What is corn
Corn also referred to as maize is a domesticated grass that originated approximately 10,000 years ago in what is now Mexico 1. Ancient Mexican farmers took the first steps in domesticating corn when they simply chose which kernels (seeds) to plant. These farmers noticed that not all plants were the same. Some plants may have grown larger than others, or maybe some corn kernels tasted better or were easier to grind. The farmers saved kernels from plants with desirable characteristics and planted them for the next season’s harvest. This process is known as selective breeding or artificial selection. Corn cobs became larger over time, with more rows of kernels, eventually taking on the form of modern corn.
Corn (maize) was spread across the world shortly after the European discovery of the Americas. Regardless of origin, corn has proven to be one of the most adaptable crops. Its evolution apparently occurred mainly under domestication and resulted in biotypes with adaptation ranging from the tropics to the north temperate zone, from sea level to 12,000 feet altitude, and growing periods (planting to maturity) extending from 6 weeks to 13 months 2. Currently, the United States, Brazil, Mexico, Argentina, India, France, Indonesia, South Africa, and Italy produce 79% of the world’s corn production 3. Between 1990 and 2011, the number of millions of corn hectares harvested ranged from 129.1 to 163.9. During the same period the production of corn in metric tons per hectare increased from 3.7 to 5.1, and total corn production increased from 482.0 to 832.5 million metric tons. Worldwide, 60–70% of corn production is used domestically as livestock feed, and the remaining 30–40% is used for production of items for human consumption 4.
Corn is the main cereal grain as measured by production but ranks third as a staple food, after wheat and rice. The reasons for this fact are varied, but some of them are related to cultural or social preferences and also because in some countries, corn is cultivated as livestock feed. More recently, the use of corn as a biofuel has generated great concern about rises in the market price of corn for consumption, the need to increase cultivable areas, as well as water quality and other ecological damages.
Corn kernel anatomy
The corn kernel is composed of four primary structures from a processing perspective. They are endosperm, germ, pericarp, and tip cap, making up 83%, 11%, 5%, and 1% of the corn kernel, respectively (Figure 1) 5. The endosperm, the main energy reserve, makes up about 83 percent of the total weight of the kernel. The endosperm is about 90 percent starch and surrounded by a protein matrix ~7 percent gluten, with the remainder consisting of small amounts of oil, minerals and some trace constituents. Two main types of starch include hard or vitreous, and soft or opaque. Vitreous endosperm is negatively related to starch degradability and in vivo starch digestibility in ruminants 6.
The embryo or germ of the corn kernel contains a miniature plant made up of a root like portion and five or six embryonic leaves. In addition, there are present
large quantities of high energy oil (33.3%), enzymes and nutrients to feed the tiny plant when it starts to grow, as well as many substances required during germination and early development. The germ also contains vitamins from B complex and antioxidants such as vitamin E. Corn germ oil is particularly high in polyunsaturated fatty acids (54.7%), which are subject to oxidative and other forms of rancidity resulting in off or objectionable flavors from full-fat corn products.
Pericarp is a high-fiber (8.8% crude) semipermeable barrier surrounding the endosperm and germ, covering all but the tip cap. The tip cap is the structure through which all moisture and nutrients pass through during development and kernel drydown. The black or hilar layer on the tip cap acts as a seal.9 The term bran is also used to refer to the fiber-rich outer layer (pericarp) that contains B vitamins and minerals and the tip cap.
Corn variations may be artificially defined according to kernel type as follows: dent, flint, waxy, flour, sweet, pop, Indian, and pod corn. Except for pod corn, these divisions are based on the quality, quantity, and pattern of endosperm composition, which defines the size of the kernel, and are not indicative of natural relationships. Endosperm composition may be changed by a single gene difference, as in the case of floury (fl) versus flint (FI), sugary (su) versus starchy (Su), waxy (wx) versus nonwaxy (Wx), and other single recessive gene modifiers that have been used in breeding special-purpose types of corn 7.
Figure 1. Components of the corn kernel5]
Corn nutrition facts
Raw, yellow, sweet corn kernels are composed of 76% water, 19% carbohydrates, 3% protein, and 1% fat (Table 1). In a 100-gram serving, corn kernels provide 86 calories and are a good source (10-19% of the Daily Value) of the B vitamins, thiamin, niacin, pantothenic acid (B5) and folate. In moderate amounts, they also supply dietary fiber and the essential minerals, magnesium and phosphorus whereas other nutrients are in low amounts
Corn has suboptimal amounts of the essential amino acids tryptophan and lysine, which accounts for its lower status as a protein source 8.
As can be observed, the corn bran is a significant contributor to corn vitamin and mineral content. The wet milling of corn separates much of its nutrient content away from the starch component.
In addition to chemical composition, physical characteristics of corn in the commercial market place influence the value of the grain or the final product. Often, countries will have grading standards for corn entering the supply chain to assist buyers and sellers assessing corn value. Test weight, moisture content, foreign material, and damage are among typical measures of corn quality and value 9.
Corn color is important to specific consumer groups in Central and South America as well as in Africa, and white corn is preferred for food consumption. In some areas of the world, such as North America, the desired color depends on the region or the food use. For example, in the eastern United States, cornmeal, grits, and homily are white, while in the northern part of the country, corn meal, and corn products used for breakfast cereals and snack foods are expected to be manufactured with yellow corn.
Corn meal or flour composition can also be driven by regional preferences, with some preferring whole ground corn rather than degerminated or partially degerminated corn products. The nutrient composition, including vitamins, minerals, and antinutrient factors, are influenced by local product preferences, which include not only the way the corn product is consumed, but also what other food items or additives are part of a complete meal. Local and regional standards for both the form and composition are to be evaluated as part of planning a corn flour or cornmeal fortification program.
Corn products and processing methods are as diverse as the corn crop itself.
Table 1. Corn sweet yellow (raw) nutritional facts
Value per 100 g
cup 145 g
ear, large (7-3/4″ to 9″ long) yields 143 g
ear, medium (6-3/4″ to 7-1/2″ long) yields 102 g
ear, small (5-1/2″ to 6-1/2″ long) 73 g
|Total lipid (fat)||g||1.35||1.96||1.93||1.38||0.99|
|Carbohydrate, by difference||g||18.70||27.11||26.74||19.07||13.65|
|Fiber, total dietary||g||2.0||2.9||2.9||2.0||1.5|
|Vitamin C, total ascorbic acid||mg||6.8||9.9||9.7||6.9||5.0|
|Vitamin A, RAE||µg||9||13||13||9||7|
|Vitamin A, IU||IU||187||271||267||191||137|
|Vitamin E (alpha-tocopherol)||mg||0.07||0.10||0.10||0.07||0.05|
|Vitamin D (D2 + D3)||µg||0.0||0.0||0.0||0.0||0.0|
|Vitamin K (phylloquinone)||µg||0.3||0.4||0.4||0.3||0.2|
|Fatty acids, total saturated||g||0.325||0.471||0.465||0.331||0.237|
|Fatty acids, total monounsaturated||g||0.432||0.626||0.618||0.441||0.315|
|Fatty acids, total polyunsaturated||g||0.487||0.706||0.696||0.497||0.356|
|Fatty acids, total trans||g||0.000||0.000||0.000||0.000||0.000|
Table 2. Vitamin content of whole kernel, crude bran, and corn starch of yellow corn
Table 3. Mineral content of whole kernel, crude bran, and corn starch of yellow corn
Industrial corn processing
There are two basic categories of industrial processing employed for transforming corn into products for human consumption. They are known as dry and wet milling. In the wet milling process, corn is separated into relatively pure chemical compound classes of starch, protein, oil, and fiber. The products and coproducts obtained from wet corn milling are not typically directly used by the consumer and often require further industrial processing before consumption. The products of wet corn milling are not typically produced on a small scale commercially or in the home. The primary product, starch, can be processed into a variety of starch products or further refined into a variety of sweeteners sold in liquid and dry forms.
Industrial dry milling includes particle size reduction of clean whole corn with or without screening separation, retaining all or some of the original corn germ and fiber 11. Because of the high-fat content, these whole or partially degerminated corn products are not particularly shelf stable. Degermination of corn involves mechanical separation and processing, resulting in dry shelf-stable products with a majority of both germ and fiber removed. Much of the particle size reduction and separation is accomplished with equipment similar to that employed in wheat flour milling, including hammer mills, stone mills, roller mills, screeners, sifters, specific gravity separators, and aspirators. Specialized equipment, such as degerminators and de-hullers or peelers, may be employed in corn processing.
Generally, whole, partially degerminated, and degerminated corn products require additional processing before consumption. These processing steps may be accomplished in a large-scale industrial setting, small-scale local processor, or in the home. These secondary processes may include addition of other ingredients along with thermal processing, including boiling, drying, frying, or baking, all of which can affect the nutritional attributes of the finished product.
A second type of industrial dry corn processing is alkali processing or nixtamalization in which whole corn is cooked with an excess of water treated with calcium oxide 12. The corn kernel may be ground whole, fractionated, or have other corn components added. Unlike wheat flour milling, processing equipment for alkali-treated corn is specialized to handle the moisture, chemicals, and heat required for wet processing. Conventional dry bulk material handling and processing equipment is employed with raw corn and dry finished product. The resulting intermediate product may be dried for commercial sales of further processed consumer food product. In North America and Mexico, dry alkali-processed corn flour is known as masa flour, a name that is not used in Spanish-speaking countries of Central and South America or non-Spanish speaking countries of Africa. These products are referred herein as alkali processed. The alkali process improves flavor, starch gelatinization, and water uptake. The process partially removes some of the germ and most of the pericarp, but the amount varies. In some cases, pericarp may be added into the process for visual product enhancement. The heating in the process causes loss of thiamine, riboflavin, niacin, fat, and fiber. As might be expected the calcium content increases owing to the alkali processing.
In the nixtamalization process, there are several stages. First, dried corn is soaked in a solution of water with lime, often with ashes mixed in. The grain is then cooked, steeped, drained, and rinsed multiple times. The grain is then ground to make a wet dough from which tortillas are formed or allowed to dry into flour. Currently, there is an important diminution in production of homemade tortillas because they are now prepared from commercial instantaneous flour or bought as packaged tortillas 11, 13. Nixtamalized corn has several benefits compared to unprocessed grains: they are more easily ground and have a higher nutritional value (increased bioavailability of niacin, improved protein quality, increased calcium) and reduced mycotoxins content 14.
A staple corn product in South America, particularly in Venezuela and Colombia, is arepa, which is a fried or baked bread prepared from precooked refined corn flour.
Traditionally, arepas are made by dehulling and degerming previously soaked whole kernels by manually grinding corn kernels in a pilon, a wooden mortar. The bran and germ are removed by repeatedly rinsing the mixture containing the endosperm with water. This fraction is then cooked and milled to prepare a dough that will be shaped and cooked (baked or fried) to obtain the arepa 15. The traditional process for preparing homemade arepas involves soaking, cooking, cooling, draining, grinding, and forming a dough piece for additional grilling or baking. The process takes 18 or more hours to complete in the home.
The traditional method has been modified with the introduction of precooked corn flour 16. This process includes conditioning, cooking, flaking, drying, grinding, and sifting to produce dry instant precooked refined arepa flour. The flour can be transported and stored easily until used in the home. Preparation is reduced to less than an hour, making it more convenient for consumers 17. Although arepa is prepared from flour that is 100% corn, there are commercial presentations of mixtures of corn with rice, corn with wheat, and corn with oat and wheat bran.
Fermented corn products, such as ogi, are prepared by soaking the corn kernel for 1–3 days until soft. It is then grinded with a stone and dehulled and degermed by repeated washes with water. The filtered endosperm is fermented for 2 or 3 days, producing a slurry that becomes the ogi porridge when boiled. Fermented products that are similar but prepared with different corn varieties or minor preparation changes include uji in Kenya, kenkey, banku, ogi, and koko from Ghana and Nigeria 18, 19. Nixtamal is another fermented corn product, prepared from dehulled kernels ground to a coarse dough and wrapped in banana leaves to ferment for 2 or 3 days 20.
Figure 2 provides a schematic of three of the corn dry-milling processes: whole or refined, nixtamalized, and precooked corn flours. There is a common, similar initial process between household and industrial preparation of nixtamalized and precooked corn flours that is taken from the traditional way of household preparation. At the point of product 1, the masa or dough is obtained and final products could be prepared. Industrial processing from this point produces the commercial flour that only needs added water and to be cooked at the household level, to obtain the traditional product without repeating all the processing on a daily or regular basis.
Figure 2. Schematics of dry-milling maize processing5]
Separation of corn constituents (e.g., dehulling or degerming) varies depending on regional customs and consumer preference. These differences affect the vitamin and mineral content of the finished product from primary processing and should be taken into consideration when developing a maize fortification strategy. Yield, fat, and fiber content of the corn product from the primary processes will be directly proportional to nutrient content. Particle size will also be important to the fortification strategy as presented later in the article. Fortification of products from the secondary processing of corn becomes widely varied and inherently more difficult to manage.
The products derived from dry milling are numerous, with their variety depending to a large extent on particle size. In Africa, ground corn is cooked into a paste accompanied by a thick low-alcoholic beer. This corn paste could be fried or baked, depending on the region of Africa. Many Africans depend on some variation of this mush, which is made with water and ground corn. It can also be eaten as a porridge or a dumpling, depending on the thickness of the batter and the cooking method 21. In Kenya, they prepare uji, a porridge of corn flour cooked in water and sweetened with sugar 22.
Other corn preparations include humitas prepared from precooked corn flour, mote made from cooked corn and cheese, pupusas made from lime-treated corn and cheese, and patasca, which is like a lime-treated corn kernel 20. Table 4 shows a variety of maize products of global interest 12.
Table 4. Various corn products consumed globally
|Flat, unleavened, unfermented||Tortilla, arepa|
|Fermented and/or leavened||Pancakes, cornbread, hoe cake, blintzes|
|Porridges||Atole, ogi, kenkei, ugali, ugi, edo, pap,|
|Fermented, unfermented||maizena, posho, asidah|
|Steamed products||Tamales, couscous, rice-like products, Chinese breads, dumplings, chengu|
|Alcoholic||Koda, chicha, kafir beer, maize beer|
|Nonalcoholic||Mahewu, magou, chicha dulce|
|Snacks||Empanadas, chips, tostadas, popped corn, fritters|
Attempts have been made to classify and define products of corn processing; however, there is not a globally recognized terminology for dry-milled corn products 23. Table 5 identifies the commonly accepted terms used according to ranges of particle size for corn products. The fat values are for degerminated maize products 24. Some have subdivided the definition of maize meal into smaller size categories to include coarse meal (1190–730 μm), medium meal (730–420 μm), and fine meal or cones (420–212 μm) 25.
Table 5. Degerminated corn products defined by particle size and fat content
|Less than (μ)||Greater than (μ)||Fat (%)|
Table 6 provides approximate vitamin and mineral analysis for white maize, whole-grain maize flour, degerminated meal (unenriched and enriched), alkali-processed maize or masa flour (unenriched and enriched), and precooked corn flour (unenriched and enriched) 10, 26. Degerminated maize products are clearly lower in fat, fiber, and ash content when compared to whole maize or whole maize flour. The influence of alkali processing on the calcium level of the finished product is readily apparent given the higher values of calcium reported.
Table 6. Proximal analysis of vitamin and mineral content of white corn, flour, meal, and alkali-processed masa, unenriched and enriched
|Corn flour,||Cornmeal,||Cornmeal,||Corn flour,||Corn flour,||Precooked||Precooked|
|Corn flour,||degermed,||degermed,||degermed,||masa,||masa,||corn flour,||corn flour,|
Note: Data from U.S. Department of Agriculture 10 and the Flour Fortification Initiative 27.[Source 5]
Uses of corn
- Corn Oil
About 95 percent of domestically produced corn oil is from the corn wet milling industry, and all but a small fraction is used in one form or another as food.
Crude corn oil, because of the natural antioxidants it contains, undergoes little deterioration when stored for long periods, provided the temperature is kept well below 40°C (102°F) and moisture plus volatile matter level is below 0.4 percent. Since virtually all refined corn oil is utilized in foods, the need to attain a quality suited to such use guides the refining process. Steps include degumming to remove phosphatides, alkali treatment to neutralize free fatty acids, bleaching for color and trace element removal, winterization (the removal of high-melting waxes) and deodorization (steam stripping under vacuum). Crude corn oil enters the process via preliminary filtration. Degumming removes phosphatides and other materials that may be precipitated or dissolved from the crude oil by hot water. This step is usually omitted in refineries that process only corn oil, but is used in refineries that are set up to refine soybean oil as well as corn oil. Degumming is accomplished by introducing hot water at a level of 1 to 3 percent of oil volume, or by injecting an equivalent amount of steam to hydrate the phosphatides. By removing free fatty acids and phospholipids from crude corn oil, the oil refining process gives corn oil one of the qualities consumers value most: its excellent frying quality and resistance to smoking or discoloration. Corn oil is regarded highly for its functionality, exceptional flavor, economy, and health benefits. It is a concentrated source of energy, is very digestible, provides essential fatty acids and Vitamin E, and is a rich source of polyunsaturated fatty acids, which help regulate blood cholesterol levels and lower elevated blood pressure.
Corn oil has replaced a significant amount of saturated fat in numerous food products. It is also a top choice for trans fat reduction. In addition to snack food applications, corn oil can be an effective component in reducing trans fats in restaurant settings. Laboratory frying tests show that corn oil performed close to parity with cottonseed oil when used to fry frozen potatoes. Corn oil can also be interesterified with fully hydrogenated vegetable oil to produce trans free margarines.
Table 7. Corn oil
Value per 100 g
|Total lipid (fat)||g||14.00||100.00|
|Carbohydrate, by difference||g||0.00||0.00|
|Fatty acids, total saturated||g||2.001||14.290|
|Fatty acids, total monounsaturated||g||4.000||28.570|
|Fatty acids, total polyunsaturated||g||8.000||57.140|
|Fatty acids, total trans||g||0.000||0.000|
- Corn Starch
Starch is one of nature’s major renewable resources and a mainstay of our food and industrial economy. Basic consumer necessities such as paper and textiles are major uses for corn starch in sizing, surface coating, and adhesive applications 28. Corn starches, and their derivatives dextrins (a roasted starch), are used in hundreds of adhesive applications. Special types of starches are used in the search for oil as part of the “drilling mud” which cools down superheated oil drilling bits 28. Other key uses of starch in American industry are as flocculating agents, anticaking agents, mold-release agents, dusting powder, and thickening agents 28.
Literally, thousands of supermarket staples are produced using both regular and specially modified starches. Many of today’s instant and ready-to-eat foods are produced using starches which enable them to maintain the proper textural characteristics during freezing, thawing and heating. You will also find starch in a wide variety of household items such as batteries, matches, cleaners, and trash bags. Many personal and health care items including cosmetics, deodorant, hair styling products, asprin, cough drops, and medicines, include starch in their ingredients.
Clyclodextrins are enzyme-modified starches that have the ability to encapsulate ingredients such as vitamins, flavors, and medicines, which allows controlled release of active ingredients and protects ingredients that might otherwise be incompatible in product formulations 28. Cyclodextrins are used in products such as laundry sheets, which release their fragrance and softener in the dryer. They can also be used to remove cholesterol from milk and eggs. Corn starch can be manipulated in a variety of ways to produce products that benefit the environment.
Corn starch combined with polymers creates a super absorbent used in disposable diapers, sanitary napkins, bandages, and baby powders, and can be used to remove water from fuels and to clean up pesticide spills. Extrusion, the same process used to make snack foods, can alter the physical structure of corn starch to make totally biodegradable packaging peanuts. Starch granules can be broken down into nanoparticles to form adhesives that can replace petroleum-based acetates and alcohols used to help laminate graphics onto cardboard and latexes used as binders in paper coatings. The most promising new market for corn starches is as raw material for the production of industrial chemicals and plastics which are today made from petroleum feedstocks. As petroleum supplies dwindle or become less reliable, the importance of an abundant source of basic industrial chemicals takes on new proportions. Corn industry scientists are at work on new systems for producing industrial necessities from the versatile corn plant.
- High Fructose Corn Syrup Sweeteners
High fructose corn syrup (HFCS), a sweetener made from corn, comes in two primary compositions—HFCS-42 and HFCS-55. This means it is composed of either 42 percent or 55 percent fructose, with the remaining sugars being primarily glucose and higher sugars (chains of glucose) 29.
In terms of composition, high fructose corn syrup is nearly identical to table sugar (sucrose), which is composed of 50 percent fructose and 50 percent glucose. Glucose is one of the simplest forms of sugar that serves as a building block for most carbohydrates. Fructose is a simple sugar commonly found in fruits and honey.
A simple comparison of the percentage of glucose and fructose reveals its similarities to table sugar.
- HFCS-42 = 42% fructose + 58% glucose
- HFCS-55 = 55% fructose + 45% glucose
- Table sugar = 50% fructose and 50% glucose
High fructose corn syrup and sugar (sucrose) have almost the same level of sweetness. High fructose corn syrup was made to provide the same sweetness as sugar (sucrose) so that consumers would not notice a difference in sweetness or taste. In fact, one type of high fructose corn syrup commonly used in foods (HFCS-42) is actually less sweet than sugar (~92% sweetness compared to sugar) 29.
What benefits does high fructose corn syrup provide other than sweetness ?
- Provide texture and enhance “mouth-feel”
- Act as preservatives that protect the flavor, aroma and color of fruit used in jellies, jams and preserves
- Help brown baked foods
- Provide fermentable sugars that help bread rise
- Retain moisture so high fiber products taste better and baked goods stay fresh
- Contribute to the bulk or volume of ice cream, baked goods, preserves and jams
- Reduce the harsh vinegar or acid bite in non-sweet foods such as salad dressings, sauces and condiments
- Control freezing, melting and boiling points of products.
- Animal Feed 30
Through different combinations of steepwater, corn germ residues, fiber, and corn gluten, corn refiners produce four major feed products: gluten meal, gluten feed, corn germ meal, and condensed fermented corn extractives (steepwater). Steepwater is the warm water (125 – 130 F) containing small quantities of dissolved sulfur dioxide that is used soaking cleaned corn kernels in a large tank called a steep. The steepwater controls fermentation and assists in separation of the starch and protein. During steeping, the soluble components are extracted from the intact kernel. At the conclusion of steeping, the water is drained from the kernels and concentrated in multiple effect evaporators to yield concentrated steepwater. This protein rich extract may be used as a nutrient for microorganisms in the production of enzymes, antibiotics and other fermentation products. The major portion, however, is combined with fiber and gluten in the production of animal feed ingredients 30.
Corn gluten meal supplies vitamins, minerals, and energy in poultry feeds; pet food processors value it for its high digestibility and low residue. Steepwater is a liquid protein supplement for cattle and is also used as a binder in feed pellets, and corn gluten feed provides protein and fiber for beef cattle.
Corn derived feed ingredients are one of America’s leading agricultural exports. More than $760 million of corn gluten feed and corn gluten meal are exported each year.
The term bioproducts designates a wide variety of corn refining products made from natural, renewable raw materials, which replace products made from non-renewable resources or which are produced by chemical synthesis. Fermentation of corn-derived glucose has given rise to a multitude of bioproducts including organic acids, amino acids, vitamins, and food gums. Citric and lactic acid from corn can be found in hundreds of food and industrial products.
They provide tartness to foods and confections, help control pH, and are themselves feedstocks for further products. Amino acids from corn provide a vital link in animal nutrition systems. Most grain feeds don’t have the amount of lysine required by swine and poultry for optimal nutrition. Economical corn based lysine is now available worldwide to help supplement animal feeds. Threonine and tryptophan for feed supplements also come from corn.
Vitamin C and Vitamin E – vital human nutritional supplements – are now derived from corn, supplanting old production systems which relied on chemical synthesis. Even well-known food additives such as monosodium glutamate and xanthan gum are now produced by fermenting a glucose feedstock. Biopolymers are a more recent development in the category of bioproducts, and an area that shows great promise.
Corn-based polymers including polylactic acid (PLA), polyhydroxyalkanoates (PHAs), and 1,3 propanediol (Bio-PDO) are high-performance, biodegradable alternatives to petroleum-derived materials. Biodegradable and energy efficient caps, cups, paper coatings, fabrics, carpeting, and a host of other products are all possible today because of corn-based biopolymers.
Henry Ford first suggested running cars on ethanol from corn, but it took the oil shortages of the seventies and the environmental problems of the eighties to turn ethanol into an important component in the American fuel supply.
Ethanol is made by fermenting sugars produced from corn starch. Many corn refining factories produce both ethanol and other corn products like starches and sweeteners so that capital and manufacturing costs can be kept as low as possible. While they are making ethanol, corn refiners also produce valuable coproducts such as corn oil and corn gluten feed.
Ethanol, blended with gasoline at a 10 percent level or in the form of ethyl tertiary butyl ether (ETBE) made from ethanol, is effective in reducing carbon monixide levels, ozone pollution, and greenhouse gas emissions from automobile exhaust.References
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