. Good steeping is a necessity for achieving a good quality starch product. Freshly made steep acid is added to the steep tank where the corn has been in the steeping process the longest. The steep acid is circulated through the steep tanks towards the tank where the newest corn is being added. From this point, some of the steep liquor must be removed from the system. The amount of steep water removed is critical for producing a quality starch product as this is the only point in the system where soluble impurities can leave the system. The dissolved solids content of this stream is usually in the range of 10%. Water in this light steep water stream needs to be evaporated until the solids content reaches about 50%. Doing this will allow the "heavy" steep water to be mixed with the end fiber product to increase the nutritional content of the feed product. The condensate from this evaporator, being high in impurities, cannot be utilized back into the process and therefore makes up the majority of the waste water leaving the plant. After steeping, the corn is conveyed to the milling area via sluice water. This water flows between the outlet of the steep tank to the dewatering screen prior the first stage milling where it is continuously recycled back. In between the steep tanks and the dewatering screen, the slurry if fed to a Stone Cyclone.DESTONINGThe stone cyclone protects the grind mills and other downstream process equipment from damage or excessive wear resulting from stones, sand, pieces of metal, or other high specific gravity contaminants that enter the corn slurry during the washing, conveying, or steeping processes. The centrifugal forces within the cyclone force the heavy contaminants to the outside of the cyclone towards the underflow. A reject pot on the bottom of the cyclone collects the contaminants which are then purged from the system with a pair of actuated valves on either side of the reject pot.The overflow of the cyclone, free of contaminants, is then directed to the corn dewatering screen.CORN DEWATERINGBefore the first grind process step, the corn slurry from the destoning cyclone is dewatered by a gravity screen. Dewatering the corn slurry prior to milling reduces the hydraulic load on the grind mill and improves the milling efficiency.
FIRST GRIND MILLThe steeped corn, after being dewatered by the corn screens, enters into the first grind. The purpose of the first grind is to crack the corn kernels and free the germs. The devils tooth grind plates commonly used in the first grind mill have a pattern of large interlocking teeth that the corn kernels must pass through before reaching the machine discharge. In the grind mill, one set of the grind plates are rotating, while the other set of plates is fixed. The gap between these two sets of plates are adjusted so that first grind mill cracks most of the kernels and frees up most of the germ without damage to the soft germ particles. In every handful of slurry coming from the first grind mill, there should only be about 7-8 uncracked kernel. Any corn not cracked in the first stage, will be cracked in the second stage. Dilution water is added to the first grind mill so that the starch freed from the kernel can enter into slurry without pasting up in the grind mill. Factors affecting the capacity and efficiency of the first grind mills include the density of the feed slurry, the plate clearance, the applied horsepower, and the steep processes employed. After leaving the first grindmill, the slurry is gravity discharged into the first grind tank where it is mixed with recycle streams from the germ separation and other millhouse recycle streams.SECOND GRIND MILLThe purpose of the second grind is to crack any of the kernels that were missed in the first grind. Like the first grind, the slurry feeding the second grind is dewatered just prior to milling. From the second grind mill, the slurry is again diluted and then dumped into the second grind tank, which feeds the secondary germ separation system.A handful of discharge from the second grind mill should not contain any whole kernels, on average. Factors affecting the efficiency of the second grind include the steep processes, the efficiency of the first grind, the feed slurry density, the grind plates used, the clearance between the plates, and the applied horsepower. Like the first grind, devil tooth plates are used in the second grind. The clearance between the plates should be set closer than in the first grind. Setting the plates too close, would result in cracking germs and tearing fibers more than necessary. Setting the plates too far would result in a lower overall oil recovery, as any germ not recovered in the second grind will be lost later in the process.PRIMARY GERM SEPARATIONThe slurry from first grind tank is fed to the primary germ separation system, where the main separation of the germs from the slurry occurs. The primary germ separation system consists of a two stage system, where the underflow of the first stage is fed to a second stage. Usually the two stages are directly connected with only one feed pump for the entire system. Typically, the Baume in the first grind tank is around 8. At this Baume, the germs, with lighter specific gravity than the starch slurry, will start to float on the top of the slurry. The centrifugal force inside the cyclone, driven by the pressure drop across the cyclone, accelerates the floating of the germs, so that the overflow of the first stage cyclone contains many of the germs in the slurry.The overflow of the first stage cyclone is controlled with pressure to ensure that most of the germs with a minimum amount of fiber is leaving the system with the germs. The underflow of the first stage cyclone is then fed to a second stage to recover more of the germs remaining in the slurry. The overflow of this second stage is directed back to the first grind tank, and the underflow of this stage proceeds to the second grind step.GERM WASHING SYSTEMThe overflow of the first stage of the primary germ separation system contains all of the recovered germ, some fiber, and some starch. The purpose of the germ washing system is to wash as much of the starch as possible from the germ and from the fiber with the germ. The system typically consists of a three stage counter current system, with the feed entering into the first stage and the wash water entering into the feed of the third stage. The overflow from the third stage is the germ which proceeds to further dewatering and drying stages. The first stage underflow is a starch/gluten stream which proceeds back to the first grind tank for dilution. GERM PRESS / GERM DRYINGAfter the germ stream is washed in the germ washing system, the slurry, which still contains some fiber, then proceeds to a dewatering press to remove as much free water as possible. At the outlet of the germ dewatering press, the germ product is dried in the germ dryer and then to Expeller section to extract oil.
FIBER WASHING SYSTEMThe fiber wash system consists of a series of 120 degree pressure screens configured to counter-currently wash the fiber stream coming from the third grind mill. The goal of the fiber wash system is to wash all of the free starch off of the fiber pieces.Typically, a 6 stage system is used where the feed enters into the 1st stage. The wash water enters in the final stage. The unders of each stage becomes the wash for the previous stage, and the overs of each stage become the feed for the next stage. The unders of the first stage becomes the feed starch slurry heading for the centrifuges while the overs of the last stage is fiber heading to the fiber dewatering step in the process. There is some starch that leaves the system with the fiber and cannot be recovered. This starch can either be bound starch (unrecoverable pieces of starch physically attached to the fiber) or free starch (starch in the water). An efficient wash system will minimize the amount of free starch leaving with the fiber to about 5-10%. Much of this free starch can be recovered prior to the fiber dewatering press with a fiber press dewatering screen. If the bound starch is high, the problem is most likely in the steeping and grinding processes. In stages 2 to 6, 75 micron screens are used, and they are fed with 0.75 nozzles. In the first stage, 0.50 nozzles and a 50 micron screen is used to prevent fine fibers from entering the starch slurry. All stages can be fed at close to 60 psi pressure for maximum capacity. To recover much of the free starch leaving with the fiber in the 6th stage overs, a fiber dewatering screen, with a 150 micron screen fed with 0.75 nozzles can be used. Using this screen to dewater prior to the press can increase the efficiency of the press and recover some of the free starch. The overs of the screen is further dewatered, and the unders of the screen is recycled back to the main fiber wash screens.FIBER DEWATERINGAfter the fiber washing system, the fiber slurry is dewatered as much as possible with an additional pressure screen. Any water removed from this dewatering screens helps to reduce the dewatering requirements of the downstream fiber press. FIBER PRESS / DRYERAfter the fiber stream is washed in the fiber washing system and dewatered, the slurry is further dewatered in a fiber press. At the outlet of the fiber dewatering press, the fiber product is dried in the fiber dryer.DEGRITTING SYSTEMThe mill starch stream, comprised of the combined filtrates from the third grind dewatering screen and the first stage fiber wash screen, will be further processed by centrifuges and starch washing cyclonettes to separate the starch and gluten fractions of the slurry. To protect the downstream equipment, the mill starch stream is fed to a degritting system to remove and small heavy particles such as sand, grit, rust, or pipe scale. These materials, when passing through the high speed centrifuges, would cause rapid wear and lead to premature replacement of expensive machine parts. This heavy material also contributes to premature wear of the cyclonettes in the starch washing system. The degritting system consists of a two stage system, with the underflow of the first stage being fed to a second stage. The overflow of the first stage proceeds to the centrifuge separation steps. The overflow of the second stage is recycled back to the feed to the first stage. Reject pots on the underflow of the second stage cyclones collect the contaminants, which are purged periodically with timed automated purge valves around the reject pot.GLUTEN THICKENERFrom the overflow of the primary centrifuge, the light gluten slurry is fed to a Gluten Thickener (GT) centrifuge. This light gluten stream contains all of the gluten to be recovered in the gluten product stream, water, and soluble proteins. The gluten thickener centrifuge concentrates the gluten stream prior to dewatering and also provides clear overflow for process water to be used upstream in the millhouse.GLUTEN DEWATERING / DRYINGAfter the GT machine has thickened up the gluten stream by removing as much water as possible, vacuum belt filters are commonly used for additional dewatering. The water removed from the gluten slurry at the vacuum filters is sent back to the feed of the GT machine to recovery and remaining gluten.
MST CENTRIFUGEThe combined filtrate streams from the third grind screen and first stage fiber wash screens will contain about 10 to 11% solids (5 to 6 Be). This slurry is comprised of the starch and protein components of the corn, along with some soluble impurities released from the corn during the steeping process. This slurry is fed to the Mill Stream Thickener (MST) centrifuge to thicken the starch slurry and provide a process water stream. The centrifuge has high rotational speed, creating significant G forces inside the machine which drives the separation of the water, solubles, and higher specific gravity starch and gluten particles. The solids are continuously discharged through nozzles around the periphery of the bowl, while the water and soluble head toward the center of the bowl and out the overflow of the machine. The MST centrifuge increases the Baume of the mill stream slurry providing a reduced flow rate and consistent feed Baume to the downstream primary centrifuge, and also separates out solubles early in the starch protein separation process. This reduces the washing requirements to remove the solubles in the downstream primary centrifuge and starch washing system. The overflow of the MST centrifuge is primarily used for the steeping system makeup water. Great care is taken in the operation of the MST centrifuge to ensure that a minimum amount of starch and gluten solids are sent back to the steeping system through the MST overflow.PRIMARY CENTRIFUGEThe primary centrifuge is fed from the MST centrifuge underflow and any direct portion of the mill starch stream that bypasses the MST. The primary centrifuge is the main separation point between the starch and the water, gluten, and solubles. Wash water from the clarifier centrifuge overflow is introduced into the primary where it washes the solubles from the starch via displacement washing. Fluid-Quip primary centrifuges are designed to accommodate high rates of displacement washing, to lower the residual soluble protein levels in the starch leaving through the underflow of the machine. The overflow of the primary centrifuge becomes the light gluten stream, which is further dewatered and dried to become the gluten meal product. Since any starch in the overflow of the primary centrifuge will be lost to the gluten meal product, and lower the important protein concentration of the gluten meal, it is very important to operate the machine with minimal starch loss to the overflow. The underflow of the primary centrifuge, which still contains some insoluble and soluble proteins, is further purified in the starch washing system.
STARCH WASHING SYSTEMTo produce a high quality starch product, the underflow of the primary is sent to a multiple stage washing system. In this system, fresh water is added to the final stage to wash the starch slurry counter currently across 13 stages. The starch reports to the underflow of each stage, getting purer and purer as it progresses towards the end of the system. The protein and soluble impurities are carried out the overflow of the first stage of the system with some starch. Sending some starch out the overflow is necessary for ensuring a high quality starch product. The underflow of the starch washing system proceeds to further processing for modification into specialty starch products, conversion to syrup for sweeteners/ethanol, or dewatering/drying to make dry starch product.
Corn Wet Milled Feed ProductsCorn Germ Meal is a by-product from the extraction of oil. It contains typically 20-21% protein and 90% dry matter. Corn Gluten Feed is a mixture of the hulls & fiber fraction with steepwater, corn germ meal and other process residuals. A typical yield per ton corn is 250 kg corn gluten feed with 18-22% protein and 89-90% dry substance. Corn Gluten Meal is the dried gluten from the gluten concentration. A typical yield per ton corn is 50 kg corn gluten meal with 60% protein and 89-90% dry substance. Corn Steep Liquor also known as condensed fermented corn extractives is a high protein ingredient. It is often a constituent of corn gluten feed, but may be sold as is with approximate 23% protein and 50% dry substance for cattle feeds or as a pellet binder.
Modified Starch Division (MSD)Native starches have certain inherent features for use in the development of foods, pharmaceuticals and industrial products. Among other advantages, they are readily available, generally low in price, and yield a simple, consumer-friendly label when listed in an ingredient panel. However, the advent of more sophisticated processing systems made it apparent that the natural properties of raw starches could not meet the demanding processing requirements of increasingly sophisticated product formulations. In order to meet such manufacturing requirements, starch chemists developed modified starches. The techniques and chemicals used to manufacture food and industrial modified starches have been thoroughly researched and tested to ensure safety and functionality. Modified food starches are strictly defined and regulated by the United States Food and Drug Administration (FDA) in 21 CFR Chapter 1, paragraph 172.892, and industrial modified starches are covered by 21 CFR Chapter 1, paragraph 178.3520.Acid-modified Corn Starch The first method used commercially to reduce the viscosity of starch pastes was the acid-modification process patented by Duryea in 1899. In this method, a starch-water suspension is agitated while being subjected to mild treatment with dilute mineral acid at temperatures elevated but below the starch gelatinization temperature, for varying16 periods of time. When tests show the desired viscosity has been reached, the acid is neutralized with sodium carbonate and the starch is filtered, washed and dried. In this manner a series of starches yielding pastes of decreasing viscosity are produced. The primary reaction taking place during acid-modification is hydrolysis of glucosidic bonds in starch molecules. This limited and controlled hydrolysis produces two important consequences. First, since the starch molecule is so large, only a small amount of cleavage is needed to markedly reduce viscosity. Second, disruption of bonds within the granule weakens the granule structure. Like the parent starch, all acid modified starch pastes have reduced viscosities when warm, yet have a strong tendency to gel when cooled. This suggests that acid-modification reduces chain length but does not substantially change the molecular configuration.Oxidized Corn Starch A second method for reducing the viscosity and altering the properties of starch is oxidation. Although oxidizing agents such as chlorine, hydrogen peroxide and potassium permanganate can be used, oxidized starches produced by the corn wet milling industry are almost exclusively made using sodium hypochlorite as the oxidizing agent. As in the case of acid-modification, aqueous starch suspensions under continuous agitation are treated with dilute sodium hypochlorite containing a small excess of caustic soda (NaOH). The reagent solution is added slowly to the starch suspension in a reactor which is maintained at about 120 o F. Cooling water in the reactor jacket or external heat exchangers remove heat generated during the oxidation reaction. When the correct amount of reagent has been added and sufficient time for reaction has elapsed, the viscosity of the starch is determined. When the desired degree of oxidation is reached, the starch slurry is treated with a reducing agent such as sodium bisulfite to remove excess hypochlorite, adjusted to the desired pH, filtered, washed and dried.
STARCH DEWATERINGThe purified starch milk is discharged to a peeler centrifuge for dewatering. The peeler filtrate is recycled to starch refining. The dewatered starch is batch-wise peeled off and discharged by gravity to the moist starch hopper. STARCH DRYING From the moist starch hopper the starch is fed by a metering screw conveyor into a flash dryer and dried in hot air. The inlet air temperature is moderate. The dried starch is pneumatically transported to a starch silo ready for screening and bagging. The moisture of cornstarch after drying is normally 12-13 %.
Glucose RefineryProduction of Glucose Syrup and Dextrose: Figure highlights the production process for glucose and liquid starch. To produce glucose, the starch slurry is first treated with acid or enzymes and heated in a conversion process to break down the starch molecule, yielding degrees, ultimately resulting in producing a wide variety of glucose. Next, the corn syrup is refined using carbon to remove residual color, odor, taste, or flavor bodies. At this point, some of the corn syrup has the water removed from it to produce some types of glucose syrup (regular corn syrup). The remainder of the corn syrup goes through a process called ion exchange to remove additional flavor and color bodies that were missed during the previous stages of production. In this process, the syrup passes through anion resin and cation resin vessels. In the case of fructose syrups, additional ion exchange steps may be necessary to remove certain additional substances. Finally, the water from this corn syrup, is evaporated to yield some additional types of glucose syrup, dextrose, and high fructose corn syrup (HFCS). To produce liquid glucose, the original starch slurry is simply fermented and distilled.
Click here to load reader