Milling Maize

by Emily Wilson
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The geographical potential for growing maize (also known as corn in many parts of the world) is greater than any other cereal crop. It can be grown at high and low elevations in the tropics and beyond 50 degrees of latitude in temperate regions.

The majority of maize is grown in North and Central America (see chart on page 20). Although maize is grown in Oceania, this region's contribution to world production is not significant.

Maize is the highest yielding cereal, with a world average of 4.3 tonnes per hectare over the last three years. (Equivalent values for paddy rice and wheat are 3.8 tonnes and 2.7 tonnes per hectare, respectively.)

Although a yield of over 20 tonnes per hectare is the average in Kuwait, major producers achieve yields much less than this. For instance, in the United States, yields average 8.26 per hectare; France, 6.23; Argentina, 5.28; China, 4.86; Brazil, 2.69; Indonesia, 2.64; Mexico, 2.42; and India, 1.72.

Maize has many uses, the major one being feed. The use to which it is put depends upon the region of the world and the affluence of the population where it is consumed. It was a staple food among Aztecs, Mayas and Incas and remains so in Mexico, Central and South America. Although introduced into Europe and then to Asia and Africa at a much later date, maize has become a staple in many parts of Africa, forming the basis of important food items and both alcoholic and non-alcoholic beverages.

TYPES OF MAIZE. The Zea mays L., subspecies mays, group comprises soft or flour maize (amylaceae), waxy maize (ceritina), popcorn (everata), dent maize (indentata), sweet corn or sugar maize (saccharata) and pod maize (tunicata). All are cultivated to a greater or lesser extent.

The variety of greatest economic value is dent corn, so called because of a concavity on the outermost face of grains. Dent corn exists in yellow and white grained forms, and the preference for one or the other depends on the consumer region. In the United States, yellow corn is most widely used for starch production while for dry milling the desirability of lighter products creates a premium market for white grains. For alkali cooked products in South and Central America, there is a preference for white grains. In South Africa, white maize forms the basis of a widely consumed porridge; yellow maize is not an acceptable alternative as it has an animal feed image.

Yellow varieties contain the pigment zeaxanthin, which contains 200 to 900 mg of beta-carotene per 100g. Feeding products of these varieties to poultry increases pigmentation of skin and egg yolks and beta-carotene is capable of conversion into vitamin A.

Other grain colors, such as red and blue, occur in dent corn. Indeed, all types of corn are capable of displaying a range of grain color, often within the same cob.

Plant breeders have made magnificent achievements in increasing yields and useful diversity in maize. Most of the commercial maize grain crop in the U.S. is grown on F1 hybrids, whereby the advantages of heterosis, or hybrid vigor, can be exploited.

In addition, the properties of grain components have been varied to suit agronomic, nutritional and industrial requirements. Facilities now exist for transferring genes from other organisms, including animals, into maize. If public opinion permits the continued application of such genetic modification, the enormous potential that exists for "growing" many molecular species with medical or industrial uses may be realized.

MAIZE PRODUCTS. Currently, separation by wet milling into component chemical constituents accounts for about 80% of grain fractionation. Starch contributes about 88% of starchy endosperm and about 70% of total grain dry mass, making it by far the greatest single component and therefore the major product of wet milling.

Starch and modified starch are used mainly in the food industry and the paper industry, and dextrins are used mainly as adhesives. However, the majority of maize starch (85%) is converted to refinery products. Of these, 73% are high fructose syrups, with glucose syrups, dextrose, corn syrup solids and maltodextrins making up the balance.

Most corn starches are composed of about 25% amylose and 75% amylopectin. The two glucose polymers differ in molecular size and degree of branching, but breeders have produced varieties in which the relative proportions are dramatically varied.

Varieties with 100% (or almost 100%) amylopectin are described misleadingly as waxy, while types with unusually high amylose contents are known as amylomaize. High amylopectin content is associated with high starch paste viscosities and thermal and pH stability. Amylomaize starch does not gelatinize in the conventional way and it has applications in textiles, adhesives and gum candies.

Because ungelatinized granules are not easily digested, amylomaize shares some properties with dietary fiber. Its inclusion in cereal products can contribute higher fiber characteristics without changing its eating properties.

In sweet corn, or sugar maize, conversion of sugar into starch during grain maturation is delayed. The water soluble polymer phytoglycogen is synthesized so that, at the state of ripeness when consumed, sweet corn contains four to six times the percentage of sugar of other types.

A major component of variations among maize types is the texture of the starchy endosperm. Flint maize is characterized by a complete peripheral layer of corneous (or horny) endosperm, which can extend almost to the center, where a core of floury endosperm exists. Flint maize is the predominant type grown in Europe, Asia, Central and South America and parts of tropical Africa.

In popcorn, which may be regarded as a special type of small-grained flint maize, the starchy endosperm is virtually all corneous, and it is this feature that leads to impermeability to steam produced from moisture in the interior of the grain on heating. Ultimately, steam pressure builds up and leads to popping. At the other extreme, soft or flour corn has endosperm that is entirely floury. It is grown in western South America, South Africa and drier parts of the United States.

Dent maize has a cup-shaped area of horny endosperm at the embryo end of the grain with floury endosperm filling the "cup." When dry milled, the floury part breaks down into small particles but the horny part remains as "grits" of relatively large particle size. Of all the products of dry milling, it is these grits that have the highest value. They are used in the manufacture of ready-to-eat breakfast cereal, extruded snacks and fermented products.

Dry milling accounts for only a small proportion of corn processing, the volume in the U.S. having changed little over the last 15 years, during which time wet-milling volume has doubled.

The second valuable component of the maize grain is the embryo. In both wet and dry milling processes, the embryo is separated as germ. Its value lies in the high quality oil that is extracted, and breeders have successfully increased the total oil and protein contents of grains by producing lines with larger than normal embryos.

Increased protein and oil also are of value in stock feeding as the nutritional balance is improved. Proteins stored in the endosperm of all cereals are deficient in some essential amino acids both for human and farm animal nutrition. Breeders have consequently bred lines with improved amino acid balance combined with acceptable textural properties and yield.

Even varieties of corn with unmodified protein balance make a valuable, if not complete, contribution to the nutritional needs of people and animals, whether used directly as food and feed or fractionated by wet and dry milling into co-products of starch production. Zein proteins recovered from co-products also have potential for modification to meet industrial requirements for adhesives, varnishes and coatings.

MEETING A VARIETY OF NEEDS. It is clear that maize is an extremely versatile cereal, with many types and many applications. Consumers and processors select from the many types that exist, either naturally or through the efforts of breeders and biotechnologists, to suit their own specific needs. Processors require samples that give high yields of the highest value components, suit their process and permit easy separation of components under the conditions that they use.

In addition, traditional preferences, either rational or otherwise, may influence choice and acceptability. With such diverse needs as exist among purchasers of maize grain, authorities that set trading standards can do little more than reflect those requirements that are common to most who buy grain.

They wish to buy whole grains that are true to type, well filled and free from disease, heat damage contaminants and toxins. It is these criteria that form the basis of grading. The U.S. Standards for Corn specify grades on the basis of test weight and content of damaged kernels, broken kernels and foreign matter, but samples may be refused a grade if they have an unacceptable odor or are considered of low quality in other ways.

Classes exist for white, yellow and mixed corn, and special classes exist for flint corn, flint and dent corn and waxy corn.

Tony Evers, based in London, is a former head of the Cereals and Milling Department at Campden and Chorleywood Food Research Association (U.K.) Ltd. He is now a consultant for the cereals industry.