Corn is an essential crop and one of the most consumed staple foods consisting of about 20% of human’s calorie intake globally. Scientifically named as Zea mays. Indigenous populations in Mexico first domesticated corn around 10,000 years ago, it has evolved through thousands years of intensive breeding and shown powerful usage for food, feed, fiber, and fuel. Known as the mother grain of Americans and the driver of the US economy, the United States of America produce about thirty-five percent of the total world maize production.
Zea is an ancient Greek word, means ‘sustaining life’, and mays is a word from the Taino language meaning ‘life giver’. The genus Zea consists of four species, of which Zea mays L. is the economically significant. When the maize reached Europe, it was referred to as mays or maizium, but the common names varied when it spreads to different countries. Corn is a general term for all types of cereals, and so it was called ‘Indian corn’. Wheat is referred to as corn in England, whereas oats are called corn in Scotland. Generally, corn had been used to indicate any small particle-like grains or any small, round object by extension. The other Zea species, like teosintes, are essentially wild grasses and ancestors of various corn cultivars.
The earliest archaeological maize found in the highland Mexico dated back to five to six thousands years old. The transition of maize from a weed species to a cultivated crop species was accomplished by the early maize breeders, who created selection pressure using simple mass selection based on the performances of individual plants that are less-branched with increased yield and larger kernels. Around 3000 years ago, corn breeders made selection primary on traits with increased protein levels and better starch qualities.
The early mass selection used by native Americans has been very successful with a large accumulation of corn ancestors with diversified genetics, not just primary corn types that are adapted to different climate and agricultural systems, but also valuable pure lines had been maintained, which altogether provided abundant germplasm pool for open-pollinated corns. Corn is an excellent genetic model plant, a single corn cob can produce 500-1200 kernels and each kernel represents a potential offspring thus no need to plant thousands of individual plants. Also, it’s relatively easier to separate offspring plants by the color and shape of kernels. Corn bear terminal male panicles with solitary ‘ears’, whereas the female inflorescences have numerous spikelets in rows on a thick axis enclosed within a ‘husk’, from which only the long silky styles emerge. These are followed by a ‘cob’ of fleshy kernels. Open-pollinated corn is developed by allowing cross pollination naturally. From traditional mass selection breeding to open-pollinated corn, the selection evolved from the simplest type of mass selection for intra-population improvement to the complex procedures of reciprocal selection for inter-population improvement. The hybrid innovation, also known as heterosis or hybrid vigor allows offspring plants achieve significant improvement in yields and size when two different parents are crossed. The hybrid revolution is the foundation for modern corn, in which molecular marker-assisted selection, and transgenic techniques are integrated for fine selection. At this stage, breeders are able to select traits that have better pest resistance and stronger stress tolerance.
Corn can grow almost everywhere, from north to south, moist to well-drained soil. It can be propagated from the seed in late winter or early spring. There are six major types of maize, which are dent corn, flint corn, pod corn, popcorn, flour corn, and sweet corn. Sweet corn is a sugar-rich variety, usually grown for human consumption. The field corn variety is mostly used as animal feed. The animal feed is made of dried kernels taken off the cob, by process known as shelling. Various corn varieties can be grind into cornmeal or Masa Harina, pressed into corn oil or used in fermentation and distillation into alcoholic beverages like bourbon whiskey. Maize is also used in making ethanol and other biofuels production.
Inflorescence and pollination
This robust annual grass grows up to twelve feet tall with arching, lance-shaped, waxy leaves. The terminal panicles of male flowers is up to eight inches and female inflorescence of the same length. Many other types of grass produce perfect flowers in a single inflorescence, yet the maize inflorescence is a naturally cross-pollinated plant that has two distinct and separated inflorescences referred to as the tassel and ear, from tassel pollen grains are produced profusely that can be easily blown far away by the wind. The ears, which is the separate ovuliferous inflorescences catch pollen easily that yield kernels or seeds. Male inflorescences in maize and teosinte are distichous, arranged alternately in two opposite vertical rows. During the condensation, some male inflorescences appear as polystichous in terms of rows of spikelets. Female inflorescences in the teosintes are always distichous, whereas in maize, they are always polystichous, and this is the main characteristic that separates the ear phenotypes of teosinte and maize.
Pollination occurs when a pollen grain lands on silk and germinates a pollen tube that must navigate tissues of the pistil to reach the embryo sac and achieve fertilization. The pollen tube in maize first grows along with silk hair, penetrates the silk, and then grows within one of the two transmitting tracts of the silk. Multiple pollen grains typically land on each silk, yet only one pollen grain ultimately fertilizes the embryo sac. Thus, the competition between pollen grains imposes an intense election for growth and development. The formation of the seeds (corn) is the result of an almost complete cross-fertilization. The flowers are followed in the late summer or early fall by cobs with usually yellow, sweet, edible grains.
Maize pollinates through the wind, which can carry the pollen for many miles. Therefore, it’s almost impossible for wild maize to avoid hybridization with the cultivated maize that grew in nearby fields. Hybridization with cultivated maize may lead to lose of its own essential primitive characteristics and eventually unable to survive on its own as the repeated contamination of wild maize by its cultivated counterpart could have genetically ‘swamped’ the former. Some wild type maize have became rare or endangered, so it’s important to take measures to protect the natural sites for wild maize.
There are tens of thousands of different types of corn varieties, and some have been lost due to monoculture and industrial farming. Corn is widely used in culinary for their taste, flavor, or color, which the corn can be grouped based on the kernel structure that gives each corn unique culinary traits. Sweet corn (pale yellow mixed with white kernels) is the most common corn best picked at its early or ‘milk’ stage when it is still tender and juicy. Sweet corn is the ideal corn to be eaten fresh in its green or milk stage. ‘Purple Hopi’ variety of flour corn is a brightly colored corn composed mainly of soft starch, making it much easier to grind and digest and the most suitable to be processed into fine cornmeal. ‘Hopi Blue’, ‘Seneca White’, and ‘Apache Red’ are other varieties of flour corn.
Besides, flint corn has a hard and glassy outer layer that protects the small and soft endosperm inside the kernel. The popcorn made of flint corns can also be popped when heated. However, the kernels will crack open rather than fully explode. Its hardness property made the flint corn kernel an excellent variety to store very well that is less susceptible to post-harvest damage by insect and rodent predation.
‘Golden Beauty’ is a much-admired cultivar. The maize is a potential source of human nutrition and health. The maize kernel is an edible and nutritive part of the plant. The composition of maize kernel contains vitamin B (B1 Thiamine, B2 niacin, B3 riboflavin, B5 pantothenic acid, B6 pyridoxine), C, E (alpha and gamma), as well as K, folic acid, selenium, N-p-coumaryl tryptamie, and N-ferrulyl tryptamine. The B-complex vitamins in maize are good for skin, hair, heart, brain, and proper digestion. Vitamin E in maize oil, which is known as a critical chain-breaking antioxidant, prevents the promulgation of oxidative stresses in biological membranes and prevents the development of atherosclerosis through the intervention of maize oil in the diet.
The most recognizable types of food corn are sweet corn and popcorn, where flavor and kernel quality are of the highest importance. Another example, baby corn, is an immature ear harvested as silks begin developing.
Phytochemicals and nutritional values
The endosperm of the maize has high starch content that is made up of seventy percent of the maize kernel and provides energy for the germinating seed, allowing the embryo growth and development until the seedling can start the photosynthesis process. Due to transposons known as jumping gene, which is expected to be observed epigenetically, there will be different colors of kernels with different anthocyanin pigments expressed differentially by cells of the aleurone tissue through mosaicism. The mosaic of kernel colors, ranging from brown to colorless white or yellow due to an absence of anthocyanins, purple, spotted, or streaked that are beneficial to human health. Though the color does not represent the sweetness of the corn kernels. The abundant phytochemicals such as carotenoids, phenolic compounds, and phytosterols are also considered as valuable nutraceuticals.
Starch constitutes approximately seventy percent of the maize kernel and provides energy for germination, ensuring embryo development until photosynthetically active parts such as leaves emerges. Starch is deposited in the endosperm tissue as insoluble granules in the amyloplast stroma. Corn starch are pure carbohydrates without protein; therefore, it is a gluten-free and excellent alternative to flour thickeners in gravy and sauce recipes. In addition, it has the advantage for food thickener as the resulting gel is transparent instead of opaque.
Popcorn is one of the most favorite snack in the world. The unique trait that the kernel explores upon exposure to heat and the subsequent formation of giant “flakes”. The moisture kept inside form expand force throughout the kernel popping. The starch of the hard endosperm gelatinizes with the released steam and expands due to heat, followed by drying and hardening into flakes. Flake production is related to a higher ratio of hard to soft starch and a thicker pericarp that able to withstand building pressure from steam. The kernel colors generally range from yellow and white, which are the commercially important species. There are other color kernels including red, blue, purple, and nearly black. Interestingly, there are two main flake shapes: mushroom flakes are round with only a few wings and butterfly flakes that are irregularly shaped but with many wings.
Corn flour, which is derived from the corn grain is a common food ingredient. It is often used to thicken the marinades, gravies, glazes, casseroles, pies, and other desserts, as well as sauces or soups, or used to coat the meat before stir-frying to help the sauce adhere. It has a versatile property easily modified in many industries like paper products as an anti-sticking agent, textile manufacturing, and glucose supply in the medical industry. In addition, it is used to make corn syrup and other sugars.
Corn starch can be manufactured into bioplastics and downstream products such as airbags. Bioplastics are plastics that are made of plant-based products that are biodegradable over time or be compostable under certain microorganism-rich conditions.
Corn oil is widely used in cooking and as dressing for salads. The average corn kernel oil content in commodity maize is about four to five percent on a dry weight basis. Several high-oil maize feeding trials for poultry, hogs, and dairy cattle have shown increased growth rates and feed efficiency. Apart from its importance in feed applications, maize oil is refined vegetable oil, a valuable co-product derived from approximately ten to fifteen percent of maize grain processed for human consumption and industrial uses. In the wet milling process, oil is extracted from embryos after being separated from the starch-rich endosperm.
Corn oil is good frying oil with a very high smoking point about four hundred and fifty Fahrenheit or two hundred and thirty-two degrees Celsius. A tablespoon of corn oil that is about fifteen milliliters provides one hundred and twenty-two calories. The total amount of maize oil produced in the United States in the year 2015, which was 4.7 billion pounds. However, maize is not considered an oil-seed crop because of its relatively low oil content, but is qualify because of the large volume of maize production.
Corn oil is pure and contains no protein or carbohydrates. The oil contains fourteen percent saturated fatty acids, thirty percent monounsaturated fatty acids, and fifty-six percent polyunsaturated fatty acids. The refined maize oil contains linoleic acid, oleic acid, palmitic acid, stearic acid, and linolenic acid. Many vitamins and minerals are lost during corn oil extraction, which leaves a fair amount of vitamin E.
Furthermore, corn oil can be used as an industrial cleaner and lubricant, which can be involved in fuel for gasoline- and diesel-powered engines production. Moreover, it is found in many cosmetic products, liquid soaps, and shampoos.
There are some common corn pests and diseases especially when planted in high density. Anthracnose (Colletotrichum graminicola) is a fungal infection that caused the infection parts to show the signs of rot with large necrotic patches on leaves and stalk. Corn grey leaf spot, is another fungal disease infected by Cercospora zeae-maydis. Charcoal rot infected by Macrophomina phaseolina that produce black fungal fruiting bodies at the vascular strands of the corn stalk showing black stalk from inside to outside. When the foliage of the corn shows a rusty brown pustule, which is known as common rust (Puccinia sorghi), the leaves will have fungal mold that is resulting in chlorosis and stunting, then mottling, chlorotic streaking, and lesions as well as white striped leaves in a later stage. This disease is commonly referred to as downy mildew that many types of fungus can cause.
Corn will grow galls that are tumor-like tissues in which it is silvery white or greenish-white in color smut that turn into darker color contributed by internal black spores. The scientific name of the corn smut is called Ustilago zeae, which the fungus spores can survive in the soil for several years. In addition, Giberella zeae is the fungus that causes ear rot on corn that showed pink color mycelium in the ear of the corn, which causes the corn husk to adhere to the ear. Then, corn foliage will be infected by various bacteria and fungi that have the symptoms of having brown colored margin, water-soaked linear lesions with narrow stripes between the veins. If a group of the corn showing dwarf symptom, which means that they had been infected by maize dwarf mosaic virus. Other diseases are maize lethal necrosis, corn lethal necrosis, and pythium root rot that caused significant crop production constraints. Pests involved microorganisms like slugs, nematodes, and insects such as corn earworms, cutworms, armyworm, flea beetles, and mites.
Maize provides approximately fifty percent of calories for human and other animals. It is widely processed into various products such as cornmeal, grits, starch, flour, tortillas, snacks, and breakfast cereals. In addition, maize flour is used for male chapatis or flatbreads, consumed mainly in a few Northern states of India. Besides, roasted maize kernel can be used as a coffee substitute.
Corn silk is an important herb used traditionally by the Chinese, Native Americans, Turkey, the United States, and France to treat many diseases. The yellowish beard-like threads, which are the stigma of the female flower, has a long history used as a therapeutic remedy for various illnesses such as cystitis, edema, kidney stones, diuretic, prostate disorder, urinary infections, bedwetting, and obesity. It’s also an excellent antioxidant that has shown useful for anti-fatigue, anti-depressant, and kaliuretic. Corn silk is rich in phenolic compounds, particularly flavonoids, and consists of proteins, vitamins, carbohydrates, calcium, potassium, magnesium, sodium, volatile oils, steroids like sitosterol and stigmasterol, alkaloids, and saponins.
Huitlacoche, also known as cuitlacoche, corn smut, or Mexican truffle, is a fungus that grows on ears of corn. Although American farmers consider the fungus as a disease, Mexicans love the fungus as a delicacy. The fungus affects every part of the corn and causes the kernels to swell up into mushroom-like growths called galls. Huitlacoche is very popular in Mexico for their special flavor and taste. They are harvested when the galls are still immature, gathered two to three weeks straight away after the ear of corn is infected that still retain moisture. The mature corn smut gave a flavor that is mushroom-like, sweet, savory, woody, and earthy when it is cooked. The flavor compounds of the corn smut consist of stolon, vanillin, and glucose.
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