Organisation:Food and Agriculture Organization of the United Nations (FAO), AGST
Author: Danilo Mejía, PhD, AGST.
Edited by AGST/FAO: Danilo Mejía, PhD, FAO (Technical), Emanuela Parrucci (HTML transfer)

CHAPTER XXIII MAIZE: Post-Harvest Operation

1 Introduction
1.1 Economic and Social Impact
1.2 World trade
1.3 Maize primary products
1.4 Secondary and derived products from maize
1.5 Requirements for export and quality assurance
1.6 Consumer preferences
1.7 Others

1. Introduction

The maize (Zea mays L.) is a monoic annual plant which belongs to maideas tribe and the grass family of gramineae, and their cells have 2n chromosomes. Is the only cereal, which was grown systematically by American Indians. Christopher Colombo encounters that maize was cultivated in Haiti, where it was named "mahiz". He carried the maize from America to Europe and later was carried by Portuguese and others Europeans to Africa and Asia, during 16th and 17th centuries. The maize is the most domesticated and evolutioned plant of the vegetal kingdom, however the origin and evolution of the maize is a mystery, since it has arrived to us highly evolutioned without intermediate forms, while the cereals from the old continent have intermediate wild varieties which are identified and preserved by nature. Notwithstanding, since the 19th century diverse theories have been exposed to explain the origin and evolution of the maize, of which one of the most accepted is that the direct predecessor of the maize is the Teosintle, figure 1.

Fig. 1. Teosintle (Zea mays ssp mexicana)
(Source: Gay J-P., Fabuleux Mais, A.G.P.M. 1984, Kalda, M., MPI Kšln)

The plant of maize has distichous leaves (two ranks of single leaves borne in alternate position). The leaf blades tend to be held at right angles to the sun by stiff mid-ribs. The external surface of the leave blade is adapted for the absorption of solar energy by little hairy structures and the internal surface is shiny and hairless with numerous stomata for breathing.

Very often it is said that productivity of maize is due to its large leaf area and to a modification of its photosynthetic pathway. In fact, this modification common in others tropical species resistant to drought periods, is known as the "C4 syndrome", which consists of an efficient mechanism to exchange water vapour for atmospheric carbon dioxide. Under these conditions, C4-species can produce more dry matter per unit of water transpired than normal plants endowed with the conventional (C3) photosynthetic pathway.

The maize plant, exhibits a single predominant stem with some few basal branches (tillers), they serve as feeder for the root system. Longer tillers may compete with the main stem and tillers of intermediate length may have terminals inflorescences which are structurally intermediate between tassels (male inflorescence) and ears (female inflorescences). The male inflorescence terminates on the uppermost spike branched arranged in a loose panicles. On this structure, the flower are organised into paired spikelets into each spikelets and there are two functional florets and each one has three anthers. The anthers are the structure which contains the pollen. Each male tassel may produce around 25 000 000 pollen grains. It means that there are available for each kernel to be fertilised an average of 25 000 pollen grains on an average of 1 000 kernels per ear. Additionally there is also female inflorescence on one or more lateral branches, their terminal ear usually borne on half-way up of the main stem and it remains enclosed into a mantle of many husk leaves. Therefore, the plant is unable to disperse the seeds as a wild plant and instead it requires the intervention of the man to remove the husk, shell and sow its grain to complete the reproductive cycle. The styles (silks) are exposed to pollination as is showed in figure 2 below.

Fig. 2 The maize plant and their parts (C.Geigi)

Maize varies widely in height, some varieties may range from 0.5 to 5 meters standing at flowering and produce 1 to 4 ears per plant. A normal average in height is 2.4 m. Maize is cultivated at latitudes 50 degrees north and south, and even slightly higher from the Equator, also from sea level to 3600 meters elevation (a.i Andean), in cool and hot weathers, and with growing cycles oscillating from 3 up to 13 months. It is a versatile crop, and it has tremendous genetic variability, which enables it to thrive well under lowland tropical, subtropical, and temperate climates. It is grown in more countries than any other cereal. In the middle of 19th and beginning of 20th centuries respectively, U.S. farmers and seeds men develop outstanding open-pollinated varieties, and intensive research in plant breeding offers spectacular improvement in crop yields. Hybrid maize is the greatest practical achievement of plant genetics to date.

Furthermore, the maize exists in different forms in respect to size and colour of plant and ear, type and size of the kernel, as is showed in figure 3.

Fig. 3 Different types of maize cobs.
(Source: Gay J-P., Fabuleux Mais, A.G.P.M. 1984, Kalda, M., MPI Kšln)

Likewise, of relevance for nutritionist, food technologist, and others scientists, is the structural parts which form the mature kernel of maize which are showed in the following figure 4.

Fig. 4. The maize kernel and their parts.
(Source: maize.agron.iastate.edu)

In line with the figure above, the kernel parts indicated include: The pericarp or hull (thin covering which enclose the kernel). Endosperm (starch section of the kernel both soft and hard starch. The germ (embryo), portion which contain a high proportion of oil, 4.5 percent w/w and it is a large part of the side of the kernel. The endosperm, the largest portion of the kernel represent about 82.3 percent of the weight of the grain and consist largely of the starch along with the gluten the bound protein 9,4 percent. The germ represents 11.5 percent and contains the maize vegetable oil. The hull or pericarp about 5.3 percent and the pedicel or tip cap 0.8 percent. The hardness of the starch in the kernel is associated to gluten. The average caloric content of the whole meal from maize is 3,578 Calories per kilogram.

On the other hand, the main classes of kernel of maize are summarised in the following table 1 according the type of endosperm and others important characteristics.

The endosperm composition is the variable feature of maize that relates most closely with its food uses, and a common and useful classification of maize based on endosperm characteristics distinguishes five types: 

a. The Pop kernel has almost all starch hard. The kernels contain 12 to 13 percent of moisture and it explode when is heated about 170 o C. This popping effect is caused because the water in the endosperm turn to steam suddenly and exploding.

b. Flinty kernels are almost impossible to grind by hand when it is dry, but may be softened by boiling in lime water and then wet-grinding to prepare the dough named masa. The entire outer portion of the kernel is composed of "hard" starch, which not easily forms a paste with water. The starch composition gives the kernel a shiny surface. It makes a good quality cornmeal (dry milling). It exhibits less risk of spoilage in shipping and storage than dent maize since hard kernel absorbs less moisture. Also this maize is more resistant to fungi and insect damage when is compared with the dent maize. Likewise, flint varieties mature earlier, and its seeds germinate much better in cold and wet soils. It can grow easily to higher latitudes than other forms of maize. This maize can be found in different colours, such as white yellow, red-blue, etc. 

c. Floury kernels, is soft when dry and have the advantages for being grind by hand, however a floury kernel called Opaque-2, which is high in lysine, have the disadvantage that may mould on the mature ears in wet areas and therefore, destroy the crop before harvesting. This type is recommended to grown in dry areas. 

d. The dented kernel is an intermediate structure between the flinty and the floury types. The surrounded side of the maize dented kernel are flinty, but the central core is floury. Due to this soft structure on the crown it contract more during drying than the hard sides. 

e. There is another type of Maize named as waxy due to somewhat waxy appearance of the kernel. China was the original source of the waxy gene (wx), but waxy mutations also happened in USA with dent strains. This Waxy maize is composed entirely of amyl pectin, in contrast with common maize which contains approximately 78 percent of starch and 22 percent of amylose. This type of hybrid maize is used for specialty products of the wet-milling starch industry.

In the case of the sweet maize, the sugary gene retards the normal conversion of sugar within starch during development of the endosperm, causing a dry sugary wrinkled and glassy kernel.

Although there are more than 200 races of maize, in this literature review on maize the idea is to provide a general description of those most relevant species. This document is be more addressed to aspects related to post harvest system of the maize, but is convenient also to give the reader some important issues and details to familiarize with this fabulous food-grain.

1.1 Economic and Social Impact.

The maize represents to all maize-based groups a source of life. Although maize is original from Mesoamerica, it is very adaptable to different weathers, and nowadays its consumption is worldwide. In fact, the maize is the most widely grown cereal crop. In the global production of cereals crops, the maize rank first after rice (paddy) and wheat. Likewise, in countries with developing economies, such as Latin-American and Africa the maize rank first. In Asia rank third after rice and wheat. The table 2 below shows important data about area, yield and production of maize in the world.

In the period from 2000 to 2002 about six hundred millions tons of maize was produced in the world on 139 millions hectares, of which 70 percent of this area is in developing countries, but only 50 percent of the global maize production is harvested there. The differences on yield are due mainly to environmental, technological, economic and organizational factors. In most developed countries the climate is temperate; likewise they use sufficient inputs and a well mechanised system for the maize production.

As complementary information, the average production in tons per hectares for industrialised countries is 7.9, in contrast in developing countries is only 2.5.

Table 3 shows the 20 larger maize producer countries which accounted for 86 percent of the world production and 77 percent of the total maize area during years 2000 to 2002, as is shown in table 3.

Due to its worldwide distribution and relative lower price to other cereals, the maize has an ample uses than any other cereals. In many developing countries the maize is a major staple food and the consumption percapita is very high. The maize can be processed in different products at traditional level as well as industrial scale. Moreover, although products derived from maize in developing countries are obtained by traditional methods of processing, the high bulk of demand for industrial process occurs in developed countries. Notwithstanding, currently important changes are happening throughout developing countries in the process of maize for major uses. For example, there is a tendency to adopt simple equipment and processing machines during the phase of post harvest for operations such as shelling, cleaning, grading, dry and wet-milling, etc. These technologies are of especial interest for developing countries where most of the maize is produced by small and medium farmers and it is for direct human consumption.

On the other hand, a large number of maize varieties for direct human consumption are available, including local and new varieties which are grown by commercial and subsistence farmers. The consumers may choice the type of maize to prepare the more acceptable food products in a given area. Traditional or commercial products from maize are based on the properties of the endosperm of the grain and others parameters such as physicochemical, organoleptic and reological properties. By fortune, these properties can could be modified by breeding or applying others agronomic and processing practices.

Some special advantages of the maize beside the broad global distribution, its lower price, the diverse type of grains and the biological and industrial properties which make the maize an adequate product for its utilisation. The maize has a very wide range of uses than any other cereal. It can be used as staple food for human consumption, animal feed and for many industrial uses. The highest rate per caput supply of maize occurs in countries where most of the grain is for feeding animals or where the maize represents the preferred staple food, as is shown in table 4.

Of the total maize harvested in the world during 2000, about 65 percent is to fed livestock, the 19 percent is for direct human consumption, 8 percent as processed, 4 percent for waste, 3 percent other uses and 1 percent as seed, such as is showed in the following figure 5.

Fig. 5. Estimation on maize use in developed and developing countries.
(Source: FAOSTAT, 2003)

The maize as human food consumed directly in quantities higher than half of its production is found in Andean countries of South America, Mexico, Central America and the Caribbean, Africa and South and Southeast Asia. The maize grain used for human consumption in these regions use mainly varieties of white maize rather than yellow maize. In fact, maize account for at least 15 percent of the total calories daily intake in 28 developing countries, almost all of them in Africa and Latin America, such as showed in the next table 5

The maize has a big impact in the economies of developed countries as well as developing countries. For example, in USA the maize dominates agriculture with a production more than double that of any other crop. The simple grain of maize in USA finds its way into the life as edible and inedible products such as rubber, plastics, fuel, clothing, food additives and many others. Just to mention an example about the maize utility (most yellow corn of USA), one kernel well sowed may produce a plant with one ear which produce an average of 800 kernels per cob. Likewise, for producing 100 Kg of maize in the form of kernel, 364 cobs are necessary of which may yield approximately what it is described in the next figure 6.

Fig. 6. What is expected from 100 Kgs of maize.

Is convenient highlight that poor and subsistence farmers grow mainly white maize in mixed cropping system which is highly recommended, especially if the complementary crop is legumes like beam. In some areas of Latin-American and even Africa where maize is the main source of caloric intake, mixed of legumes crop with maize is very desirable, since legumes like phaseolus bean and other nitrogen fixer legumes, help to prevent the exhausting of the nitrogen contained in the soil, because the legumes are in general good nitrogen fixer. Therefore, is important to appoint out that due that white maize is an important food in those countries and even, it can supply the minimum daily caloric requirements for a person.

The maize by itself, is a poor source of the essential amino acids such as lysine and tryptophan. A diet where maize is predominant may cause deficiencies diseases such as pellagra and kwashiorkor. However, in some developing countries of Latin-American where the consumption of maize, very often, is complemented with legumes such as phaseolus beam, the protein profile for this mix exhibit a very similar to that of milk. A very common and ancestry practice for the preparation of the maize grain is the cooking in an alkali batch named "Nixtamalizar" or in others parts "Nezquizar", which results in a greater availability for vitamin niacin, which its deficiencies in the diet may cause diseases such as pellagra. Some research indicates that although the lime cooking process to convert maize in tortillas induces some important losses in nutrients, this treatment also causes important and positive changes in nutrients availability, such as increasing of the calcium content (Lime is Calcium hydroxide), better release of aminoacids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine and tryptophan) from tortilla as from maize. 

1.2 World trade. 

The growth in maize production in the world market has increased considerably particularly in countries with temperate environment where hybrids and high yielding agronomic practices are used. In 1950 the maize production in the world was about 16 millions of tons and by 1980 it had increased to about 80 millions of tons, after this peak of maize world production and due to rising of production in developing countries and shortage of foreign exchange in many countries, the maize trade tended to diminish.

Since the 1990s the maize trade has fluctuated between 70 to 90 millions of tons. Most of the maize movement for trading is used to fed livestock and poultry. The main maize exporters are: United States, Argentina , France, China P.R., Hungary, Canada, South Africa, such is showed in table 6. China is a relatively new exporter being the main suppliers of Asian neighbor countries.

On the other hand, 28 countries in the world imported an average each during 1999 to 2002, of more than 500 000 tons of maize. All these countries accounted together 75 percent of the world total maize imports, table 7.

Furthermore, from 1988 to 2002 the world maize supplies expanded slowly compared as was before this period, however developing countries tend to increase their productivity as developed countries does, figure 7. The global maize production currently is around 600 millions of tons and it seems that there is a tendency to increase for the next years due mainly to growth population. However, the increase in productivity owing to the use of improved varieties and agronomic practices has had a big impact in the lowering maize prices. Since United States is the predominant maize trade in the world, the best indicator of world maize prices is the FOB US price.

Fig. 7. World Maize Production (1988-2002).
(FAOSTAT, 2003)

The trends in maize prices indicate that maize supply as well demand has changed at about the same rate. In regards, the supply side, cost-reducing technologies, especially in developed countries has doubled the maize production without increase of the real prices. At the same time, the increased demand of maize as animal feed has absorbed the increased production, due mainly to a rising income and growth of the population.

The intervention of governments in the market of maize in producing and consumers maize countries has had (and often adverse) a big influences on international maize prices. In developed countries, the protectionism generally favours agriculture which is paid by taxpayers, consumers and producers of other tradable products in the economy. In contrast, in developing countries and in central planned economies, protectionism penalizes agriculture, in favor of urban consumers and others sector of the economy. The USA and EU with direct and indirect price support have had a significant depressing effect on the international maize price. The governments in most developing countries are removing the discriminatory maize police in response to the pressure from world bank, international monetary fund and others bilateral donors agencies. This lower trend for maize price has worsened the economic situation of maize exporting countries.

1.3 Maize primary products.

Meal is a primary product obtained from maize. The meal from maize can be obtained by manual or mechanically milling. There are different ways to make manually the maize meal, for example in traditional culture of Central America they use traditional tools to ground the grain slowly between two stones figure 8 (molcajete) or piedra de mano (metate) figure 9.

Fig. 8. Manual maize grinder (Molcajete)

Fig. 9. Piedra de mano or metate for grinding maize
(Source: mexico.udg.mx)

In some part of Africa pounding of the maize with a pestle and a mortar is also widely used as traditional method. Likewise, dry milling and wet milling are the most common system to obtain maize meal, for hard varieties like flint-type maize is recommended to use wet milling process. In Africa and in some countries of Latin-American at village level there are small-scale mechanical dry-maize milling. The grain mills hand-operated or engine-powered are available for maize grinding and these equipments can be produced nationally or imported figure 10.

Fig. 10. Hand grain milling

Depending of the use of maize there are some other products of maize which can be considered as primary products. As for example, sweet grain maize grown for green ears, normally this is consumed after boiling or roasting. At the moment of the harvest, the grain has about 70 per cent of water and has not started to harden. These grains have high sugar content and it is sweet in taste. The green ears are generally boiled with or without the husk leaves in water with or without salt or lime. In some part of Africa boiled ears are sun dried and stored for later use after re-boiling and heating. Another manner is to extract the juice from the kernel flavoured, cooked and allowed to become a jelly. This type of product is used in western and eastern Africa. Also fresh kernel removed from green ears are ground into a paste and mashed or slurred without fermentation. Then, this is used to make soup or various porridges or baked products, such as cachapas in Colombia and Venezuela, humita and mingau in others countries of South America and Atole in México and Central America. Likewise, this mashed paste can be allowed to ferment for a few days to make various porridges or pudding dishes. Another type of maize which is popular in some places is the baby ear shot or "baby corn". The young ear shoots are harvested before pollination occurs and it is used as vegetable, popularly known as cooked "Chilote" in Central America. It is also consumed fresh or canned. Tropical environment are suitable for ear baby shoots.

1.3.1 Mature dry grain.

1.3.1.1. Whole grain.

In Africa the grain is usually parched and eaten. Likewise, the hard flint or pop maize grain is popped in hot sand (Africa and Asia) or mostly in a hot plate and eaten hot as popped maize.

In Andean regions and even in USA floury grain is roasted and eaten as "corn nuts". In some countries of Africa the grain is boiled and eaten whole or the grain is beaten and pulped to make a product similar to boiled rice, this product is also consumed in Asia. The grain can be cooked in lye or lime water in USA and it is called hominy in Mexico after removing the per carp, it is used to make soup or a traditional dish known as "pozole".

A very popular product named Ogi or Uji is consumed in Africa, this is prepared by steeping and fermenting then milled and made into slurry. Then it is fermented and made into porridge.

1.3.1.2. Dry milled grain.

The maize whole grain can be dry milled to produce a coarse maize meal or fine flour and it is used in a variety of ways. In Africa, for instances, they used to make cooked paste, fermented or unfermented. The flour is used to make a dough adding water. This dough is made for preparing unleaved bread and flat thin called chapattis in Asia. In Ethiopia, the dough is fermented and cooked in a hot plate to prepare the Enjera of maize.

In some regions of Central America especially in Nicaragua, dry grains are toasted and then milled. This product it seems like flour and is known as Pinol (in Nicaragua) or Pinole (in Mexico). When cacao seed is added and grinded together with the maize, it is called Pinolillo. These products are used as the base for the preparation of a traditional daily beverage, just by adding water, very often sugar, and shaking. The addition of sugar or others flavour is optional for this product and it is a typical beverage consumed specially in towns and rural areas.

1.3.1.3. Soaked grain

The grain is soaked and cooked in a water or lime solution then the grain is ground to make dough which is used as the base for different preparations. Eventually the grain soaked and cooked is dehulled and the germ removed partially or totally. This product can be pounded to obtain the grit and then cooked and eaten as the rice boiled or can be used to prepare special type of breads such as the arepas in Venezuela or the sopas how is called in Paraguay. Likewise, maize gruel can be transformed into sweet or sour drinks. Fermented drinks are popular in Africa and Latin America. Un example of this product is the chicha and also the pozol, a fermented masa used in Central America used to prepare a typical beverage, just by adding to the portions of fermented masa water, or milk, sugar and shaking. Pozol is made in Nicaragua with maize purple colour grains varieties (called pujagua), although any maize variety can be used.

1.3.1.4. Nixtamalized maize.

This process similar to the previous one described was developed by Native Americans Indians. The kernel are soaked and cooked in lime water (in some parts of Latin America ash is used instead of lime) and then dehulled and ground to form dough called masa. This masa is the base for preparing several traditional products such as tortillas, tamales, etc. Furthermore, the masa can be dried and converted into maize flour by grinding the masa dried, sieving, classifying and blending to obtain dried masa flour. This dry masa flour can be also used to make tortillas (by rehydration), tortillas chips, etc. The dry masa flour can be also used to prepare composite flour.

The nixtamalization consist of mixing one third part of whole maize with two thirds parts of a lime (calcium sulphate) solution between 1 to 2 percent of concentration. In general terms, the cooking time may vary from 15 to 45 minutes and the temperature of cooking is held above of 68 0 C degree. The grinding of the nixtamalized kernels are carry out by simple pounding with a hand operated or electric kitchen grinder-mixer, with a semi commercial grinder for cottage industry or with commercial grinders for mass-scale masa production. The dry masa flour is more stable against rancidity and the shelf life can be until one year, in comparison with the whole dry kernel ground maize flour does. Is important to remark that the nixtamalization treatment has the following advantages: it facilitates the pericarp be removal, controls microbial activity, enhance water uptake, increase gelatinization of starch granules and improve nutritional value through an increased availability of Niacin.

1.3.1.5. Composite flour

The use of composite flours to supplement wheat flour for making bread and biscuits is not a new concept. Due to the increased global wheat production since the green revolution has caused a decrease in the price of wheat and consequently it has boosted wheat consumption in tropical countries where there is no grown of wheat. These countries have depended on imported wheat or wheat flour received as food aid or purchased from countries with surplus of wheat. However, many tropical countries at the present are pressed for foreign exchange and therefore are restricting the import of wheat or wheat flour.

Furthermore, milling and baking research have shown that technically is feasible to substitute in a limited extent flours of crop from maize, sorghum, millet and cassava for wheat flour. Tastes and flavour of such composite flour for making bread is technically feasible.

Some countries in sub-Saharan Africa have little wheat production and the increased demand for wheat create a potential market for composite flour. Notwithstanding, composite flour are only used commercially in Zambia (6 percent maize flour substitution) and in Zimbabwe (10 percent maize flour substitution). In Latin-American only Brazil use composite flour made with cassava and maize. In India when soft wheat imported from Mexico was used, consumers did not like the leathery, chapattis.

According to a FAO publication on precise techniques of composite flour, 25 percent as maize flour can be mixed with 75 percent without appreciable difference in the quality of the composite bread. In other products the substitution could be even higher.

1.3.2 Special types of maize and their use as food

Flint and dent maize type are the most used for human consumption, however there are some special types which are used for specific purpose, as for example:

1.3.2.1. Floury maize.

This type of maize is used in the Andean highland for food. The green ears are roasted and the mature floury kernels toasted becoming partially popped. Some native products from this maize are the kancha and the chicha. Another product is the sopa which is very popular in Paraguay. Corn nut is also popular from floury maize and kernels from the large grain floury race Cuzco Gigante are normally heated in an alkaline solution, washed to remove the pericarp and then blanched in warm water for a few hours. Finally the product is fried to develop texture, colour and flavour. Some floury maize is being used to extract natural food colours from the pericarp.

1.3.2.2. Puffed and popped maize.

The most used kernel is hard flint which is subjected to high temperatures, either in hot sand or in a hot plate in order the kernel puff and pop. This snack is very popular over the world. Some hard flint varieties have been modified and improved by selection in order to obtain a maximum popping expansion up to 30 to 40 fold expansion respect the original volume of the uncooked grains. For an adequate popping of the grain a temperature around 177 0 C is required and at this temperature, the water within the endosperm of the kernel is turned into steam which provides the force and pressure for the endosperm to burst and puff out. The moisture content of the grain is a very important factor to be controlled and it makes the packaging and storage of maize for popping more expensive.

1.3.2.3. Baby ear shoots.

Known also as "baby corn", it was first developed and promoted in Thailand. It is popular in Southeast Asian countries. The immature ear shoots are harvested when the silks have just emerged, but before the silks are pollinated. To ensure that not pollinization take place, the plants are detasselled before pollen sheds. The baby shoots are marketed with husk leaves and the silks. Baby ears shoots cleaned are used fresh in salads, as a vegetable, for making soup or pickled and canned.

The table 8 shows the nutritive value for the baby ears shoots and it is compared with other typical vegetable. After baby ears shoots are harvested, the green maize plant is used as fodder for livestock. In tropical countries there is an advantage, since baby ears shoots can be produced all year round and supplied fresh.

1.3.2.4. Green ear maize.

A favourite snack food in almost every country where maize is grown is green ears roasted or boiled. The grain is eaten on the cob and it is an expanding street snack, especially when a topping like chilli, mayonnaise, butter, etc is added (very common in Mexico and Central America). The most used maize for this purpose is the normal flint maize ears, and it is a good source as food and energy. In some West African countries more than 50 percent of the area planted with maize is harvested for green ears. Among some advantages for this practice are that maize harvested for green ears does not face the problem of ears rots and grain insect damage in the field. Likewise, it makes a very useful source of energy and food between two main crop harvests. The green ear maize once it is cooked is known as "elote" in Central America.

The roasted or boiled green ears are largely consumed by children and women. The grain at the milk stage is more nutritious than rice. Also the fresh kernel separated from the green ears is also used as a vegetable, for preparation of sweet or sour pudding. A traditional tortilla named guirila and an especial tamale called yoltamal is prepared and consumed popularly in Nicaragua, and they are made from the kernel separated from the green ears and then ground.

The plants of maize are still green when ears are harvested and it provides better fodder for livestock than dry Stover left over after the harvest of mature maize. Likewise, the green ear maize is a crop with shorter duration and occupies the field for few days allowing more intensive cropping patterns.

1.3.2.5. Quality Protein Maize (QPM).

 This type of maize has the opaque-2 gene (o2). The dull appearance of this kernel and others undesirable characters have been overcome with the accumulation of genetic modifiers and extensive selection efforts carried out by scientist at CIMMYT in Mexico. The protein quality in QPM is much better than in normal maize. The zein fraction is reduced between 10 to 13 percent in QPM as against 39 percent in normal maize. In contrast, glutelin and glutelin-like fractions are increased. The nutritional and biological superiority in QPM has been demonstrated in the diets of infants, small children and adults, particularly women. At the present only Brazil, China, Ghana and South Africa are making serious effort to grown QPOM; there is also evidence that this QPM variety may be more suitable for use as greens ears and for production of composite flour.

1.3.2.6. Maize for fodder

The plant of maize is an excellent fodder for milk cattle as well draft cattle. It can be used as fodder at different stages of the plant growth particularly from tasselling onward. The plant does not have problems of prussic acid or hydrocyanic acid; therefore it can be used before flowering or in dry weather. The plant with ears at dough stage of grains development is best for use as fodder. Compared with others fodder it surpasses in dry matter production and digestibility of nutrient per hectare. Even this stage also is the best time for preparing silage. Usually, the grains varieties planted at higher densities have best results as fodder crop.

In some countries of Asia and in Egypt, farmers plant maize a very high density and progressively these are removed for using as fodder. In Mexico and Central America, the stalks above the ears are cut for fodder after ears development is well-advanced. Likewise, the green stalks left after the harvest of baby ear shoots and ear green maize are also used as fodder. Also QPM green ear silage or corn cob mix (CCM) is becoming very popular in Northern European countries, where maize cannot be grown to maturity. The (o2) CCM is equal in yield but superior in nutritional quality than normal maize (CCM) for feeding pigs.

1.3.2.7. Maize as livestock and poultry feed.

The maize grain gives the highest conversion ratio to meat, milk and eggs when is compared with others grains used as livestock feed, this is due its high starch and low fibre content which make it a very concentrated source of energy for livestock production. Although there is not available statistic for maize and livestock use, it is believed that greater portion is used as poultry feed in tropical countries. Yellow maize is preferred for livestock feed and it is used as whole grains, cracked or coarse ground, dry or wet or steamed and generally supplemented with vitamins and others proteins. Is expected that use of maize in formulated feed will increase in the future.

The use of QPM as animal feed promise good potential and it still remains to be exploited particularly for swine production. So far, there is some use of QPM for pig feed and it has been reported that the use of QPM as an ingredient in pig feed could help to reduce costs. However, it is possibly that the unavailability of sizeable quantities of QPM grains in the market, and the fact that cultivation of QPM has not been taken up on a commercial scale.

1.4 Secondary and derived products from maize

There are many products from maize that have been taken over by industry and manufactured and marketed at commercial scale. Several of these products already mentioned are now industrialised on a small or large scale. In the USA over 1 000 different items can be found on the shelves of a typical supermarket and they are derived wholly or partially from maize. These products include: tortillas, maize flours (masa), chips and several types of snack, breakfast cereal, thickness, pastes, syrups, sweeteners, grits, maize oil, soft drinks, beer, whisky, etc.

Basically, there are two milling process used for the maize industry for making various food, feed or industrial products. They are:

2. The dry milling process.

In both process, there are some common operations applicable for the maize to be used, such as the handling system, storage structures, drying and cleaning operations, inspection, etc. Some useful parameters to control in the maize grains are: heaping bulk angle 27 degree, the specific volume 1.2-1.3 m 3 /ton or specific gravity which is 0.72-0.85 grs/cm3. For the maize flour is 0.65 grs/cm3. The moisture content of the grains should be 13 percent and for the maize flour 11.5 percent. The temperature of drying recommendable for the grain is 60 0C and it takes about 12 hours for drying the maize from 35 percent to 15 percent .

According to some experts, a plant for maize flour processing be profitable, the investment for this type of plant is only justified on the basis of processing a minimum of 70 tons/day of maize grains, otherwise it is not profitable.

1.4.1 Wet Milling.

The wet milling process normally produce pure starch, sweeteners (dextrose, fructose, glucose and syrups including high fructose syrups), proteins, industrial starch, fibres, ethanol and maize oil from the germ. The most important by-products are animal feed and this industry usually uses the flint and dent maize types.

Likewise, some special maize such as the waxy maize and high amylose maize are handled by the wet milling industry to produce tapioca-type high-grade starch and high amylose maize (also called amylomize) starch respectively.

Starch of maize is the most important product of the wet milling process, and it is widely used for food and industrial applications. The starch and oil extraction account for about 70 percent of the product, and the remaining 30 percent is formed principally by proteins and fibres (consisting mainly of cellulose and hemicelluloses) which is converted in animal feed.

The flow diagram for this process is described in figure 11.

Fig. 11. Wet-millingProcess flow diagram
(Source: Corn Refining Assoc.)

The process of maize wet milled is to obtain starch, oil, cattle feed (gluten feed, gluten meal, germ cake) and the hydrolysis products of starch, liquid and solid glucose and syrups. The process includes the followings main operations:

1.4.1.1. Cleaning.

The maize received is cleaned before is storage. The selection and cleaning is applied by vacuum and it eliminates undesirable materials or particles such as dust, wastes, and pieces of ears, stone and insects.

Wet milling differs from dry milling in being a maceration process in which physical and chemical changes occurs in the nature of the basic constituents of the endosperm (starch, protein and wall cell material in order to cause a complete dissociation of the endosperm cell contents which release the starch granules from the protein network, where they are enclosed.

1.4.1.2. Drying.

For a safe storage the maize must be dried since the moisture content at harvest is generally higher than the desirable moisture content for storage. The temperature of drying should not exceed 54 0C (130 F), because higher temperatures may cause change in the protein, whereby it swells less during steeping and tend to tie the starch in a stronger manner, than in grain not dried or dried at lower temperatures. Likewise, if is dried above 54 0C, the germ become rubbery and tend to sink in the ground maize slurry (the germ separation depends on the floating of the germ) and the starch tends to retain high oil content.

1.4.1.3. Steeping.

The cleaned maize is steeped at a temperature of about 50 0C (122 F) for 28-48 hrs in water containing 0.02 to 0.03 percent of sulphur dioxide. The steeping is carried out in a series of tank through which the steep water is pumped counter- current. The moisture content in the grain increase rapidly to 35-45 percent , and more slowly to 43-45 percent. The steeping softens the kernel and assist the separation of the hull, germ and fiber from each other. The sulphur dioxide may disrupt the -S-S- bond in the matrix of the protein (Glutelin), facilitating the starch/protein separation. After steeping, the steep water is drained off. It contains around 6 percent of solids of which 35-45 percent is protein. The protein in the steep water is recovered by vacuum evaporation, allowed to settle out of the water in tanks, and dried as "gluten feed" for animal feeding. The water recovered is re-used as steep water or, after concentration, as a medium substrate for the culture of organism from which antibiotics are obtained.

1.4.1.4. De-germing.

The maize grain is coarsely ground freeing the germ from the remainder of the grain without breaking or crushing the germ. A fuss-mill is used for this purpose, it has a bronze-lined chamber housing two upright metal plates studded with metal teeth. One plate rotate at 900 rev/min the other one is stationary. Water and grains are fed which crack and open the grain, releasing the germ. By addition of starch-water suspension (1.06-1.08 sp. gr) the germ floats, whereas the grits and hulls sink.

1.4.1.5. Germ separation.

The ground material flows down separating the hulls and grits settled, while the germ overflows. Modern plants use hydro cyclones which use less space and are less costly to maintain than flotation equipment. Likewise, the germ separated on hydro cyclones is cleaner than the one separated by flotation.

The germ is washed and freed of starch on reels, dewatered and squeeze presses and dried on rotary steam driers. The dry germ is cooked by steam, and the oil extracted by hydraulic presses or by solvent extraction. The oil is screened, filtered and stored. The residual germ cake is used for cattle feed.

1.4.1.6. Milling or grinding.

The de-germed underflow from the germ separators is strained off from the liquor and finely milled on impact mills, such as entoleter or attrition mills like the Bauer mill. After this, the starch and protein of the endosperm are in a very finely divided state and remain in suspension. The hulls and fiber, which are not enough reduced in size, can be separated from protein and starch on reels fitted with 18-20 mesh screens. Fine fibers, which interfere with the subsequent separation of starch from protein, are removed by gravity shakers fitted with fine nylon cloth.

1.4.1.7. Separation of starch from protein.

In the raw grain the starch is embedded in a protein web which swells during steeping and tend to form tiny globules of hydrated protein. The dispersion of the protein, which frees the starch, is accelerated by the sulphur dioxide in the steep water. The suspension of starch and protein from the wet screening is adjusted to a density of 1.04 sp. gr. by de-watering over Grinco or string filters, and the starch separated from the protein in continuous high speed centrifuges, such as the Merco centrifugal separator. The starch is re-centrifuged in hydro cyclones to remove residual gluten protein and is then filtered and dried to 10-12 percent moisture in kiln or ovens, or tunnel or flash driers. The separated proteins is filtered and dried in rotary or fresh driers.

Then the protein is filtered and dried in rotary or flash driers. Further, fractioning to obtain the alcohol-soluble protein (zein), which is 50 percent of the maize gluten by solvent extraction and precipitation, can be carried out. Zein has ample uses as a water protective coating material for nuts and confectionary, and as a binder for pharmaceutical.

1.4.1.8. Separation of gluten from the starch.

The paste of starch containing between 5 to 8 percent of gluten is passed through high speed centrifugation by using a centrifuge type Merco. Firstly, the good quality of gluten is separated from the starch and then it is concentrated in other centrifuge. The gluten is then filtered and dried in an instant rotatory drier. The gluten out coming is used as one of the major components of many food products.

The starch obtained in the first centrifugation still contains between 2 to 2.5 percent of gluten proteins and is also centrifuged with hydrocyclons. The hiyrocyclons centrifugal equipment used to separate the starch from the gluten is composed of many little tubes within a divided compartment and washing to countercurrent is applied to obtain a good separation of gluten and starch.

1.4.1.9. Zein separation (Optional).

Some companies use to separate from the fraction gluten a portion of the most important protein is the zein. The zein (prolin) is soluble in alcohol and it is contained into the gluten in about 50 percent. The zein is extracted by solvents and then precipitated. The main use of the zein protein is used in the food industry like an especial cover to prevent humidification on the surface of candies. Also it has good glutinous properties in the manufacture of pharmaceutical products.

1.4.1.10. Products derived from starch.

Due to the nature of the starch from it, it is possible to obtain other products by some specific chemical treatment, such as.

1. Syrups and sugars.

Approximately the half of the starch produced is converted into syrups and sugars depending of the extent of the treatment and the degree of purity desired in the final product. The conversion of starch into sugars can be done by enzymatic or acid hydrolysis. The syrups are produced by partial hydrolysis and the sugar dextrose by total hydrolysis. The acid hydrolysis process requires boiling the starch paste with a certain quantity of diluted acid. Generally, the acid most used is hydrochloric acid and the process ends when the degree of hydrolysis is get up. The chemical reaction is stopped by neutralizing agents like sodium carbonate. The impurities and solids particles are removed by filtration, then the syrup is blanched and concentrated until a final specific gravity desired.

The sugar dextrose is obtained by a complete or total hydrolysis of the starch, and then the liquor is neutralized, filtered, clarified and concentrated. Finally, the product is crystallized.

2. Uses for wet-milled maize products.

a. The maize starch.
Include paper manufacture, textile, adhesives and packed foods, and as the starting material for the manufacture of syrups and dextrose sugar by hydrolysis. The starch obtained from the wet milling of waxy maize, also called "amioca", which consist mainly of amyl pectin, is non-jelly and has clear, fluid, adhesive properties. Heated and dried maize starch/water slurries yield pre-gelatinized starch, known as "instant starch" as it thickness upon addition of cold water.

b. Glucose and Dextrose.
Are used in beer, cider, soft drinks, pharmaceuticals, confectionary, baking and jams.

c. Corn Gluten.
Is used mainly as animal feed. Also can be used as cork binding agent, additive for printing dyes, and pharmaceutical.

d. Dextrins.
The dextrins are products obtained by the breakdown of the solid dry starch, which is heated with chemical products like mineral acid almost always hydrochloric acid. The dextrins are used mainly as adhesives, dressing or glutinous agents.

1.4.2 The dry milling process

The process of dry milling is also used to produce a wide variety of food and non food products. In general, the process of maize starts with milling, even the maize which is used at household level. Except the maize eaten as kernel on the cob and popcorn all other products from maize are based on milled maize

A particular problem exist with rodent excreta pellets. Some time because of the maize grain size it overlap with rat’s excreta pellets. Removal of this is more difficult than with other cereals, however removal of mouse excreta pellets is less of a problem. The specific gravity table, air separator and wet flotation each remove 50 to 70 percent of rat’s pellets with a loss of 0.5 to 1 percent of the maize.

Milling separator, length and width separations and scourer-aspirators were considered less efficient.

1.4.2.1 Conditioning.

This step is for a better de-germining. It allows loosen and toughen the germ and bran to mellow the endosperm, so as to obtain maximum yields of grits and a minimum yield of flour in the subsequent milling. This is achieved when the germ is somewhat damper than endosperm. The conditioning involves the addition of 2 to 3 percent of cold or hot water, or of steam for de-germining by rollers or entoleters. A moisture content of 20 to 22 percent is raised if a Beall degerminators is to be used. The conditioned grain stand for 24 hours, but generally it remains for only 1 to 2 hours.

1.4.2.2 De-germining.

This process includes de-germining de-hulling and it is carried out in one of the followings ways:

a)   With a Beall de-germinator (d-germer and corn huller)

b)   With roller mills and sifters

c)   With impact machines, such as an entoleters, and gravity separators.

The Beall de-germinator is a cast iron cone at about 750 rev/min within a conical stationary housing fitted partly with screens and protrusions on the inner surface. The maize with 20 to 22 percent of moisture is fed in at the small end and works along to the large end, between the two elements. The protrusions on the rotors and the housing rub off the hull and germ by abrasive action, and break the endosperm into two o more pieces per grain.

In this method, it produce a maximum yield of large particle size grits (hominy) with low fiber content (about 0.5 percent ) and low fat content, suitable for the manufacture of corn flakes. This vitreous part of the endosperm yields the hominy grits which come from the tail end of the machine. The germs are flattened, and cannot be separated from the comminuted mealy endosperm and bran, which pass through the screen mixed. The germ is heavily damaged and it has 15 to 18 percent of fat. The power consumption is high due to need of drying the hominy grits.

The method using roller mills is the simple one and the germ separation, endosperm and bran is relatively inefficient. The content of fat in the grits and flour is 1.5 to 2 percent and the germ is obtained with a fat content of 15 percent. The roller mills used for de-germining have rolls flutted 15 to 23 cuts per centimeters (6 to 9 cuts per inches) and rotate at a differential speed of 1 1/4:1 or 1 1/2:1.

De-germing by impact machine, for example entoleters or turbo-crushers are used in Europe and it is carried out with the maize with natural moisture content and has lower power consumption than other process. The separation of germ from endosperm on gravity tables is efficient. However, separation of endosperm from bran by aspiration, and of the vitreous endosperm from mealy endosperm is less efficient than with the Beall de-germinator. The following products are obtained from this entoleter process:

Maize germ, 1-4,5 mm, with 18-25 percent of fat

Maize grits, 1-4,5mm, with 1-1.5 percent fat, 0.8-1,2 percent crude fiber: about 60 percent of the original maize

Semolina and flour, which may be made by size reduction of the grits.

The mealy endosperm of higher fat reduces more readily than does the vitreous endosperm, therefore, the flour has a higher fat content (about 3 percent , the semolina of lower fat content) 0.8-1.3 percent than the grits.

1.4.2.3 Drying and cooling.

The product from the Beal degerminators is dried to 15-15.5 percent moisture in a rotary steam tubes at a temperature of 60-710 centigrade degree (140-160 F) and cooled to 32-380 C (90-100 F) by aspiration with air.

1.4.2.4 Grading

The dried stock is sifted to produce a number of particles size fractions (large medium and fine hominy, germ roll stock, and meal). These stocks are fed to the mill, each one entering at a specific point, the large and medium at first break, the fine and germ roll stock at the second roll.

1.4.2.5 Milling.

The milling is carried out on roller mills using fluted rolls. The products are sifted on a plan sifter and are aspirated. The mill is divided into a break station, a series of germ and a series of reduction and quality rolls. The break system releases the rest of the germ as well entire particles and cracks the large grits to produce grits of medium size. The finished grits, meal and flour product are dried to 12-14 percent moisture content on a rotary steam tube driers.

1.4.2.6 Oil extraction.

Solvent extraction and mechanical pressing are used. The germ from the mill is first dried to about 3 percent moisture and then extracted while at a temperature of about 121 0C (250 F). The oil content in the germ is reduced by extraction from 18-25 percent to about 6 percent leftover in the germ cake; the extracted oil is filtered through a cloth using a pressure 552-690 kN/m2 (80-100 lbs/inch.2). The oils have a Specific gravity of 0.922-0.925 and is rich in essential fatty acids. Its high some point make it suitable for use as cooking oil and salad oil.

The following table 9 shows particle sizes range and yield of dry-milled maize products obtained by dry-milled process.

1.4.2.7. Uses for dry-milled maize products.

1. Flaking grits.

Are used for the manufacture of breakfast cereal corn flakes and grits from yellow maize are preferred.

2. Coarse grits and medium grits.

These are used in the manufacture of cereal products and snack foods.

3. Fine grits.

It is used as a brewing adjunct, providing up to 40 percent of the mash. At domestic level, maize grits or hominy grits are used to prepare porridge by boiling with water. In Italy, the maize porridge, made from fine grits or coarse meal and flavored with cheese is called "Polenta". Also maize grits can be used for the manufacture of wallpaper paste and glucose by chemical hydrolysis.

4. Coarse or granulated maize.

This is used in pancake and muffin mixes, corn snacks, cereals products and other baking products.

5. Fine meal or corn (maize cones).

Used to make maize bread, bakery mixed, infant’s foods and breakfast cereal.

6. Corn (maize) flour.

Is used for make bread and pancake mixed, infants foods, biscuits, wafers, as filler and carriers in meat products, and in breakfast cereals. Dry-milled maize flour it should not be confused with "Corn flour", this term is used in the U.K. for maize starch obtained as a product of wet milling.

Furthermore, dry milling and wet milling process are used for the production of ethanol or gasohol from maize. Approximately one third of gasohol is produced by the dry milling process and two third by the wet milling process. In these two processes only a little over 70 percent of the product is in the form of starch and it is used for ethanol production. The remaining material, which comprises about 11 percent of cellulose, hemicelluloses, leftover starch and sugars, is used to make animal feed supplements. In USA the National Renewable Energy Laboratory has developed a technology using sophisticated biotechnological tools to increase gasohol production. They have selected certain strain of fungi and develop genetically engineered bacteria that can hydrolyse cellulose and hemicelluloses and produce alcohol from these complex carbohydrates. This new technology or biotechnology could increase the ethanol production from maize by about 13 percent and make gasohol more cheap and competitive. Also maize cob and stover, although they do not have starch, however they have cellulose and consequently may become feedstock for ethanol production. These technologies are of special interest in the tropics.

1.5 Requirements for export and quality assurance.

The term quality applied to food material refers to those attributes of the food which make it agreeable to the person who eat it. In a broaden context, attributes of quality involve color, flavor, texture, nutritional value and free of harmful substances such as microorganisms, insects, pest and their products. For these reasons is very important to have and implement a regulatory system to control production and commercialization of food materials in order to protect the public from harmful and poisonous food, prevent the sale of substandard foods containing substances which may not be harmful, but do not describe the food correctly, and eliminate false and fraudulent trade practices. Applications of regulations can be effective only if thoroughly tested standards are set up and implemented.

The quality of food grains is assessed in line to the circumstances prevailing in different part of the world. Usually, they take into considerations attributes like size, color, texture and extraneous materials. Chemical parameters such as oil content, acidity, moisture and presence/absence of toxins may also be considered in quality assessment. Some quality legislation in force in some developing countries was reviewed and detailed analysis of these legislations indicated relevant issues as follows:

• Each country has its own standard of quality, and the quality parameters become more comprehensive for major commodity (ies) than for others.

• The quality standards are seldom based on an objective scientific basis and are often arbitrary.

So that is important to select the most useful indicators of quality and to apply them for minimizing the losses in nutrient available to the population of developing countries. Agreements at the international level on which indicators for using would be most beneficial. In any case, the parameters chosen must be able to be applied uniformly, quickly and cheaply. Likewise, to keep in mind that use of sophisticated equipment and techniques only when their are unavoidable (e.g. pesticides analysis), the indicators chosen should be those which ensure a safe and wholesome supply of grain. Also is recommendable to include factors which affect the market value of the produce. At the present, actions addressed to regulate updating issues like production of cereals genetically improved or modified, etc require regulations. This situation is the great interest for export and import countries and design of parameters on upgrading qualities for trading cereals is required.

A country which deserve be mentioned in this regards is India. It as one of the largest grain producing nations in the developing world, implemented through a coordination of the Food Corporation of India (FCI) and the Prevention of Food Adulteration Act (PFA) a reasonable standard grain system. This system since 1955 has evolved through years and it appears to be working satisfactorily and it applies to purchase, storage and sale of food grains and their products. According to these provisions, food grains meant for human consumption shall fulfill the followings standards of quality:

• Grain shall be free from deleterious material and insecticide residues in excess of the prescribed limits

• >Foreign matter including sand, gravel, dirt, stones, pebbles, straw, stem, chaff, cockles, oilseeds and other non-poisonous seeds, but excluding non-food grains, shall not exceed 4 percent by weight

• Grain that is damaged by fungus, moisture or heating wherein the damage is not superficial, but grain is affected internally, shall not exceed 5 percent by weight

• Uric acid content arising as a result of insect damage shall not exceed 10 mg per 100 g of the sampled grain

• The loss in weight due to moisture content shall not exceed 16 percent .

Other useful and typical standards have been developed by FAO/WHO Codex Alimentarius Commission and may serve as an appropriate reference for trading of maize in developing or developed countries as is showed in the next table 10.

Likewise, the next table 11 can serve as a model and it also shows some specifications for maize in India.

The next table 12 shows some grades for maize in India as useful reference showing Maximum Tolerance Limit to grains.

1.6 Consumer preferences.

As it was described in previous section 1.3, maize in general is used in more ways than any other cereal. White maize in particular is preferred in developing countries as human food due to the organoleptic properties. In contrast, yellow maize is used in developed countries for feeding livestock and poultry. The yellow maize is desirable, for instances, to increase the yellow colour characteristic of the eggs yolk. In any case, maize is used either home cooked and industrial, as fodder, feed animals and fermentation in various industrial products.

1.6.1 Some nutritional aspects of maize.

Maize nutritionally is superior than others cereals in many ways, except in protein value. The following table 13 shows the nutritive composition of maize, wheat and rice of various parts of the kernels.

Maize compared with wheat and rice is higher in fat, iron and fibre content. A weak nutritional aspect of maize is the quality of its protein since around a half of its protein is made up of zein, which is low in two essential amino acids, lysine and tryptophan. Fortunately this deficiency nowadays has been corrected with the development of the quality protein maize (QPM), which is nutritionally the most superior cereal grain.

1.6.2. Regional consumption of maize.

In most tropical countries that produce maize on a commercial scale, it is used mainly as food for human consumption, such as is shown in the following table 14.

1.7 Others.

1.7.1 Maize production and the need for adopting technology.

Technological components are useful if they are used integrated in the production system for sustainable crop or animal production. Most farmers, particularly at the subsistence level, rarely adopt complete production packages, especially for crops such as maize, in zones when it has been produced for many years as a staple food and has become part of their traditional culture. It is deficient for them to afford drastic changes in their traditional technologies, unless they accept the risk of the innovations.

However, there is a need for developing single production components which can be individually adopted by farmers as a mini package of technologies that can secure a noticeable and immediate yield increase (i.e. nitrogen, weed control, mulch, ridging system, manure, intercropping, etc)

The production components varies in specifity such as a nutrients levels, cultivars, tillage system, date of planting, intercropping components or run off and erosion control system.

In the following section, some basic concepts of tropical maize agronomy are presented as a background source of information.

1.7.2 Soil preparation (Basic concepts).

  Tillage can be defined as the chemical, physical or biological manipulation of soil to optimize germination, seedling emergence and crop establishment. This definition includes operations involved in producing a crop, such as chopping of residues, planting, applications of pesticides, fertilizers and harvesting.

Although, there are many tillage systems, including zero-tillage, that are undoubtedly play a very important in determining the time-efficiency of a farmer, a system that optimize production and productivity should be chosen depending of the soil and agronomic and climatic conditions.

Since the intervention of the plough, has been justified for many persons that the preparation of the soil is necessary based on reasoning which has not been completely scientifically proven. Some of the main justifications for soil preparation with manual and mechanical implements include: efficient weed control, incorporation of plant residues, soil aeration improvement, seedbed preparation, disease and/or insect control, improvement of the physical condition of the soil, fertilizer incorporation, plough pan elimination and improvement of root development. However, nowadays is easy to refute these arguments since it is well known that:

• weeds are controlled with herbicides and even with mulch alone

• It is better to leave crop residue on the soil surface than to incorporate them, because crop residue prevent soil erosion. Mulch also protects the soils against excessive water loss caused by evaporation and maintain soil moisture close to the surface and present crusting, which avoid the infiltration of water

• mulch dissipate the kinetic energy of raindrops, which upon impact with the bare soil would otherwise loosen soil particles and cause crusting

• The aeration of the soil not constitutes a problem in untilled soils, except in cases of excess of moisture in the soil.

• An opening made on the soil with a pointed stick or a cut made with a disc of a maize planter is enough to prepare a seed bed. Remotion of 7 000 tones of soil/Ha during seed bed preparation cannot be justified for the only purpose of providing a place to deposit the seeds.

• Soil preparation has a coadjutant effect on maize disease and insect control, however in temperate zones; however the situation in the tropics is controversial. An adequate solution can be found by integrated pest and disease control.

• it is normally accepted that the soils are prepared to improve the physical structure, paradoxically however, in some cases the more the soil is worked, the more pore structure is destroyed. Plough pan and soil compaction are direct consequences of the use of ploughs and harrows.

• Due to compaction of the soil, there is limited water and nutrient availability to plants. Compaction also limits plant growth and yield, by affecting water infiltration, aeration, and plant disease and yield quality.

1.7.3 Conventional and conservation tillage.

Traditional soil preparation methods, almost at the same time, began to change when the mould-board plough was replaced advantageously by the disc plough. This new implement left a good proportion of the crop residue on the soil surface. However, both types of ploughs, and even the offset discs contribute to plough pan formation since they pack the lower furrow-slice. In some cases though, this hard layer may originate from other original causes during the process of soil formation. The advantage of chisel plough, as well as subsoilers, brought the advantage of being able to break the plough pan on naturally well drained-soils, resulting in better root penetration through the cracks, permitting the plants to reach nutrient lying below the arable layer.

During 1950 to 1960, a new generation of herbicides with residual effects and inhibitor effects on photosynthesis, such as triazine, revolutionized maize production. This allowed the implementation of a herbicide-based (atrazine) system known as zero-tillage, which combined various operations into a single pass of machinery, in such a way, that only the planting furrows was opened and the fertilizer incorporated at the same time. The zero-tillage can be performed by smallholders, even without herbicides, through an appropriated management of plant residues, such as mulch cover, which by means of its shading effect checks the development of weeds in the maize field.

This new system, which is a conservation system, includes operations that create an appropriate environment for the development of the plants of maize, while optimizing water and soil conservation. Sometimes, conservation tillage is confounded with minimum or reduced tillage, but the latter simply means that a farmer who normally compresses the soil in eight or even more passes with tilling and other farm equipment reduces a sustainable system that involve growing crops without plowing, harrowing or discing. It is characterized by the least amount of soil disturbance (since tillage is restricted to the minimum necessary for preparing the seedbed) a maximum retention of crop residues o the surface of the soil and at lower costs.

1.7.4 Time of planting.

The time of sowing will be determined by the actual date when appropriated rain is received. For a given cultivar, the exploitation of the best weather conditions in terms of temperature and rainfall, require precise information from experimentation. In some regions, the pre-soaking of the seed in lukewarm water overnight hastens the germination, which is of importance when temperature is low at the time of sowing. For landraces, the traditional dates of planting used by farmers derived from many years of trials and errors and should be highly considered and checked through experimentation. Usually, their planting

Dates avoid periods of excessive rainfall or drought at the time in which maize is most susceptible, such as around flowering.

An important problem in planting maize is the erratic onset of the rains in some regions, where intermittent showers tend to encourage weed growth before rains are in fact established.

If planting is done at the first onset of rainfall and the rain disappear immediately for more than five days or more, germination can be seriously affected. Sometimes the rains take so long to resume that farmers realize, and later there will be the need to replant, which is an expensive exercise.

1.7.5 Planting depth.

Independently of the tillage system used, the seed (treated with fungicides and/or insecticides when deemed necessary) should be placed at the correct depth, generally 5 to 10 cm. It will assure good contact with the moist of the soil to prevent desiccation and secure that the coleoptiles will not have problems in reaching the surface of the soil. Deeper planting is recommendable in areas with high soil temperature and when no mulch is present. Shallow planting in soils with poor moisture must be avoided, since it may not only endanger germination, but may also cause to be very uneven with lagging seedling poorly competing with plants from earlier germinating dates.

Cloddy soils prevent good contact between the soil and seed and are responsible for poor and uneven germination. Soils prone to crust formation should be planted under a zero-tillage system and with a good mulch cover. If planted under conventional tillage, crusting should be disrupted just prior to seedling emergence by means of a very superficial till.

1.7.6 Planting system.

In conventional tillage, maize seeds can be planted by making holes in the soil with a stick, a cutlass or a hoe, or by putting in a furrow opened with a small wooden or mould-board plough. There is several mechanized equipment for planting. They can be operated manually as a planting machine or a planting-fertilizing machine, and it is known in Brazil as "matraca" which has two small containers one for the seed and once for the fertilizer, see figure 12.

Fig. 12. The "Matraca"planter with a seed and a fertilizer deposit (A.D.Violic).

Another model with a zero tillage single planter and with fertilizer attachment is suitable for small and medium farmers , such as is showed in figure 13 below.

Fig. 13. Zero-tillage single planter with fertilizer attachment (A.D.Violic).

Planting machines also apply fertilizers, and some models include adjustable mechanism for the application of pesticides. Zero tillage planting machines must be equipped with additional parts suitable for the type of terrain being planted and for the presence of mulch, since the furrow-opener function is hindered by crop residue on the soil surface. CIMMYT has developed a small, low cost direct-drill planter known as "Chiquita" which can be animal or tractor drawn.

1.7.7 Planting density and spacing.

According to CIMMYT a good density is achieved by using an optimum density (for example, 85 000 plants/Ha.) and suing a simple calculation, where this quantity is reduced by 30 percent. The calculations are as follows:

85 000 - (85 000 x 0.3) = 60 000 plants/Ha.

So that, if 20 percent of the total expected plants are going to be lost between planting and harvesting, the recommended seeding rate should be adjusted upwards:

60 000 x 120 = 72 000 seeds/Ha.
100

Therefore, if 1Kg of maize seed variety contains 3 000 kernels, 25 Kg of seed per hectares will be necessary.

1.7.8 Fertilization.

For the improvement of agricultural sustainability it requires to reduce dependency on external purchase and non-renewable resources and minimizing the environmental harmful. However in order to improve their productivity and profitability, the use of agrochemical nutrients including insecticides, herbicides and fungicides, these may be unavoidable for farmers in tropical and subtropical areas, taking in considerations that the best ones are chosen and for each specific case in the correct way. The following table 15 shows some experimental data from several sources estimating the quantity of nutrients that maize plants extract from the soil to yield different levels.

These data can be used as a guide in order to estimate how much nutrients are necessary to attain certain yield, but taking into considerations that other biotic and abiotic factors have a minimum interference or influence on the maize crop.

1.7.9 Weed Control.

During maize production weeds, insects and diseases are important factors which can cause loss of maize. Generally the losses due to weed surpass those caused by diseases and insect together. Some studies have demonstrated a negative correlation between weed dry weigh and maize yield, with an actual grain weight reduction of up to 95 percent. Some report of yield losses in maize due to weeds ranging from 20 to 100 percent in Philippines, Brazil, and Gambia, Sierra Leone and Nigeria and 30 to 56 percent in Ethiopia.

Although land area is a constraint to small-scale farmers, the labor is often even more limiting, since it is estimated that weeding may utilize 35 to 70 percent of total agricultural labor in Africa, except in areas where animal traction is abundant, most weeding during the cropping season is done by hand.

There are three ways which weed affect the maize crop, in terms of light utilization, some weeds grow faster and taller than maize during the first growing stages of the crop, therefore depriving the maize crop from an adequate light supply for photosynthesis. However, early weeding give maize back its natural advantage, which is its tall size.

Likewise, water is a limiting factor in many tropical and subtropical maize growing areas and consequently a few days of water stree may be responsible for severe yield losses. An early drought during the first vegetative growth stage may kill young plants and therefore reduce severely the plant density/Ha. Weeds at this stage will increase water stress during this critical growth period between two weeks before and two weeks after flowering of maize, and consequently the crop will respond with lower yield.

In regards to the competition of nutrients, some weeds absorb up to twice as much nitrogen and phosphorus and up to three times more potash than maize on a plant dry matter basis. Nitrogen is usually the first nutrient to become deficient due to weed competition, and it is recognised by the pale colour of maize seedlings in the heave presence of weeds. In addition to the three types of competition, a biochemical type of competition known as allelopathy is the effect which some weeds have on maize by liberating growth inhibitors structure into the soil and may result in total crop loss.

The weeds which affect maize crop can be controlled by the followings methods:

· Cultural methods, such as crop rotation

· Mechanical methods which could be since hand pulling and hoeing to machine tillage and

· Chemical methods such as herbicides.

1.7.10 Water management.

The maize crop requires from 550 to 650 mm rainfall per unit area for a good growth, and it reduces weed competition for water throughout the growing cycle. However, if the profile of the soil is at field capacity at the time of planting, 350 to 450 mm of well-distributed rain through the growing cycle is enough to produce a good crop.

Good deep soil permitting the roots to grow down to 1.5m deep may have a moisture capacity of 1cm of water per 6cm of soil, i.e. about 250 mm of water. The coefficient of transpiration for maize is 280 to 350 litres of water per Kg. of grain produced. The ratio of evapo-transpiration from the maize field to open pan evaporation is around 0.35 at the seedling stage and increase to 0.80 at the silking stage before declining again. During the early growth stages, most water loss is due to evaporation from the base soil, since only about 2 to 2.5 mm of water per day is needed until the crop reaches the five-to-six-leaves stage. The water requirements also vary with plant density, it means than less water is needed with low stand than with high stand. But, this relationship is not always linear since there is a point at which increasing density will increase water need at a lower rate since the utilization of water in evapo-transpiration does not increase. Considering other losses not related to photosynthesis, and the fact that for every Kg of grain there is about another 1.2 kg of Stover, the total water needed to produce 1kg of maize grain is about 600 litres.

The main sources for the maize crop may come from the moisture retained in the soil before planting, the rainfall during the crop season, from irrigation and in less amounts from dew condensed on the leaves that is funnelled by the leaf bleads and stems to the base of the plant.

Most tropical cropping areas are rain fed, which capture and storage of as much water as possible in the soil profile imperative. The solution is irrigation, which although expensive, should be important in the tropics.

When irrigation is available, it should be a priority to provide water during the period two week before to two weeks after silking.

The most common system of irrigation and where water is abundant should consider borders, furrows, level furrows, terraces and sprinklers. Likewise, in order to choose an irrigation system some factors should be taking into account such as the slope, soil texture and depth, topography and costs.

Among the main features of irrigation system are:

· The borders are strips of land surrounded by border which run in the same direction of the slope. The borders guide the water over the field, when it leaves the head ditch.

· Contour furrows are used in steep terrain. The furrows laid out on the contours and the slope of the furrow is less than 3 percent (this is the terrain falls less than 3 m in 100 m).

· Terraces are level beds where maize crop could be irrigated by borders or furrows; it is suitable on soils which do not crack too much and with slopes up to more than 30 percent .

· Sprinklers, which are very used for maize seed production due to its high cost is suitable for uneven fields with slopes even higher than 30 percent .

By using special attachments sprinklers irrigation allow to make fertigation, which is the application of fertilizers and pesticides together with the irrigation water.

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