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The chemical components and nutritive value of maize do not lose their susceptibility to change when the grain is harvested. Subsequent links in the food chain, such as storage and processing, may also cause the nutritional quality of maize to decrease significantly or, even worse, make it unfit for either human and animal consumption or industrial use.
Maize harvesting is highly mechanized in developed countries of the world, while it is still done manually in developing countries. The mechanized system removes not only the ear from the plant but also the grain from the cob, while manual harvesting requires initial removal of the ear, which is shelled at a later stage. In both situations, maize is usually harvested when its moisture content is in the range of 18 to 24 percent. Damage to the kernel (usually during the shelling operation) is related to moisture content at harvest; the lower the moisture content, the less the damage.
Changes in the physical quality of the grain are often a result of mechanical harvesting, shelling and drying. The first two processes sometimes result in external damage, such as the breaking of the pericarp and parts around the germ, facilitating attack by insects and fungi. Drying, on the other hand, does not cause marked physical damage. However, if it is carried out too rapidly and at high temperatures, it will induce the formation of stress cracks, puffiness and discoloration, which will affect the efficiency of dry milling and other processes (Paulsen and Hill, 1985).
In tropical countries, drying is sped up by bending down the upper part of the plant holding the ear, a practice that also prevents the kernels from becoming soaked when it rains. In either mechanical or manual harvesting, the shelled kernels contain too much moisture for safe storage, and they must be dried to safe moisture levels of about 12 percent at 30°C and about 14 percent at 10°C (Herum,1987). Storage stability depends on the relative humidity of the interstitial gases, which is a function of both moisture content in the kernel and temperature. Low moisture content and low storage temperatures reduce the opportunity for deterioration and microbial growth. Aeration therefore becomes an important operation in maize storage as a means of keeping down the relative humidity of interstitial gases.
Significant maize losses have been reported in tropical countries. Losses of up to 10 percent have been found, not including those losses caused by fungi, insects or rodents. If these were included, losses could go up to 30 percent in tropical humid areas or 10 to 15 percent in temperate areas. Schneider (1987) reported post-production losses in Honduras of 6.5 to 8.7 percent in the field and of 7.4 to 13.9 percent in storage. Losses due to fungi (mainly aspergillus and penicillium) are important for both economic and health reasons because of aflatoxins and mycotoxins (de Campos, Crespo-Santos and Olszyna-Marzys, 1980).
In a survey on maize sold in rural markets in Guatemala, Martinez-Herrera (1968) found considerable contamination by several fungi. Among these, some Aspergillus species, well known as aflatoxin producers, were frequently present. There is evidence that maximum aflatoxin contamination of maize in Guatemala is during the rainy season. Samples analysed 20 days after maize was harvested had levels of 130 µg aflatoxin per kg of total maize. The same samples analysed 60 days later showed a great increase of up to 1 680 µg per kg. These data as well as data from several other studies strongly indicate the need to dry maize before storage. Diverse drying systems and equipment are available, using various sources of energy including solar energy (Herum, 1987). A number of factors must be considered such as temperature and air velocity, rate of drying, drying efficiencies, kernel quality, air power, fuel source, fixed costs and management. Drying is an important step in ensuring good quality grain that is free of fungi and micro-organisms and that has desirable quality characteristics for marketing and final use.
Drying Methods
Layer drying. In this method, the harvested grain is placed in a bin one layer at a time. Each layer of grain is partially dried, before the next is added, by forcing air through a perforated floor or through a duct in the bottom of the bin. To improve efficiency, the partially dried grain is stirred and mixed with the new layer. An alternative is to remove the partially dried grain and dry it completely in batches. One of the problems with this and other methods of drying is in finding a way to mix low-moisture grain with high-moisture grain to get the desired equilibrium in the final product. Spoilage often occurs in this attempt. Sauer and Burroughs (1980) reported that equilibrium was more than 80 percent complete in 24 hours. Methods have been developed to detect highmoisture maize in mixtures with artificially dried maize.
Portable batch dryers. Since drying installations are costly, few maize producers, particularly small farmers, can afford to have their own. Portable batch dryers are useful since they can be moved from farm to farm. These dryers operate with air heated to 140 to 180°F (60 to 82°C).
Continuous flow dryers. The principle behind these dryers is the continuous flow of grain through heated and unheated sections so that it is discharged dry and cool. The equipment is the central point in grain storage depots.
Biotic and non-biotic factors
The efficient conservation of maize, like that of other cereal grains and food legumes, depends basically on the ecological conditions of storage; the physical, chemical and biological characteristics of the grain; the storage period; and the type and functional characteristics of the storage facility. Two important categories of factors have been identified. First are those of biotic origin, which include all elements or living agents that, under conditions favourable for their development, will use the grain as a source of nutrients and so induce its deterioration. These are mainly insects, microorganisms, rodents and birds. Second are non-biotic factors, which include relative humidity, temperature and time. The effects of both biotic and nonbiotic factors are influenced by the physical and biochemical characteristics of the grain. Changes during storage are influenced by the low thermal conductivity of the grain, its water absorption capacity, its structure, its chemical composition, its rate of respiration and spontaneous heating, the texture and consistency of the pericarp and the method and conditions of drying.
Nutrient losses have been reported in maize stored under unfavourable conditions. Quackenbush (1963) showed carotene losses in maize stored under different temperature and moisture conditions. In other studies common and QPM maize were stored in different types of containers with and without chemicals. After six months samples were examined for damage by insects and fungi and for changes in protein quality. In both types there was some damage to the unprotected maize but not to that stored with chemicals. Protein quality was not affected (Bressani et al., 1982). Other changes subsequent to drying and storage included a decreased solubility of proteins; changes in nutritive value for pigs; changes in sensory properties (Abramson, Sinka and Mills, 1980); and changes in in vitro digestibility resulting from heat damage (Onigbinde and Akinyele, 1989).
Although damage caused by insects and birds is of importance, a great deal of attention has been paid to the problems caused by micro-organisms, not only because of the losses they induce in the grain, but more importantly, because of the toxic effects of their metabolic by-products on human and animal health.
Studies on the nutritional effects of insect infestation of maize are not readily available. Daniel et al. (1977) and Rajan et al. (1975) have reported losses in threonine and in protein quality of maize infested with Sitophilus oryzae. In the first study, protein efficiency ratio (PER) decreased after three months from an initial value of 1.30 to 0.91. In the second study, threonine decreased from 3.5 to 2.9 g per 16 g N and PER decreased from I .49 to I .16. These researchers also reported that the damaged maize was less efficient in complementing food legumes.
Also of nutritional significance was an increase in uric acid from 3.5 to 90.6 mg per 100 g after three months. Thiamine losses were detected as well.
Bressani et al. (1982) evaluated five chemicals and three types of containers for their effectiveness in protecting QPM's nutritional quality against insect damage. About 38 percent of the untreated grain (control) was damaged by insects. This did not, however, affect its protein quality.
Several research studies have identified an association between insect damage and toxin contamination (e.g. Fennellet al., 1978; Perez, Tuite and Baker, 1982).
Christensen (1967) measured selected changes in United States No. 2 maize stored for two years with moisture contents of 14.5 and 15.2 percent and at temperatures of 12, 20 and 25°C. Changes in condition were evaluated by appearance, fungal invasion, germination percentage and final fat acidity value. Samples stored at 25°C deteriorated rapidly at both levels of moisture content. The samples with 15.2 percent moisture changed slightly after six months at 12°C but appreciably after two years. The maize stored with 14.5 percent moisture content retained its original condition when kept at 12°C for the twoyear period and changed only slightly in 18 months at 20°C. However, large variability in the insect-fungi interaction was observed. Some maize-growing regions have experienced extensive insect damage to maturing ears with no occurrence of aflatoxin, while other areas with equivalent insect damage have exhibited relatively broad incidences of the toxin in kernels at harvest.
Many studies have been conducted to assess the nutritional value of mouldy maize. Although some increase in B-vitamin content has been reported, possibly as a result of the metabolites of the micro-organisms, the damage to animal health far exceeds any beneficial change in chemical composition. Several researchers have studied the impairment in nutritive value of mould-damaged maize. For example, Martínez et al. (1970a) found significant negative effects in poultry and laboratory rats fed mouldy maize. It is difficult, however, to decide whether these effects were caused by fungi-produced toxins or by a loss in nutrients in the substrate because of their utilization by the micro-organisms.
Christensen and Sauer (1982) reviewed the effects of fungal invasion on cereal grains. They found that it reduced both the quality and grade of the grains through loss of dry matter, discoloration, heating, cooking, mushiness and contamination by mycotoxins. Microbial indices of fungal invasion and seed deterioration include visible damage, seed infection, number of fungal propagules, evolved carbon dioxide and decrease in seed germination and ergosterol content.
Inhibition of atlatoxin contamination
Two ways of preserving maize from being destroyed by aflatoxin contamination have been under investigation. One is to inhibit growth of Aspergillus flavus or Aspergillus parasiticus and the other is to remove the aflatoxins after they have been produced by the Aspergillus infection. Most researchers have concentrated on the inhibition of fungal growth, and some chemicals have already been found effective in storage conditions. This, however, does not solve the problem of field contamination by moulds, since the airborne spores of the organisms are readily available in the environment. The spores can germinate on the cob and infect the inner tissues under optimum temperature and moisture conditions. Therefore, other researchers have pursued the possibility of detoxification.
Roasting has been shown to be effective in reducing aflatoxin levels, depending on the initial level of the toxin as well as on roasting temperatures (Conway and Anderson, 1978). Higher temperatures may cause up to 77 percent aflatoxin destruction; however, it is well known that heat also destroys the nutritive value of the material. Tempering aflatoxincontaminated maize with aqua ammonia and then roasting it may be a simple and effective way to decontaminate it. Valuable results using ammonia have been reported. It is difficult, however, to remove the smell of ammonia from the treated grain. Other more complex methods have been tried. For example, Chakrabarti (1981) showed that aflatoxin levels could be reduced to less than 20 ppb using separate treatments with 3 percent hydrogen peroxide, 75 percent methanol, 5 percent dimethylamine hydrochloride or 3 percent perchloric acid. These treatments, however, induced losses in weight and also in protein and lipids. Other methods include the use of carbon dioxide plus potassium sorbate and the use of sulphur oxide.
A process that has received some attention is the use of calcium hydroxide, a chemical used for lime-cooking of maize (Bressani, 1990). Studies have shown a significant reduction in aflatoxin levels, although the extent of reduction is related to the initial levels. Feeding tests with mouldy maize treated with calcium hydroxide have shown a partial restoration of its nutritional value.
Appropriate harvesting and handling can do much to reduce fungal contamination of maize and can thus prevent the need for chemical decontamination measures, which not only increase the cost of the grain but cannot completely restore its original nutritional value. In this respect, Siriacha et al. (1989) found that if shelled grain was immediately sun-dried the chance of contamination was reduced as compared with that of undried maize shelled mechanically or by hand. Shelling encourages fungal contamination as it causes damage to the kernel base, which is rough compared with the rest of the grain. Corn on the cob, even with its high levels of moisture, resists fungal contamination relatively well.
To facilitate marketing and to identify the best uses for the various types of maize produced throughout the world, measures of grain quality have been identified, although they may not be accepted by all maize-producing countries. In the United States maize is classified into five different grades, based on several factors. Minimum test weight is expressed in pounds per bushel, pounds per cubic foot or kilograms per cubic metre. The higher the test weight the higher the grade. The maximum permitted amount of broken maize and foreign material (BCFM) varies from 2 percent for Grade I to 7 percent for Grade 5. There is a classification for damaged kernels that includes heat-damaged kernels. Maize is also classified as yellow, white or mixed maize. Yellow maize must have no more than 5 percent white kernels and white maize must not have more than 2 percent yellow grain. The mixed class contains more than 10 percent of the other grain.
Although the moisture content of maize, an important part of its chemical composition, is not considered a quality factor, it has much influence on composition, quality changes during storage and processing and economics. High-moisture maize with a soft texture is easily damaged in storage, while maize with low levels of moisture becomes brittle. The most commonly accepted moisture level for marketing purposes is 15.5 percent. Density of maize - weight per unit volume - is important in storage and transportation since it establishes the size of container for either purpose. Moisture content and density or test weight are related; the higher the moisture level the lower the specific density test weight. This characteristic of maize is also important for milling.
Another important quality characteristic of maize is its hardness, since this influences grinding power requirements, dust formation, nutritional properties, processing for food products and the yield of products from dry and wet milling operations. Hardness of maize is genetically controlled, but it can be modified by both cultural practices and post-harvest handling conditions. Many investigators have proposed methodologies for measuring hardness for a number of different applications (Pomeranz et al., 1984, 1985, 1986). Maize varieties with a horny endosperm such as flint and popcorn types, have hard kernels, while starchy and opaque maize varieties are soft. Some flint types are intermediate.
Finally, freedom of the kernel from fungi is recognized as a quality characteristic.
Maize is consumed in many forms in different parts of the world, from maize grits, polenta and corn bread to popcorn and products such as maize flakes (Rooney and Serna-Saldivar, 1987). The grain is fermented to give ogi in Nigeria (Oke, 1967) and other countries in Africa (Hesseltine, 1979) and is decorticated, degermed and precooked to be made into arepas in Colombia and Venezuela (Instituto de Investigaciones Tecnológicas, 1971; Rodriguez, 1972).
In Egypt a maize flat bread, aish merahra, is widely produced. Maize flour is used to make a soft dough spiced with 5 percent ground fenugreek seeds, which is believed to increase the protein content, improve digestibility and extend the storage life of the bread. The dough is fermented all night with a sourdough starter. In the morning the dough is shaped into small, soft, round loaves, which are left for 30 minutes to "prove". Before baking the loaves are made into wide, flat discs. Aish merahra keeps fresh for seven to ten days if it is stored in airtight containers. A similar product called markouk is eaten in Lebanon.
Maize is also widely used to make beer. In Benin, for example, malt is obtained by germinating the grain for about five days. The malt is then exposed to the sun to stop germination. The grains are lightly crushed in a mortar or on a grinding stone. The malt is cooked and the extract is strained off, cooled and allowed to stand. After three days of fermentation it is ready to be drunk as beer (FAO, 1989).
The lime-cooking process for maize is particular to Mexico and Central America (Bressani, 1990), although today the technology has been exported to other countries such as the United States. A dough prepared from limecooked maize is the main ingredient for many popular dishes such as atole, a beverage with a great variety of flavours, and tamalitos, made by wrapping the dough in maize husks and steam-cooking it for 20 to 30 minutes to gelatinize the starch. This form is usually prepared with young chipilín leaves (Crotalaria longirostrata), the flowers of loroco (Fernaldia pandurata) or cooked beans mixed with the dough, thus improving the nutritional quality of the product and its flavour (Bressani, 1983). The dough is also used for tamales, a more complex preparation because of the number of ingredients it contains, in most cases with chicken or pork meat added to the gelatinized dough. It is also used to provide support for enchiladas, tacos (folded tortillas containing meat, etc.) and pupusas, the latter made with fresh cheese placed between two layers of dough and baked like tortillas. When the dough is fried and flavoured, it yields foods such as chips and chilaquiles. If the dough is allowed to ferment for two days, wrapped in banana or plantain leaves, it provides a food named pozol from which a number of drinks can be made. It has been claimed that this preparation is of high nutritional quality.
There are many ways to convert maize into interesting and acceptable forms which, if presented in attractive and easily prepared products, could to some extent counteract the trend toward greater consumption of wheat derived foods in arepa- and tortilla-eating countries and elsewhere.