Gross chemical composition

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Information on the gross chemical composition of maize is abundant. The variability of each major nutrient component is great. Table 8 summarizes data on various types of maize taken from several publications. The variability observed is both genetic and environmental. It may influence the weight distribution and individual chemical composition of the endosperm, germ and hull of the kernels.

TABLE 8 - Gross chemical composition of different types of maize (%)

Source: Cortez and Wild-Altamirano, 1972

Starch

The major chemical component of the maize kernel is starch, which provides up to 72 to 73 percent of the kernel weight. Other carbohydrates are simple sugars present as glucose, sucrose and fructose in amounts that vary from 1 to 3 percent of the kernel. The starch in maize is made up of two glucose polymers: amylose, an essentially linear molecule, and amylopectin, a branched form. The composition of maize starch is genetically controlled. In common maize, with either the dent or flint type of endosperm, amylose makes up 25 to 30 percent of the starch and amylopectin makes up 70 to 75 percent. Waxy maize contains a starch that is 100 percent amylopectin. An endosperm mutant called amylose-extender (ae) induces an increase in the amylose proportion of the starch to 50 percent and higher. Other genes, alone or in combination, may also modify the amylose-to-amylopectin ratio in maize starch (Boyer and Shannon, 1987).

Protein

After starch, the next largest chemical component of the kernel is protein. Protein content varies in common varieties from about 8 to 11 percent of the kernel weight. Most of it is found in the endosperm. The protein in maize kernels has been studied extensively. It is made up of at least five different fractions, according to Landry and Moureaux (1970, 1982). In their scheme, albumins, globulins and non-protein nitrogen amount to about 18 percent of total nitrogen, in a distribution of 7 percent, 5 percent and 6 percent, respectively. The prolamine fraction soluble in 55 percent isopropanol and isopropanol with mercaptoethanol (ME) contributes 52 percent of the nitrogen in the kernel. Prolamine 1 or zein 1 soluble in 55 percent isopropanol is found in the largest concentration, about 42 percent, with 10 percent provided by prolamine 2 or zein 2. An alkaline solution, pH 10 with 0.6 percent ME, extracts the glutelin fraction 2, in amounts of about 8 percent, while glutelin 3 is extracted with the same buffer as above with 0.5 percent sodium dodecyl sulphate in amounts of 17 percent for a total globulin content of 25 percent of the protein in the kernel. Usually a small amount, about 5 percent, is residual nitrogen.

Table 9 summarizes data by Ortega, Villegas and Vasal (1986) on the protein fractionation of a common maize (Tuxpeño-1) and a QPM (Blanco Dentado-1). Fractions II and III are zein I and zein II, of which zein I (Fraction II) is significantly higher in the Tuxpeño-1 variety than in the QPM. Similar results have been published by other researchers. Amounts of the alcohol-soluble proteins are low in immature maize. They increase as the grain matures. When these fractions were analysed for their amino acid content, the zein fraction was shown to be very low in lysine content and lacking in tryptophan. Since these zein fractions make up more than 50 percent of the kernel protein, it follows that the protein is also low in these two amino acids. The albumin, globulin and glutelin fractions, on the other hand, contain relatively high levels of lysine and tryptophan. Another important feature of the zein fractions is their very high content of leucine, an amino acid implicated in isoleucine deficiency (Patterson et al., 1980).

Quality protein maize differs from common maize in the weight distribution of the five protein fractions mentioned above, as shown in Table 9. The extent of the change is variable and affected by genotype and cultural conditions. It has been found, however, that the opaque-2 gene reduces the concentration of zein by some 30 percent. As a result, lysine and tryptophan content is higher in QPM varieties than in common maize.

TABLE 9 - Protein fraction distribution of Tuxpeño-1 and Blanco Dentado-1 QPM (whole grain)

Source: Ortega, Villegas and Vasal, 1986

The nutritional quality of maize as a food is determined by the amino acid make-up of its protein. Representative amino acid values are shown in Table 10 for both common maize and QPM. To establish the adequacy of the essential amino acid content the table also includes the FAD/WHO essential amino acid pattern. In common maize, deficiencies in lysine and tryptophan are evident as compared with QPM. An additional important feature is the high leucine content in common maize and the lower value of this amino acid in QPM.

Oil and fatty acids

The oil content of the maize kernel comes mainly from the germ. Oil content is genetically controlled, with values ranging from 3 to 18 percent. The average fatty acid composition of the oil in selected varieties from Guatemala is shown in Table 11. These values differ to some extent; it may be expected that oils from different varieties have different compositions. Maize oil has a low level of saturated fatty acids, i.e. on average 11 percent palmitic and 2 percent stearic acid. On the other hand, it contains relatively high levels of polyunsaturated fatty acids, mainly linoleic acid with an average value of about 24 percent. Only very small amounts of linoleic and arachidonic acids have been reported. Furthermore, maize oil is relatively stable since it contains only small amounts of linoleic acid (0.7 percent) and high levels of natural antioxidants. Maize oil is highly regarded because of its fatty acid distribution, mainly oleic and linoleic acids. In this respect, populations that consume degermed maize benefit less in terms of oil and fatty acids than populations that consume whole-kernel products.

TABLE 10 - Amino acid content of maize and teosinte (%)

TABLE 11 - Fatty acid content of Guatemalan maize varieties and Nutricta QPM (%)

Source: Bressani et al., 1990

Dietary fibre

After carbohydrates, proteins and fats, dietary fibre is the chemical component found in the greatest amounts. The complex carbohydrate content of the maize kernel comes from the pericarp and the tip cap, although it is also provided by the endosperm cell walls and to a smaller extent the germ cell walls. The total soluble and insoluble dietary fibre content of maize kernels is shown in Table 12. Differences in soluble and insoluble dietary fibre are small between samples, even though QPM Nutricta has higher levels of total dietary fibre than common maize, mainly because of a higher level of insoluble fibre. Table 13 shows values of fibre expressed as acid and neutral detergent fibre, hemicellulose and lignin in whole maize. The values shown in the table are similar to those reported by Sandstead et al. (1978) and Van Soest, Fadel and Sniffen (1979). Sandstead et al. found that maize bran was composed of 75 percent hemicellulose, 25 percent cellulose and 0.1 percent lignin on a dry-weight basis. Dietary fibre content in dehulled kernels would obviously be lower than that of whole kernels.

TABLE 12 - Soluble and insoluble dietary fibre In common and quality protein maize (%)

Source: Bressani, Breuner and Ortiz, 1989

TABLE 13 - Neutral and acid detergent fibre, hemicellulose and lignin in five maize varieties (%)

Source: Bressani, Breuner and Ortiz, 1989

Other carbohydrates

When mature, the maize kernel contains carbohydrates other than starch in small amounts. Total sugars in the kernel range between I and 3 percent, with sucrose, the major component, found mostly in the germ. Higher levels of monosaccharides, disaccharides and trisaccharides are present in maturing kernels. At 12 days after pollination the sugar content is relatively high, while starch is low. As the kernel matures, the sugars decline and starch increases. For example, sugars were found to have reached a level of 9.4 percent of kernel dry weight in 16-day-old kernels, but the level decreased significantly with age. Sucrose concentration at 15 to 18 days after pollination was between 4 and 8 percent of kernel dry weight. These relatively high levels of reducing sugar and sucrose are possibly the reason why immature common maize and, even more, sweet maize are so well liked by people.

Minerals

The concentration of ash in the maize kernel is about 1.3 percent, only slightly lower than the crude fibre content. The average mineral content of some samples from Guatemala is shown in Table 14. Environmental factors probably influence the mineral content. The germ is relatively rich in minerals, with an average value of 11 percent as compared with less than I percent in the endosperm. The germ provides about 78 percent of the whole kernel minerals. The most abundant mineral is phosphorus, found as phytate of potassium and magnesium. All of the phosphorus is found in the embryo, with values in common maize of about 0.90 percent and about 0.92 percent in opaque-2 maize. As with most cereal grains, maize is low in calcium content and also low in trace minerals.

Fat-soluble vitamins

The maize kernel contains two fat-soluble vitamins: provitamin A, or carotenoids, and vitamin E. Carotenoids are found mainly in yellow maize, in amounts that may be genetically controlled, while white maize has little or no carotenoid content. Most of the carotenoids are found in the hard endosperm of the kernel and only small amounts in the germ. The betacarotene content is an important source of vitamin A, but unfortunately yellow maize is not consumed by humans as much as white maize. Squibb, Bressani and Scrimshaw (1957) found beta-carotene to be about 22 percent of total carotenoids (6.4 to 11.3 µg per gram) in three yellow maize samples. Cryptoxanthin accounted for 51 percent of total carotenoids. Vitamin A activity varied from 1.5 to 2.6 µg per gram. The carotenoids in yellow maize are susceptible to destruction after storage. Watson (1962) reported values of 4.8 mg per kg in maize at harvest, which decreased to 1.0 mg per kg after 36 months of storage. The same loss took place with xanthophylls. Recent studies have shown that the conversion of beta-carotene to vitamin A is increased by improving the protein quality of maize.

TABLE 14 - Mineral content of maize (Average of five samples)

Source: Bressani, Breuner and Ortiz, 1989

The other fat-soluble vitamin, vitamin E, which is subject to some genetic control, is found mainly in the germ. The source of vitamin E is four tocopherols, of which alpha-tocopherol is the most biologically active. Gamma-tocopherol is probably more active as an antioxidant than alphatocopherol, however.

Water-soluble vitamins

Water-soluble vitamins are found mainly in the aleurone layer of the maize kernel, followed by the germ and endosperm. This distribution is important in processing, which, as will be shown later, induces significant losses of the vitamins. Variable amounts of thiamine and riboflavin have been reported. The content is affected by the environment and cultural practices rather than by genetic make-up. Variability between varieties has, however, been reported for both vitamins. The water-soluble vitamin nicotinic acid has attracted much research because of its association with niacin deficiency or pellagra, which is prevalent in populations consuming high amounts of maize (Christianson et al., 1968). As with other vitamins, niacin content varies among varieties, with average values of about 20 µg per gram. A feature peculiar to niacin is that it is bound and therefore not available to the animal organism. Some processing techniques hydrolyze niacin, thereby making it available. The association of maize intake and pellagra is a result of the low levels of niacin in the grain, although experimental evidence has shown that amino acid imbalances, such as the ratio of leucine to isoleucine, and the availability of tryptophan are also important (Gopalan and Rao, 1975; Patterson et al., 1980).

Maize has no vitamin B12, and the mature kernel contains only small amounts of ascorbic acid, if any. Yen, Jensen and Baker (1976) reported a content of about 2.69 mg per kg of available pyridoxine. Other vitamins such as choline, folic acid and pantothenic acid are found in very low concentrations.

Changes in chemical composition and nutritive value during grain development

In many countries, immature maize is often used as a food, either cooked whole as corn on the cob or ground to remove the seed-coat, with the pulp used to make thick gruels or foods like tamalitos. The changes in chemical composition that take place upon maturation are important. All relevant studies have shown a decrease in nitrogen, crude fibre and ash on a dry-weight basis and an increase in starch and ether extract (e.g. Ingle, Bietz and Hageman, 1965). The alcohol-soluble proteins increase rapidly as the kernel matures, while acid- and alkali-soluble proteins decrease. During this biochemical process arginine, isoleucine, leucine and phenylalanine (expressed as mg per g N) increase, while lysine methionine and tryptophan decrease with maturation. Gómez-Brenes, Elías and Bressani (1968) further showed a decrease in protein quality (expressed as protein efficiency ratio). Thus, immature maize should be promoted during weaning or for infant nutrition.

Nutritional value of maize

The importance of cereal grains to the nutrition of millions of people around the world is widely recognized. Because they make up such a large part of diets in developing countries, cereal grains cannot be considered only as a source of energy, as they provide significant amounts of protein as well. It is also recognized that cereal grains have a low protein concentration and that protein quality is limited by deficiencies in some essential amino acids, mainly lysine Much less appreciated, however, is the fact that some cereal grains contain an excess of certain essential amino acids that influence the efficiency of protein utilization. The classic example is maize. Other cereal grains have the same constraints but less obviously.

A comparison of the nutritional value of maize protein with the protein quality of eight other cereals is given in Table 15, expressed as percentages of casein. The protein quality of common maize is similar to that of the other cereals except rice. Both opaque-2 maize and the hard-endosperm QPM (Nutricta) have a protein quality not only higher than that of common maize, but also significantly higher than that of other cereal grains.

The reasons for the low quality of maize proteins have been extensively studied by numerous investigators. Among the first were Mitchell and Smuts (1932) who obtained a definite improvement in human growth when 8 percent maize protein diets were supplemented with 0.25 percent lysine These results have been confirmed over the years by several authors (e.g. Howe, Janson and Gilfillan, 1965), while others (e.g. Bressani, Elías and graham, 1968) have shown that the addition of lysine to maize causes only a small improvement in protein quality. These differing results may be explained by variations in the lysine content of maize varieties. Work in this field led to the discovery by Mertz, Bates and Nelson (1964) of the highlysine maize called opaque-2.

TABLE 15 - Protein quality of maize and other cereal grains

Some researchers (Hogan et al., 1955) have reported that tryptophan rather than lysine is the first limiting amino acid in maize, which may be true for some varieties with a high lysine concentration or for maize products modified by some kind of processing. All researchers have agreed that the simultaneous addition of both lysine and tryptophan improves the protein quality of maize significantly; this has been demonstrated in experimental work with animals.

The improvement in quality obtained after the addition of lysine and tryptophan has been small in some studies and higher in others when other amino acids have been added. Apparently, the limiting amino acid after lysine and tryptophan is isoleucine, as detected from animal feeding studies (Benson, Harper and Elvehjem, 1955). Most researchers who reported such findings indicated that the effect of isoleucine addition resulted from an excess of leucine which interfered with the absorption and utilization of isoleucine (Harper, Benton and Elvehjem, 1955; Benton et al., 1956). It has been reported that high consumption of leucine along with the protein in maize increases niacin requirements, and this amino acid could be partly responsible for pellagra.

When a response to threonine addition has been observed, it has been attributed to this amino acid's correction of amino acid imbalances caused by the addition of methionine. A similar role can be ascribed to added isoleucine resulting in improved performance. Similarly, the addition of valine, which results in a decrease in protein quality, could be counteracted by the addition of either isoleucine or threonine.

In any case, isoleucine seems to be more effective than threonine, producing more consistent results. A possible explanation for these findings is that maize is not deficient in either isoleucine or threonine. However, some samples of maize may contain larger amounts of leucine, methionine and valine, end these require the addition of isoleucine and threonine besides lysine and tryptophan to improve protein quality. In any case, the addition of 0.30 percent L-lysine and 0.10 percent L-tryptophan easily increases the protein quality of maize by 150 percent (Bressani, Elías and graham, 1968). Many of the results of the limiting amino acids in maize protein are influenced by the level of protein in the maize. As was indicated previously, protein content in maize is a genetic trait that is affected by nitrogen fertilization. The observed increase in protein content is highly correlated with zein, or the alcohol-soluble protein, which is low in lysine and tryptophan and contains excessive amounts of leucine. Frey (1951) found a high correlation between protein content and zein in maize, a finding that has been confirmed by others. Using different animal species, various authors have concluded that the protein quality of low-protein maize is higher than that of high-protein maize when the protein in the diets used is the same. However, weight for weight, high-protein maize is slightly higher in quality than low-protein maize. The levels of dietary protein, then, affect the response observed upon amino acid supplementation with lysine and tryptophan in particular but with other amino acids as well, such as isoleucine and threonine.

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