Contents - Previous - Next
World maize production increased from 1979-1981 to 1987, as shown by continent in Table 4. The land area planted with maize increased from 105 million ha in 1961 to about 127 million ha in 1987. Although part of the increase resulted from additional land area planted, significant increases in production resulted from genetic improvement and more efficient technological field practices and fertilizer applications, as well as from the introduction of new, more highly reproductive varieties.
TABLE 4 - World maize production
|Region and year||Area harvested (1 000 ha)||Yield (kg/ha)||Production (1 000 MT)|
|1979-81||18 193||1 554||28 268|
|1985||19 099||1 522||29 069|
|1986||19 580||1 575||30 840|
|1987||19 512||1 395||27 225|
|North and Central America|
|1979-81||39 399||5 393||212 384|
|1 985||40 915||6 092||249 258|
|1986||37 688||6 116||230 511|
|1987||35 187||5 690||200 211|
|1979-81||16 751||1 928||32 369|
|1985||17 813||2 182||38 859|
|1986||18 799||2 021||38 001|
|1987||19 413||2 143||41 595|
|1979-81||36 815||2 296||84 531|
|1985||35 246||2 628||92 629|
|1986||37 474||2 729||102 274|
|1987||37 399||2 788||104 269|
|1979-81||11 738||4 668||54 792|
|1985||11 556||5 423||62 673|
|1986||11 539||6 207||71 621|
|1987||11 405||6 039||68 901|
|1979-81||3 063||2 989||9 076|
|1985||4 482||3 214||14 406|
|1986||4 223||2 955||12 479|
|1987||4 600||3 217||14 800|
|1979-81||126 035||3 345||421 751|
|1985||129 235||3 771||487 367|
|1986||129 411||3 757||486 198|
|1987||127 605||3 584||457 365|
Source: FAO, 1988
The developing countries have more area given to maize cultivation than developed countries, but yield in the latter is about four times higher. Since 1961, yields per ha in the United States, for example, have increased significantly, while yields in Mexico, Guatemala and Nigeria (selected as countries where maize intake by the human population is high, particularly in the first two) have increased only slightly. While most of the production in developing countries is for human consumption, in the developed world it is mainly for industrial use and animal feed. The high yields and production in North and Central America are mainly attributed to the United States, which outproduces countries such as Mexico where maize is the most important staple cereal grain. With changing rural-to-urban populations and lifestyles in developing countries, there is a continuous shift to the consumption of wheat, which may influence maize production. There is a slow increase in its use in industry and as an animal feed, particularly for poultry and other monogastric animals. A comparison of the available data for wheat, maize and rice put maize as the second most important cereal grain, after wheat and before rice. In terms of yield per hectare, however, maize outyields the other two. The only food crop outyielding maize in tonnes per hectare is potatoes in their unprocessed state, though not on an equal moisture basis.
As indicated in previous sections, maize has three possible uses: as food, as feed for livestock and as raw material for industry. As a food, the whole grain, either mature or immature, may be used; or the maize may be processed by dry milling techniques to give a relatively large number of intermediary products, such as maize grits of different particle size, maize meal, maize flour and flaking grits. These materials in turn have a great number of applications in a large variety of foods. Maize grown in subsistence agriculture continues to be used as a basic food crop. In developed countries more than 60 percent of the production is used in compounded feeds for poultry, pigs and ruminant animals. In recent years, even in developing countries in which maize is a staple food, more of it has been used as an animal feed ingredient. "High moisture" maize has been paid much attention recently as an animal feed because of its lower cost and its capacity to improve efficiency in feed conversion. The by-products of dry milling include the germ and the seed-coat. The former is used as a source of edible oil of high quality. The seed-coat or pericarp is used mainly as a feed, although in recent years interest has developed in it as a source of dietary fibre (Earll et al., 1988; Burge and Duensing, 1989). Wet milling is a process applicable mainly in the industrial use of maize, although the alkaline cooking process used in manufacturing tortillas (the thin, flat bread of Mexico and other Central American countries) is also a wet milling operation that removes only the pericarp (Bressani, 1990). Wet milling yields maize starch and by-products such as maize gluten, used as a feed ingredient. The maize germ processed to produce oil gives as a by-product maize germ meal, used as an animal feedstuff. Some attempts have been made to use these by-products for humans in food mixes and formulations.
Although the technology has been available for a long time, the increase in fuel oil prices has resulted in much research on the fermentation of maize to produce alcohol, popular in some states of North America. Fermentation also provides some alcoholic beverages.
Finally, maize plant residues also have important uses, including animal feeds as well as a number of chemicals produced from the cobs, such as furfural and xylose. These residues are also important as soil conditioners.
There are significant amounts of data on the chemical composition of maize. Many studies have been conducted to understand and evaluate the effects of the genetic make-up of the relatively large number of available maize varieties on chemical composition, as well as the effects of environmental factors and agronomic practices on the chemical constituents and nutritive value of the kernel and its anatomical parts. Chemical composition after processing for consumption is an important aspect of nutritive value (see Chapter 5); it is affected by the physical structure of the kernel, by genetic and environmental factors, by processing and by other links in the food chain. In this chapter, the chemical nature of maize, of both common and quality protein types, is described as a basis for understanding the nutritive value of various maize products consumed throughout the world.
There are important differences in the chemical composition of the main parts of the maize kernel as shown in Table 5. The seed-coat or pericarp is characterized by a high crude fibre content of about 87 percent, which is constituted mainly of hemicellulose (67 percent), cellulose (23 percent) and lignin (0.1 percent) (Burge and Duensing, 1989). On the other hand, the endosperm contains a high level of starch (87.6 percent) and protein levels of about 8 percent. Crude fat content in the endosperm is relatively low. Finally, the germ is characterized by a high crude fat content, averaging about 33 percent. The germ also contains a relatively high level of protein (18.4 percent) and minerals. Some information is available on the chemical composition of the aleurone layer (see Figure 1), which is relatively high in protein content (about 19 percent) as well as in crude fibre. Tables 2 and 3 provide some additional details on nitrogen distribution in the maize kernel. The endosperm contributes the largest amount, followed by the germ, with only small amounts from the seed-coat. About 92 percent of the protein in teosinte comes from the endosperm. Protein in the maize kernel has been reported on by a number of researchers (e.g. Bressani and Mertz, 1958).
TABLE 5 - Proximate chemical composition of main parts of maize kernels (%)
Source: Watson, 1987
From the data shown in Tables 2 and 3 it is evident that the carbohydrate and protein contents of maize kernels depend to a very large extent on the endosperm, and crude fat and to a lesser extent protein and minerals on the germ. Crude fibre in the kernel comes mainly from the seed-coat. The weight distribution among parts of the maize kernel and their particular chemical composition and nutritive value are of great importance when maize is processed for consumption. In this regard there are two important matters from the nutritive point of view. Germ oil provides relatively high levels of fatty acids (Bressani et al., 1990; Weber, 1987). Where there are high intakes of maize, as in certain populations, those who consume the degermed grain will obtain less fatty acids than those who eat processed whole maize. This difference is probably equally important with respect to protein, since the amino acid content of germ proteins is quite different from that of endosperm protein. This is indicated in Table 6, in which essential amino acids are expressed as mg percent by weight and as mg per g N. As Table 2 shows, the endosperm represents between 70 and 86 percent of the kernel weight and the germ between 7 and 22 percent. It follows that, in considering the whole kernel, the essential amino acid content is a reflection of the amino acid content in the protein of the endosperm, in spite of the fact that the amino acid pattern in the germ protein is higher and better balanced. Germ proteins nevertheless contribute a relatively high amount of certain amino acids, although not enough to provide a higher quality of protein in the whole kernel. The germ provides some lysine and tryptophan, the two limiting essential amino acids in maize protein. Endosperm proteins are low in lysine and tryptophan, as is the whole grain protein (see Table 6, in which the FAO/ WHO essential amino acid pattern is also shown). The deficiencies in lysine tryptophan and isoleucine have been well demonstrated by numerous animal studies (Howe, Janson and Gilfillan, 1965) as well as by a few studies on humans (Bressani, 1971).
TABLE 6 - Essential amino acid content of germ protein and endosperm protein
|Amino acid||Endosperma||Germb||FAD/WHO pattern|
|mg %||mg/g N||mg %||mg/g N|
|Leucine||1 024||810||1 030||444||440|
|Total sulphur amino acids||249||197||362||156||220|
a1.16 percent N
b2.32 percent N
Source: Orr and Watt, 1957
TABLE 7 - Net protein of whole grain, germ and endosperm of Guatemalan maize varietiesa
aExpressed as percentage of case in (100%)
Source: Poey et al., 1979
The superior quality of germ protein to endosperm protein in various samples of maize is shown in Table 7, which compares the quality of the two parts as percentages of the reference protein, casein in this case. The maize varieties include three of common maize and one of quality protein maize (QPM). In all cases the quality of germ proteins is much higher than that of endosperm proteins and is obviously superior to the quality of whole kernel protein. Endosperm protein quality is lower than that of the whole kernel because of the higher contribution of germ protein. The data also show less difference in the quality of germ and endosperm proteins in the QPM variety. Furthermore, the QPM endosperm and whole grain quality are significantly superior to the endosperm and whole grain quality of the other samples. These data, again, are important in regard to how maize is processed for consumption and in its impact on the nutritional status of people. They also clearly show that the quality of QPM is better than that of common maize. The higher quality of QPM endosperm is also of significance for populations that consume maize without the germ.
Contents - Previous - Next