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Because of the great importance of maize as a basic staple food for large population groups, particularly in developing countries, and its low nutritional value, mainly with respect to protein, many efforts have been made to improve the biological utilization of the nutrients it contains. Three approaches have been tried: genetic manipulation, processing and fortification. Abundant data show great variability in the chemical composition of maize. Although environment and cultural practices may be partly responsible, the variability of various chemical compounds is of genetic origin; thus composition can be changed through appropriate manipulation. Efforts in this direction have concentrated on carbohydrate composition and on quantity and quality of oil and protein. Some efforts have also been made to manipulate other chemical compounds such as nicotinic acid and carotenoids. Processing is not widely recognized as a means of improving nutritive value; however, examples are presented to show its effects and potential. Finally, there have been many efforts to fortify maize, with outstanding results, but unfortunately fortification has not been implemented to a large extent. This approach, however, may become important in the future as more people consume industrially processed foods, which can be more easily and efficiently fortified.
The component with the greatest concentration in the maize kernel is starch. Since the plant accumulates starch in the endosperm, which is subject to genetic influence, starch can become a good source of energy. The quantity and quality of the carbohydrate fraction can be modified by breeding as described in recent reviews by Boyer and Shannon (1983) and Shannon and Garwood (1984). The waxy gene (Wx) in waxy maize has been shown to control amylopectin starch in the endosperm up to 100 percent with very low amounts of amylose (Creech, 1965). Other genes and gene combinations have been shown to be responsible for the composition of the starch in the endosperm. The amylose-extender gene (Ae) increases the amylose fraction of the starch from 27 to 50 percent (Vineyard et al., 1958). Other genes cause an increase in reducing sugars and sucrose. Sugary (Su) genes produce relatively high amounts of water-soluble polysaccharides and amylose. Maize kernels containing this gene are sweet and are important for canning. Their starch content and quality also have nutritional implications, since some starch granules have low digestibility while others have high digestibility, as demonstrated by Sandstead, Hites and Schroeder (1968). These researchers suggested that maize varieties with waxy or sugary genes could be of better nutritional value for monogastric animals because of the greater digestibility of the type of starch they produce.
Classical studies at the University of Illinois demonstrated the feasibility of changing the protein content of the maize kernel by selection. In these studies it was shown that protein content could be increased from 10.9 to 26.6 percent in the high-protein (HP) strain after 65 generations of selection. The low-protein strain contained about 5.2 percent. Dudley, Lambert end Alexander (1974) and Dudley, Lambert and de la Roche (1977) demonstrated that the protein content of standard inbred lines could be increased by crossing with the HP strain from Illinois and then backcrossing to the inbred line. Woodworth and Jugenheimer (1948) concluded that total protein content could be increased by selection in an open pollinated variety or by crossing standard inbred lines with an HP strain followed by backcrossing and selection in segregating populations.
The full expression of the protein genes in maize can be attained with appropriate levels of nitrogen fertilizers. Tsai, Huber and Warren (1978, 1980) and Tsai et al. (1983) showed that nitrogen fertilization of maize increased total protein because of an increase in prolamine content. Studies conducted by others showed, however, that the protein quality of the HP strains was lower than that of common maize since the increase in protein was due to an increase in the prolamine fraction. Eggert, Brinegar and Anderson (1953), from studies of pigs, showed that HP maize had lower biological value than common maize, which they attributed to the higher prolamine content in HP than in normal protein maize. The value of an HP maize kernel will depend on how it behaves agronomically and economically compared with maize with about 10 percent protein. The data available show that these types of maize not only require more soil nitrogen but also yield less than normal protein maize.
The low protein quality of maize stems mainly from the deficiency in the protein of the essential amino acids lysine and tryptophan. Still, variability in both amino acids has been shown (Bressani, Arroyave and Scrimshaw, 1953; Bressani et al., 1960). As early as 1949, Frey, Brimhall and Sprague were able to show the genetic variability in tryptophan content in a cross between the Illinois HP and LP strains as well as in hybrids. Biological testing in which maize strains furnished the same level of protein in the diet also showed variability. All of these data suggest the feasibility of improving the quality of maize varieties. Mertz, Bates and Nelson (1964) found that the opaque-2 gene significantly increased the lysine and tryptophan content in maize endosperm. This gene also reduced the leucine level, giving a better leucine-to-isoleucine ratio. In 1965, Nelson, Mertz and Bates showed that the floury-2 gene when homozygous could also increase the lysine and tryptophan levels in maize. Research conducted at the Centro Internacional de Mejoramiento de Maíz y Trigo (CIMMYT) eventually yielded maize lines of QPM, which agronomically behave like common maize. As shown elsewhere in this book, the protein quality of these materials is significantly higher than that of common maize as shown by tests in humans.
Although such types of maize are available, it has been difficult to grow them commercially, even though the benefits to be derived from them by large maizeconsuming populations would be high.
Genetic studies have also revealed that oil content in maize is subject to genetic influence, with diversity often found, although environment and agronomic practices can influence fatty acid composition (Jellum and Marion, 1966; Leibovits and Ruckenstein, 1983). As with protein content, mass selection over 65 years increased oil content from 4.7 to 16.5 percent. This increase was obtained through increases in the size of the germ. The problem with high-oil varieties is a low yield, although it has been reported that varieties with 7 to 8 percent oil yield as well as varieties with lower oil content. Besides total oil content, some studies have shown that the fatty acid content may also be subject to genetic control, as seen by changes in linoleic acid content in maize oil. Poneleit and Alexander (1965) suggested a single-gene or single-gene-plusmodifier effect. A multi-gene system of inheritance has been proposed by other investigators. QPM oil fatty acid composition was found to be similar to that reported for normal maize.
Because of the association of maize consumption and pellagra and the low availability of nicotinic acid in maize, efforts have been made to increase niacin in maize by genetic means. Variability in 22 varieties planted in one location ranged from 1.25 to 2.6 mg per 100 g (Aguirre, Bressani and Scrimshaw, 1953). The problem with niacin in maize and in other cereal grains is that it is unavailable to the animal organism.
The other nutrient that has received some attention is carotene, a precursor of vitamin A. Results from some investigators have shown yellow maize to vary in vitamin A activity from 1.52 to 2.58 µg per gram. Cryptoxanthein contributed 3X.3 to 57.3 percent of the total activity and beta-carotene the difference (Squibb, Bressani and Scrimshaw, 1957). Other researchers have indicated that provitamin A activity is under genetic control in the maize kernel.
Often the processing of foodstuffs stabilizes nutrients in the food, but losses may take place when optimum conditions are exceeded. There are cases, however, in which processing induces beneficial changes in the food; a classic case is the elimination of antiphysiological factors in beans.
Lime-cooking of maize as described in Chapter 4 causes some losses in nutrient content, but it also induces some important nutritional changes. Its effects on calcium, amino acids and niacin content have already been described in Chapter 4.
Besides the lime cooking process, other processes have been reported to improve the quality of maize. One such process is natural fermentation of cooked maize, which results in higher B-vitamin concentration and protein quality (Wang and Fields, 1978). Pozol, a food made from lime-treated maize allowed to ferment naturally, has been shown to be of higher quality than raw maize or tortillas. Germination of the grain has also been reported to improve the nutritional value of maize by increasing lysine and to some extent tryptophan (Tsai, Dalby and Jones, 1975; Martinez, Gómez-Brenes and Bressani, 1980) and decreasing zein content. A similar result was found with QPM.
A third approach often used to improve the nutritive value of foods, mainly cereal grains, is fortification. Because of the great nutritional limitations in maize, many efforts have been made to improve its quality, and particularly that of its protein, through addition of amino acids or protein sources rich in the limiting amino acids.
Supplementation with amino acids
Raw maize proteins have been shown to be of a low nutritive value because of deficiencies in the essential amino acids lysine and tryptophan. Many studies conducted with animals have demonstrated that the addition of both amino acids improves the quality of the protein. Some workers have even found that besides lysine and tryptophan, isoleucine is also deficient, possibly because of an excess of leucine in maize proteins (Rosenberg, Rohdenburg and Eckert, 1960). Similar data have been obtained from studies with animals when lime-treated maize was supplemented with lysine and tryptophan (Bressani, Elías and graham, 1968). These results have been confirmed in nitrogen balance studies conducted with children as shown in Chapter 6. (Selected results are shown in Table 32.) The finding that the addition of lysine and tryptophan at the lower levels of protein intake gave a nitrogen retention significantly higher than at the higher level of protein intake has often been overlooked, and the importance of protein quality has been overshadowed by that of energy intake.
Supplementation with protein sources
The results from animal and human studies in which limiting amino acids have been added to lime-treated maize have served as the basis for evaluating the ability of different types of protein supplements to improve its protein quality. Studies on protein supplementation of lime-treated maize flour have been published by many researchers using different food sources including milk, sorghum, cottonseed flour, fish flour, torula yeast and casein. Table 40 summarizes the results of adding small recommended amounts of various protein sources. The quality increase is at least 200 percent of the protein quality value of maize. In tests with young dogs, the nitrogen balances when maize was supplemented with 5 percent skim milk, 3 percent torula yeast and 4 percent fish flour were significantly higher than those measured when maize was given alone. Most of the supplements that have been tested have several characteristics in common. They all have a relatively high protein content and are good sources of lysine, with the exception of cottonseed protein and sesame oil meal. The latter is a good source of methionine. With the exception of casein and/or milk and fish protein concentrate, they are of vegetable origin.
The improvement in quality of protein in tortilla flour is in most cases a synergistic response to lysine and tryptophan enhancement and to a higher level of protein, both provided by the supplement. Since soybean protein in different forms is the supplement to tortilla flour most often tested by different investigators and because it is almost the only one also tested in children, with results comparable to those in studies with animals, its importance and effects are reviewed in this section. Figure 3 depicts the PER for combinations of common maize and opaque-2 maize with soybean flour in different ratios.
TABLE 40 - Recommended levels of protein concentrates to improve the protein quality of lime-treated maize
|Recommended level (%)
|Fish protein concentrate
|Soy protein isolate
Source: Bressani and Marenco, 1963
Studies show that maximum PER is achieved upon addition of 4 to 6 g percent soybean protein, whether from whole soy, soy flour (50 percent), soy protein concentrate or soy protein isolate (Bressani, Elías and graham, 1978; Bressani et al., 1981). For reasons of availability, cost and practical applications in developing countries, the results with whole soybean are discussed here. The 4 to 6 g percent level of supplementary protein can be provided by either 15 percent whole soybean or 8 percent soybean flour, which have resulted in comparable protein quality improvement. The advantage of using 15 percent whole soybeans is that supplementation can be carried out in the home with soybeans produced by the family; soybeans are very economical, and besides providing higher protein quantity and quality, they give some additional energy from the oil they contain.
FIGURE 3 - Protein efficiency ratio of combinations of common or opaque-2 maize and soybean flour
Whether the supplementation process is at home or at the industrial level, it has been demonstrated that nutritional quality improves, with the process being capable of destroying all trypsin inhibitor and urease activity in the soybean (Del Valle and Pérez-Villasenor, 1974; Del Valle, Montemayor and Bourges, 1976; Bressani, Murillo and Elías, 1974; Bressani et al., 1979). Tortillas made with 15 percent soybeans have been shown to be acceptable to rural consumers and have many of the properties of tortillas without soybeans, except that they are more flexible and softer. Many attempts have been made to transfer this technology at both the industrial and the home level, but this has not been a sustainable approach for various reasons such as the cost of soybeans and (possibly) changes in organoleptic characteristics.
With the relative increase in industrially produced lime-treated maize flour, fortification with protein sources and other nutrients is efficiently accomplished in a dry-mixing operation, as is done with other cereal flours. The problem is not so much the technology, but the lack of legislation, which if implemented could improve the quality of maize tortillas as is done with wheat flour in many countries throughout the world. The studies described above led to the development of a dry supplement to tortilla flour which contained 97.5 g percent of soybean flour (50 percent protein), 1.5 percent L-lysine HCl, 26.8 mg percent thiamine, l 6.2 mg percent riboflavin, l 9.3 mg percent niacin, 0.60 percent ferric orthophosphate, 0.031 percent vitamin A 250 and 0.133 percent corn starch. The quantity recommended for addition to tortilla flour was 8 percent by weight. Nitrogen balance studies in children fed this food are shown in Table 41 (Viteri, Martínez and Bressani, 1972). Maize nitrogen balance was only 42 percent of the nitrogen balance from milk. When maize with the supplement was fed, nitrogen balance was 84 percent of that from milk. All studies, in animals and children, show the same response, i.e. a significant improvement in the protein quality of maize. The effectiveness of this supplement was partially tested by Urrutia et al. (1976) and preliminary data suggested some improvement in the nutritional status of young children. Other maize-based foods such as arepas and fermented maize foods have also been shown to be improved by supplementation with soybean flour.
Supplementation with green vegetables
One form in which masa is eaten in some countries is the tamalifo. This is made by wrapping the dough in maize husks and placing it over steam.
TABLE 41 - Nitrogen balance in preschool children ted milk, normal maize and soybean/lysine supplemented maize
Tomalitos are often eaten instead of tortillas and have the advantage of remaining soft for a longer period. There are various ways to prepare them, some of which include the young leaves of native vegetables such as crotalaria and amaranthus. Chemical and nutritional studies have demonstrated that about a 5 percent contribution of these leaves improves the protein quality of the dough (Bressani, 1983). The reason is that they have relatively high levels of protein rich in lysine and tryptophan. They also provide minerals and vitamins, particularly provitamin A. Leaf protein concentrates have also been shown to improve the protein quality of cereal grains (Maciejewicz-Rys and Hanczakowski, 1989).
Supplementation with other grains
Sorghum is another grain that has been processed by lime-cooking in Mexico and Central America, particularly in areas where maize does not grow well. Sorghum tortillas, however, are not of the same organoleptic or nutritional quality as maize tortillas. Many successful efforts have been made to use blends of both cereal grains, among others by Vivas, Waniska and Rooney (1987) and Serna-Saldivar et al. (1987, 1988a, 1988b). Other approaches include the use of blends of common maize, since germination has been reported to increase lysine Mixtures of tortilla flour and rice and of tortilla flour and wheat flour have also been studied. The rice/maize products have higher nutritive value than the wheat/maize tortillas, as shown in Figure 4. These results show the superiority of rice over whole maize flour and of the latter over wheat flour. More recently, blends of amaranth grain with lime-cooked maize flour have been shown to have an improved protein quality because of the much higher lysine and tryptophan content of amaranth as compared with maize. The product has been reported to be of an acceptable organoleptic quality. Other products added include potato, rice and pinto beans, providing foods with acceptable sensory attributes.
High-quality protein foods
The nutritional value of maize, particularly maize protein, can also be improved by protein complementation. In this approach, the objective is to combine two or more protein sources with maize to maximize the quality of the product by achieving a good balance of the essential amino acids. Using this approach a number of high-quality foods have been developed. (Similar results can be obtained with other cereal grains.)
FIGURE 4 - Protein value of mixtures of two cereals
An example, complementation of both common and QPM maize with common black beans, is shown in Figure 5. Here the isonitrogenous replacement of the bean nitrogen by QPM nitrogen resulted in a constant PER increase up to a level corresponding to 50 percent of the protein from each component, with no further change as the nitrogen of the mixture was provided increasingly from the QPM. A similar result is observed with mixtures of the beans and common maize, except that as more of the dietary nitrogen is provided by maize, the protein quality drops. Further studies indicated that on the left side of the peak response the limiting amino acid was methionine, while on the right side it was lisyne. The peak was obtained through the contribution of lysine from beans to maize and the contribution of methionine from maize to beans. This response has served as the basis for formulating high-quality protein food mixtures containing 70 percent maize and 30 percent common beans.
A similar type of response is observed with mixtures of normal and QPM maize and soybean flour. The peak mixture is equivalent to 77 percent maize and 23 percent soybean flour on a weight basis. When whole soybean flour is used, however, the mixture by weight is 70 percent maize and 30 percent whole soyflour. This product is called maisoy and is commercially produced in Bolivia. It is used to improve lime-treated maize for tortillas or as a wheat flour extender for bakery products. Other oil seed flours have been used in a similar fashion, for example cottonseed flour (CSF) and maize. In this case there is no synergistic effect of complementation. Optimum quality mixtures can be obtained when CSF provides about 78 percent of the protein and maize 22 percent. This distribution by weight is equal to 40 percent CSF and 60 percent maize flour, which is the ratio for incaparina produced in Guatemala since 1960.
Many other mixtures of maize and other foods have been developed. The United States Department of Agriculture has been involved since 1957 in product and process development, and products such as instant and sweetened corn-soya milk and corn-soya bread are well known throughout the developing world. Many other mixtures have been developed with common maize or QPM and other protein sources, giving products of high nutritional value and acceptability.
FIGURE 5 - Protein efficiency ratio of combinations of common or opaque-2 maize and black beans
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