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Food science

Amaranth: The nutritive value and potential uses of the grain and by-products

Ricardo Bressani

In 1966 the National Academy of Sciences held a symposium on the Prospects of World Food Supply, at which Dr. Paul Manglesdorf presented a paper entitled "Genetic Potentials for Increasing Yields of Food Crops and Animals" [1]. Dr. Manglesdorf pointed out that, although humans during the course of their existence have used some 3,000 species of plants, only about 150 of those had entered commercial production. Apart from edible vegetables and fruits, approximately 21 species are today feeding humankind, including eight cereal grains, eight food legumes, two oil-containing food legumes, and three starchy food crops. Significant improvements in production and use have been realized throughout the years of intensive and continuous study by national and international agricultural research institutions. Still, it is recognized that this food base is narrow, and its exclusive use could lead to serious problems if the efforts against disease, insects, and environmental conditions are not continued. Furthermore, an important and vital activity in the effort to improve all these agricultural food crops is collecting their germ plasm as a source of needed genetic variability.

The relatively small number of edible grains suggests a need to introduce other food crops into production and commercial systems, and the logical choice would be those that are known to have played an important role in past civilizations and that are still produced and used today to some extent. There are several of these, and one of them is amaranth. Reports by a number of researchers indicate that amaranth was a very important crop in Aztec, Mayan, and Incan civilizations. Current thinking is that its production was significantly reduced as the result of restrictions imposed by the Spanish conquerors due to its association with religious festivities. Today, it is still produced, and foods such as alegría and alboroto are prepared on a small scale in Mexico and Guatemala respectively [2-6].

In Guatemala some native populations consume it as a porridge; however, it is considered a poor man's food. The leaves, consumed as a vegetable, are highly regarded, and their consumption is widespread throughout many countries. They are an excellent source of carotenes and have a high protein content, and supplement cereal grain efficiently [7, 8].

The research effort to bring amaranth back into production and commercial systems is a relatively recent undertaking. It has been reinforced by the economic support given by the National Academy of Sciences, through its Board on Science and Technology for International Development, to a number of researchers in various parts of the world such as Thailand, Kenya, Mexico, Guatemala, and Peru. In comparison with the funding given to other food crops and with the number of researchers involved, the support for amaranth is relatively small; however, some significant advances have been made, and recognition of the potential of this crop throughout the world is beginning to grow. Expectations for it are great because of its exceptionally high nutritional value [6, 9, 10].

Because of the present availability of high-quality maize, triticale, wing beans, quinua, and the like, the question arises whether amaranth grain will enter into commercial production in the underdeveloped world, where it is most needed. The possibilities are great if integrated research is continued and if the information obtained is disseminated at all levels within a country or region. By integrated research is meant research on all components of the food chain, as well as on their value as vegetable crops and the use of their by-products.

Agricultural production

Production is the starting point in the food chain, and most agricultural research has as its first objective to increase productivity through improved plant materials and appropriate practices within the environmental conditions prevailing at the site where the crop is to be grown. Increasing production through productivity is the result of two important components, agricultural practices and the genetic make-up of the crop. Although some important information is available for amaranth grain cultivation, a number of constraints still must be solved: seed germination, seeding methods and planting dates, control of the optimum planting density per hectare, weeding, fertilizer application and nutrient requirements, and insect and disease control, to mention a few agronomic practices that have been and are under investigation in developing countries [9-11].

As important as these elements are, plant characteristics are also of significance. Some constraints include plant height, lodging, harvest index, seed shattering, synchronous dry-down, grain size, and stem rigidity to hold a large inflorescence [12]. The structure of the inflorescence should be such as to allow homogeneous drying of the grain once it has reached physiological maturity. In many cases the inflorescence is quite compact, making water evaporation a slow process.

TABLE 1. Plant heights at harvest of groups of amaranth trials

Sources: Refs. 13-15.

The problem of plant height is readily seen in table 1, which presents results of various agronomic trials conducted in Guatemala [13-15]. For 25 selections of Amaranthus caudatus, the plant height varied from 190 to 260 cm. A wide variation can be seen for A. hypochondriacus, while less variability is found for A. cruentus. The important finding is the variability, which suggests plant height can be reduced, it is hoped without reducing grain yield [13-15]. Similar results have been reported by others [9-12]. Tall plants make harvesting difficult. Furthermore, they tend to lodge and are susceptible to winds, which make it necessary to build supports, resulting in increased costs.

Seed shattering is an additional constraint that reduces grain yield if the inflorescence is allowed to dry in the field. Therefore, varieties should be found or should be developed so as to reduce this problem as much as possible in order to obtain higher yields [12].

TABLE 2. Seed weight

Sources: Refs. 13-15.

The results of studies by various workers have shown an important variability in seed weight, which must be used to advantage (table 2). In 25 selections of A. caudatus, seed weight varied from 0.50 to 0.93 ma. Other species presented in table 2 showed similar variability, which suggests again the possibility of selecting materials with higher seed weight [11, 13, 15]. Care should be taken, however, not to decrease yield at the expense of seed weight or protein or fat content. Larger seed size has some advantages at harvest and also in providing grain less contaminated by plant particles and dirt. Seed weight for a single type within a species probably does not affect protein or fat content; table 3 shows that the two are not significantly correlated [14]. Additional studies with other species should be carried out to verify this.

TABLE 3. Correlation coefficients between seed weight and protein and fat content in 25 selections of A. caudatus

Source: Ref. 14.

In the area where selections are tested for adaptability and grain production as carried out in Guatemala, planting is usually done in June to harvest in early October, when the rainy season ends. It is then too late to plant another crop, and the land will not be used until the start of the new rainy season in May. Since in October soils still contain available water, it would be advantageous to have a crop that could give a second and even a third harvest, if possible. Types of amaranth having this characteristic would be highly acceptable to farmers and should be developed, since amaranth seems to thrive under dry conditions. A type of A. cruentus from Guatemala seems to have this feature. One experiment tested four treatments (table 4). Although it is not clear how these treatments affect grain yield, what is of interest is that it was possible to obtain a second harvest, which increased total yield by 26% for a total of 3,911 kg per hectare [16]. Agrotechnology to optimize this characteristic should be pursued, since it will result in increased economic returns to the farmer.

TABLE 4. Effect of multiple harvest on yield of amaranth grain (Gua-17 A. cruentus)

source Ref. 16

TABLE 5. Variability in protein and fat content in amaranth grain species

Sources: Refs. 13-15.

Chemical composition and yield

Comprehensive reports on the chemical composition of amaranth grain have been published [2, 4, 5]. From the nutritional point of view, the two most important chemical components are protein and fat; levels of both are relatively high in comparison with those in cereal grains. It is therefore important to know the interrelationships of these two nutrients and yield.

Several reviews have suggested that environmental factors play an important role in determining protein and fat content in grain [2, 4, 5, 9, 14, 15, 17, 18]. The results of various studies, in which a number of types of several species were planted under equal soil and other environmental conditions, indicate that variability in protein and fat levels is due to genetic characteristics and is to be expected (table 5) [1315].

TABLE 6. Correlations between yield and protein and fat content in A. cruentus, A. caudatus, and A. hypochondriacus

Source: Ref. 15.

The relationships between yield and protein and fat content for 13 cultivars representing three species are summarized in table 6. As shown in the table, yield and protein were negatively correlated; however, the correlation was not statistically significant. This could be interpreted to mean that higher yields do not reduce protein content. On the other hand, yield was positively correlated with fat content, but again this was not statistically significant. This would be expected, since protein and fat are negatively related [15]. On the basis of results obtained so far and of nutritional value, it would be ideal if the amaranth grain cultivars released for commercial production contained not less than 15% protein and not less than 8% fat. Further studies are needed to understand and establish the relationships between yield and nutrient content.

Nutritive value

Growth inhibition of raw amaranth grain

TABLE 7. Amino acid content of amaranth gain (mg/g N)

Figures in parentheses indicate percentage of adequacy.

One of the attributes always mentioned when describing amaranth grain is its high protein quality predicted on the basis of its relatively well-established essential amino acid patterns. The lysine content is given as the main reason for the high protein quality of amaranth, since the grain contains more of this essential amino acid than cereal grains [4, 5, 18-20]. It is now evident, however, that raw amaranth grain does not have biologically the protein quality suggested by its amino acid pattern. This is shown in table 8, where four amaranth selections representing three species induced a low weight gain, which if expressed as protein quality would be only 65% of the value of casein. It is important to note that some selections resulted in better animal growth when eaten raw.

TABLE 9. Nutritive value of crude oil from three amaranth species added to diet at two levels, compared with cottonseed oil

source: Ref. 27.

TABLE 10. Effect of feeding fat-free amaranth (A. caudatus) flour, raw and cooked

The same observations were made with A. cruentus and A. hypochondriacus.
Source: Ref. 27.

What is of much interest is the significant improvement in nutritive value upon appropriate thermal processing of the grain. The differences that existed when raw largely disappeared, all having essentially the same quality [21], although this observation had already been shown [22, 23]. The reasons for these effects, which include a higher food intake and protein quality, are not known. The common tryptin inhibitors, lectins, and tannins have been discarded as possible factors [14]. Possibilities under study include other kind of antiphysiological substances and amino acid availability, or even both. These findings were interpreted to mean that the low animal response to eating raw grain is more due to inhibitory substances or nutrient unavailability than to essential amino acid pattern.

Lipid digestibility

As indicated, fat content is a major nutritional component in amaranth grain because of the energy it provides and for its fatty acid composition [4, 15, 24, 25]. The quality of the raw crude lipid fraction of the grain has been studied, and some results are shown in table 9. Two levels of the crude oil, 5% and 10%, were added to a basal casein diet in comparison with the same levels of cottonseed oil. As shown in the table, the crude oil from A. cruentus and A. hypochondriacus reduced weight gain. The crude oil from all species had lower digestibility than that of cottonseed oil. The effects were not as marked at the 10% oil level, probably because the energy needs of the animals were met with less food intake [26].

Apparently the oil from A. cruentus was the least nutritious, and work is needed to learn the reasons for the lower digestibility. In any case, the oil did not show any toxicity. It should be pointed out that removing the oil did not improve the protein quality of the raw residue (table 10). Thermal processing of the lipid-free amaranth flour resulted in a significant improvement in animal performance, however, suggesting that the lipid fraction may influence nutritive value, but it is not responsible for the lower animal response observed from eating raw amaranth grain flour [27].

TABLE 11. Effect of amino-acid supplementation on the protein quality of processed amaranth grain

Source: Ref. 28.

Limiting amino acids

Literature reports indicate that the protein quality of amaranth grain is deficient in leucine [4, 5] (see table 7). The results in table 11, however, suggest that threonine is the amino acid that limits protein quality [16, 28]. This may be of importance in efforts to use amaranth flour in mixtures with cereal grain flours, for example, rice and wheat, that are deficient in the same amino acid, threonine, after Iysine [29, 30]. The Iysine contributed by amaranth grain to the total Iysine in a mixture with wheat flour, for example, would make threonine the first limiting amino acid in the mixture, with only a small improvement in protein quality. Results would be similar with mixtures of rice and amaranth, with amino acids likely to become less available due to the high temperatures during processing [31-34].

Effects of processing

Processing is important with respect to the protein quality of amaranth grain. Although it is not possible to show comparative values of different kinds of processing on all grain species, it is of interest to show the effects of flaking, light roasting, popping, wet cooking, and extrusion on two species [32, 33] (table 12). The values clearly show that all processes increase the protein quality of amaranth grain, with roasting the least effective. This process, if carried out under more extreme conditions of time and temperature, destroys the quality of the product by reducing available Iysine content [32-34]. Of interest is the extrusion process, which for A. cruentus and A. caudatus yielded cooked flour equal in protein quality to casein [35].

Other workers, rather than using the whole grain, are studying milling techniques and evaluating such products for the development of a number of food products [20-36]. Study in this area should be continued and expanded. However, the use of the whole grain as a food source merits greater attention, particularly as a weaning food, because of its excellent protein quality and relatively high energy content [37, 38]. Similarly, it is important to define better the conditions needed for optimum thermal processing of amaranth grain in view of its nutritive value when eaten raw.

TABLE 12. Effect of various types of processing on the protein quality (net protein ratio) of amaranth

Source: Ref. 33

Food products

Amaranth used alone

Food product development and use are important areas of research since they give meaning to agricultural production efforts and create the driving force for a dynamic food chain. Many efforts are being made with the whole grain and milling fractions [2, 4, 5, 9, 10, 34-39]. The technologies being applied with whole grain are shown in figure 1 (see FIG. 1. Processing of whole grain).

After cleaning, which can be done by air or with water, the grain can be processed to be popped or expanded, and if wetted to 20%-25% moisture, it can be flaked. The dry grain can be extruded alone or mixed with other products such as soybeans to give extruded flours. After being suspended in water, these can be subjected to starch hydrolysis and dried to give a soluble product. Such a product can also be obtained by 10-minute cooking followed by starch hydrolysis and final drying.

This last product would be ideal for feeding children since it can be suspended in a 15%-20% solid solution. All could make excellent foods for weaning purposes. For this use, the levels of protein and fat suggested previously, 15% and 18% respectively, would be highly desirable.

In grain mixtures

The processed flour could be used as such or mixed with other nutrient sources, for example, other grains [31, 36, 39]. The results shown in table 13 are examples of the nutritive value when the protein from maize, wheat flour, and rice is replaced with amaranth-grain protein [16]. For wheat flour, there is a continuous increase in protein quality, as amaranth protein makes a greater contribution in the diet. This is to be expected on the basis of the higher Iysine content in amaranth protein than in wheat-flour protein.

It is of interest that a complementary effect was observed with maize, a protein higher in leucine content than amaranth protein, which suggests that this amino acid does not limit the quality of amaranth protein, as was indicated previously [30]. A supplementary effect is observed as amaranth protein increases in the mixture, up to a point beyond which no additional effect is observed. The results with rice are also of interest. As amaranth protein replaced rice protein, quality was increased. These studies must be continued because of the interest in mixing amaranth grain with cereal grains.

TABLE 13. Protein quality of mixtures of cooked amaranth grain and maize, rice, and wheat flour

With other protein sources

An attractive alternative is to use amaranth grain with other sources of protein. An example is shown in table 14, in which small amounts of milk, soybean, and cottonseed were added [ 16]. With the possible exception of milk, the other protein sources did not improve protein quality; however, the protein content in all the products was higher. The development of a milk-based product would be of interest [35, 37, 38].

This research must be continued, and the changes in chemical structure and functionality in the various primary products from amaranth must be established. It would be interesting to submit one or two such products to markets, using locally available industrial capacity, with the grain provided in a subsidized form. This could break the cycle of uncertainty for producers, who would like to have an assured market with adequate economic returns, and for the industry, who would be assured of having sufficient raw materials. If this were backed by appropriate advertising and a reputable institution, it might help significantly in promoting the commercial production of amaranth gram.

TABLE 14. Protein quality of amaranth grain flour supplemented with various protein sources

1. 10.5% protein diets.
   Source: Ref. 16.

The use of by-products

At the beginning of this paper I indicated that the reintroduction of amaranth into commercial systems might be successful if research on the crop is carried out in an integrated manner based on the food chain. This concept implies also the use of the plant for animal-feeding purposes or, if grain is the main product, the use of by-products.

With respect to the use of the whole plant, table 15 shows data on the yield of dry matter and of protein per hectare with respect to time [40]. Although at 60 days the protein yield was higher, the product was probably of lower quality than that obtained at 40 days. This of course, depends on the kind of animal to be fed, with the higher-protein product for monogastric animals and the 60-day-old grain with higher fibre content for ruminants. This matter is under continued research, and could offer attractive solutions to developing countries. The problem is that undesirable factors in the dehydrated product limit its use at least in rabbits (table 16). The data show that 15% replacement of alfalfa leaf meal did not affect animal responses; however, higher levels reduced weight gain per day. Thermal treatment of the product seems to help, however, since 60% amaranth plant meal, which was blanched before drying, improved the animals' weighs gain [41].

TABLE 15. Dry biomass and protein yield of whole amaranth plants

Source: Ref. 40.

TABLE 16. Effect of amaranth plant meal on rabbit weight gain, feed efficiency, and carcass yield

a. Heat-treated.
Source: Ref. 41.

TABLE 17. Dry weight distribution of plant parts of nine selections of A. cruentus

The use of the by-products from grain harvest and cleaning is also of interest. This aspect is justified by the results presented in table 17, which shows the weight percentages of grain, of the inflorescence, and of the plant residue, with respect to the weight of the whole plant. The grain weight averaged 37.1% of the total weight of the plant, while the flower, and the stem and leaves averaged 23.7% and 39.2% respectively. The total weight of by-products is significantly higher than the weight of grain. Therefore, these products must be used based on their chemical composition. This is partially shown in table 18. The two components that are of interest are crude fibre and protein. The table shows the composition of an additional fraction, the seed calyx. The use of these byproducts would be limited by their high fibre content; however, the stems and leaves fed to young ruminants gave results similar to corn stover, and the seed calyx was comparable to wheat middlings in preliminary studies with baby chicks [42, 43].

TABLE 18. Chemical composition of agricultural residues from amaranth

The inflorescence is presently being tested, and relatively good silage has been produced. As with the whole young plant, thermal treatment of the grain calyx improves its quality. These by-products must be studied further, which will be of great value in introducing amaranth production in our countries.

To conclude, activities should be continued and expanded in the following areas:

- agrotechnology, with significant activities in genetics, with a focus on nutritive value, increased yield, and the physical and chemical characteristics of the plant and grain;
- development of an agro-industry to use the grain, particularly in developing countries;
- increased knowledge on the limiting nutritional factors of raw and processed grain;
- improved processing technologies to achieve attractive functional properties and nutritive value;
- use of the whole plant, and/or plant residues after grain harvest;
- increased dissemination of information.

References

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