Introduction 

Corn is a domesticated grass that originated approximately seven thousand years ago in what is now Mexico when humans first learned to cross-pollinate plants and slowly turned a nondescript grass called teosinte into plump and productive modern corn.  It is also referred to as maize and in general, the two terms are synonymous.  It first spread across the world after the European discovery of the Americas and has proven to be one of the most adaptable crops in the world.  The evolution of different corn biotypes has created species that can grow from the tropics to the northern temperate zone, from sea level to twelve thousand feet altitude, and with growing periods that can range from six weeks to thirteen months.  More corn is produced than any other cereal worldwide and it is used for human consumption, livestock feed, corn starch which has both food and non-food uses, and fuel.  While it is the highest produced grain, it is the third most consumed as a staple food after wheat and rice.  Yield per acre has been steadily increasing for decades, showing an estimated increase in the United States to 172.0 bushels per acre in 2020 from 167.5 bushels per acre in 2019.  It is estimated that 60% to 70% of corn produced worldwide is used for livestock feed while the remaining 30% to 40% is used for producing human food and processed corn products.  The volume of the global corn market was approximately 1.118 billion metric tons in 2020 and is expected to grow at a CAGR of 5.3% from 2021 to 2026, reaching a volume of 1.524 million metric tons by 2026.  The United States, China, Brazil, and Argentina account for over two-thirds of worldwide maize production.  Ukraine is the fifth highest producing country and the only European country in the top ten for corn production.  Strong demand for exports is expected to raise prices in 2021/22.   There is tough competition for export crops as Argentina, Brazil, and Ukraine are expected to reach a combined increase in corn exports of 19.5 million tons but the United States is still the dominant producer and exporter of corn, accounting for over 36% of global exports.  In 2018/19, more than three hundred sixty-six million metric tons of corn were grown in the United States and about 14.3 percent of this was exported to over seventy-three different countries.  The increased demand for corn is driven by the growing animal feed and corn starch processing sectors, especially in China which is the worldwide leader in corn starch production and also accounts for a significant market share in other sectors like high fructose corn syrup and polyols.  The use of corn as a biofuel is increasing as well but there are also a number of concerns, such as a rise in market price in corn produced for consumption and the need to produce more cultivable areas to meet increased demand that could cause ecological damage.  Some models project that large-scale corn ethanol production may lead to decreases in food exports, higher prices, and greater global deforestation.   

Corn Composition, Harvesting, Processing, and Refining 

There are four parts to a kernel of corn: endosperm, germ, pericarp, and tip cap.  The endosperm is about 83% of the dry weight and is the source of starch energy for the germinating seed.  A hard endosperm kernel has the starch tightly packed together while a soft endosperm has loose starch. When corn dries in the field before harvest, moisture loss causes soft endosperm to collapse and form a dent in the top of the kernel, thus the term “dent” corn.  The germ is about 11% of the dry weight and is the only living part of the kernel.  It contains the essential genetic information, enzymes, vitamins, and minerals for a kernel to grow into a corn plant.  About 25% of the germ is corn oil which is the most valuable part of the kernel.  Corn oil is high in linoleic fatty acid and has a bland taste.  The pericarp is about 5% of dry weight and is the outer covering of the kernel that protects it from deterioration and damage by water, water vapor, insects, and microorganisms.  The tip cap is about 1% of dry weight and is the only area of the kernel not covered by the pericarp. It is the attachment point of the kernel to the cob.  Components consist of approximately 61% starch, 19.2% feed, 3.8% oil, and 16% water. 

The process of planting and harvesting corn can vary greatly depending on the climate and part of the world it is grown in, but in places with moderate and seasonal temperatures, it is a warm season annual that is best planted after the soil reaches 60°F.  This generally happens around two to three weeks after the last spring frost.  Fresh seeds are sowed about two inches deep in soil and four to six inches apart.  Rows should be spaced thirty to thirty-six inches apart.  Soil pH should be between 5.8 and 6.8.  Plants should be watered well after planting.  It is important to aerate the soil and remove weeds as corn is a heavy feeding plant and cannot compete with weeds.  Deep soil penetration can sever the shallow growing roots of corn.  Depending on the variety and growing conditions, corn is usually harvested between sixty and one hundred days after planting.   

Corn is ready for harvesting about twenty days after silk first appears. When ready, the silk will turn brown but the husks are still green.  Farmers check to make sure the corn is in the milk stage which is done by puncturing a kernel and checking for milky liquid.  If the liquid is clear, the kernels are not ready.  If there is no liquid, the farmer waited too long.  Corn is harvested with a grain combine that has row dividers that pick up corn stalks as the combine moves through the field.  Corn ears are broken off from the corn stalk and dragged into the combine.  A separator inside the combine divides the husks, kernels, and cob.  The kernels are stored while the cob and husks are dropped on the ground.  They help prevent soil erosion and return plant matter to the ecosystem.   

Approximately 20% of the annual corn harvest is used by industrial corn processors to produce a variety of products such as sweeteners, starches, oils, ethanol, and animal feeds.  The two main methods of corn processing are wet milling and dry milling.  In wet milling, corn is separated into relatively pure chemical compounds of starch, protein, oil, and fiber.  Further processing is usually required to make these compounds into the final desired product.  Starch is the primary product from wet milling and is processed into a variety of starch products or further refined into sweeteners sold in liquid or dry form.  Industrial dry milling is a less versatile and less capital intensive process that focuses primarily on the production of grain ethanol.  The main objective of dry milling is to separate the endosperm from the germ and pericarp as much as possible and per the dry name, it entails physical separation based on mass and requires no use of chemicals.  Both processes begin with cleaning after corn arrives at the processing plant by truck, barge, or railcar.  Shipments are inspected and cleaned multiple times to remove pieces of cob, dust, chaff, and foreign materials.  An average bushel of yellow dent corn weighs fifty-six pounds and after cleaning, corn is moved to storage silos that can hold up to three hundred and fifty thousand bushels before processing begins.   

Wet milling begins with steeping the corn in stainless steel tanks.  Tanks can hold up to three thousand bushels of corn and they are soaked for thirty to forty hours in 50°C water.  Kernels absorb water that increases moisture from 15% to 45% and doubles the size of the kernels.  Sulfur dioxide is added at about 0.1% to prevent excessive bacterial growth.  The acidity of the steepwater loosens the gluten bonds and releases the starch.  Corn is then ground to break the germ free from the other components.  The water is condensed to capture nutrients in the water for animal feed.  The ground corn water slurry flows to germ separators that spin the low density corn germ out of the slurry.  Germs are pumped on screens and are washed to remove any excess starch.  Oil is extracted from the germs by a series of mechanical and solvent processes for further refining into finished oil.  The remaining slurry is ground through a mill to release starch and gluten from kernel fiber.  Screening catches fiber but lets the starch and gluten flow through.  Fiber is piped to a feed house for use in animal feeds.  The starch & gluten suspension (known as mill starch) is piped to a centrifuge.  Gluten has a lower density than starch and is easily spun out for use in animal feeds.  The remaining starch typically has 1%-2% residual protein which must be removed by dilution and washing in hydroclones.  After this, the starch is usually 99.5% pure and at this stage, the starch can be dried and sold as unmodified corn starch, modified into specialty starches, or converted into corn syrups and glucose.  Syrup is made by liquifying the starch suspension in the presence of acids or enzymes that convert the starch to a low-glucose solution.  Refiners can halt acid or enzyme actions at key points to produce various types of sugars.  Conversion halted at an early stage produces low to medium sweet syrups and is allowed to proceed to nearly all glucose for sweeter syrups.  The finished products are further refined with various processes and excess water is evaporated.  Syrups can be sold directly, crystallized into pure glucose, or further processed into high fructose corn syrup.  From one bushel of corn, the following can be made: thirty-one pounds of starch OR thirty-three pounds of sweetener OR 2.5 gallons of ethanol PLUS eleven pounds of animal feed, 2.5 pounds of gluten meal, and 1.6 pounds of corn oil.   

Glucose is one of the most fermentable sugars and can be converted to alcohol by traditional yeast fermentation.  However, use of wet milling will ultimately result in lower ethanol than a dry milling process because some of the fermentable starch is lost during the various separations of products during the process.  Industrial dry milling includes particle size reduction of corn with or without screening separation while retaining a good portion or all of the original germ and fiber.  Much of the particle size reduction and separation is done with equipment similar to wheat flour milling, including hammer mills, stone mills, roller mills, screeners, sifters, specific gravity separators, and aspirators.  If the kernels are processed for ethanol manufacturing, they are ground into a medium-to-fine meal and the final products are fuel ethanol and Dried Distillers Grains (DDGS), the leftover mash which is an animal feed product.  Corn flour and related products are also made from the dry milling process.  There are some dry fractionation processes that have been introduced in recent years that occur after milling to remove non-fermentable components of the kernel, but they result in co-products with less purity than those produced from wet milling and also reduce ethanol yield because some of the fermentable starch is lost.  There is no globally recognized terminology for dry-milled corn products, but commonly used definitions are based on particle size and fat divides them into four categories: grits, meal, fine meal, and flour.   

Transgenic Corn and Genetic Engineering  

Genetically modified corn has been genetically modified through the addition of a small amount of genetic material from other organisms through molecular techniques.  The gene that produces a genetic trait of interest is identified and separated from the rest of the genetic material from a donor organism.  Corn that has been genetically modified for resistance to pests and to herbicides is used in multiple countries.  The practice of engineering genetically modified crops remains controversial because of health effect concerns, impact on insects other than those targeted by the genetic modification, impact on other plants, and environmental concerns.  The first varieties resistant to glyphosate herbicides were commercialized in 1996 by Monsanto and are known as Roundup Ready Corn.  There are also Roundup Ready Sweet Corn varieties, known as the Performance Series.  Roundup Ready seeds are referred to “terminator seeds” because the produced crops are sterile and farmers must purchase the most recent seed strain every year.  Bayer CropScience has developed Liberty Link Corn which is resistant to glufosinate.  Bt corn is a variant that has been genetically modified to express one or more proteins from the bacterium Bacillus thuringiensis.  The protein is poisonous to certain pests, especially the European corn borer which causes about a billion dollars in damage to corn crops each year.  The first genetically modified corn producing a Bt protein was approved in 1996 and subsequent genes have been introduced that kill corn rootworm larvae.  When a part of the plant is ingested by the pest, the protein binds to the gut wall and the insect stops feeding, eventually causing a breakdown of the gut wall and the invasion of bacteria in the body.  The endotoxin is considered very selective and is safe for humans, mammals, fish, birds, and other insects.  Despite Bt being a popular pesticide spray since being introduced in the 1960s, the use of Bt for genetic modification is a subject of strong debate.  One concern is that large-scale planting will render the endotoxin ineffective over time.  Because of this, the EPA requires that 20% of Bt field areas be planted with non-Bt corn.  In practice, this is difficult to enforce and one report states that nearly 20% of farmers in the United States Corn Belt overplant Bt corn.  Another type of genetically modified corn is drought resistance, known as DroughtGard and first launched by Monsanto in 2013.  Cross-contamination is a big concern and one incident in 2000 involved Starlink, a variety containing Bt that was approved for use in animal feed in 1998 but not for human food because it lasts longer in the digestive system than other Bt proteins.  Starlink corn was found in food destined for consumption by humans in the United States, Japan, and South Korea.  A recall was issued which began when taco shells sold in supermarkets were found to contain the corn.  Fifty-one people reported adverse effects to the FDA and twenty-eight were determined by the CDC to possibly be related to Starlink, although blood tests for these individuals showed no hypersensitivity to the Starlink Bt protein.  As more strains of genetically modified corn and other plants are developed, controversy will continue and new issues will emerge, making the need for proper testing and screening of genetically modified corn even more prevalent.   

Corn and NIR Spectroscopy  

NIR spectroscopy has emerged as a tool for rapid, non-invasive, and cost-effective analysis of parameters of interest in corn that could potentially replace traditional reference methods.  There are a number of quality parameters in corn that have been studied and predicted with NIR spectroscopy with results suitable for process control purposes. Other parameters have shown results good enough for screening purposes and more study and calibration work could improve the prediction results.   Sweet corn and supersweet corn are considered two important varieties that are appealing to consumers and classifying different cultivars as well as distinguishing between viable and non-viable seeds were examined in two different studies using NIR spectroscopy.  Good classification accuracy for the groups was obtained in both studies.  One in-depth study using NIR spectra of over twelve thousand kernels determined the effect of different varieties, producing areas, ears, and ear positions on the NIR spectra.  Results showed that genetic differences had the greatest influence on changes in the NIR spectra with producing area having a similar but smaller difference while the ears and ear position had much smaller effects.  Many nutritive quality parameters in corn can be correlated to NIR spectra and another study created calibration models for various carbohydrates, protein, fiber, and digestibility parameters, with good correlation shown for most parameters.  A practical application used calibration models to determine different nutritional parameters in various corn hybrids and then to estimate energy and digestibility rates, showing excellent results.  Another study examined determining corn seed germination rate using NIR spectroscopy.  While the calibration model showed good correlation, the sample size was limited and more work would be necessary before using this calibration in a practical setting.  Mycotoxins are a huge concern for corn farmers and one study examined classifying healthy corn grains and grains diseased with Fusarium mycotoxin.  While the classification rate was over 99% correct, it is certain that the classification is based on something else besides the toxin concentration as the threshold of detection for such toxins is far below the normal detection level for NIR spectroscopy.  While an indirect correlation is acceptable in NIR chemometric models, further examination is necessary to determine the basis for the correlation.  Maize cob is being used as a biomass fuel in some countries and NIR spectroscopy was examined as a method for determining gross calorific value in cobs, showing good results.  Another study used mapping of a maize recombinant inbred line population with NIR spectroscopy to track the response of water deficit of traits associated with biomass quality.  The results showed that water deficit favors cell wall degradability and that the breeding varieties that reconcile improved drought tolerance and biomass degradability is possible.  Classification of different transgenic corn plants is important for breeding purposes and one study examined classifying plants based on different genetic lines, showing results considered good enough for screening purposes.  A practical application used NIR spectroscopy to determine different nutritional components in seven distinct genetic groups within a specific corn germplasm collection.  The parameters predicted from the NIR spectra were used as a basis for checking progress based on the expected genetic gain.  All these parameters and measurements have been studied using NIR spectroscopy with results showing the potential to replace traditional reference methods. 

References 

Global Corn Market Outlook 

https://www.expertmarketresearch.com/reports/corn-market 

 USDA: Corn and Other Feedgrains Market Outlook 

https://www.ers.usda.gov/topics/crops/corn-and-other-feedgrains/market-outlook/ 

 USDA Report: Corn and Soybean Production Up In 2020 

https://www.nass.usda.gov/Newsroom/2021/01-12-2021a.php 

 Best and Easiest Ways to Plant Corn 

https://www.kellogggarden.com/blog/growing/best-and-easiest-ways-to-plant-corn/ 

 NCGA Corn Curriculum 

https://nebraskacorn.gov/wp-content/uploads/2010/07/unit9_TeachersKey.pdf 

 Processing Maize Flour and Corn Meal Food Products 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4260129/ 

 Corn Milling: Wet Vs. Dry Milling 

https://blog.amg-eng.com/corn-wet-milling-vs-dry-milling/ 

 The Corn Refining Process 

http://docshare01.docshare.tips/files/26054/260549449.pdf 

 Genetically Modified Corn – Environmental Benefits and Risks 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC212689/ 

 Genetically Modified Maize 

https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/genetically-modified-maize 

 Bt-Corn: What It Is and How It Works 

https://entomology.ca.uky.edu/ef130