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Grain quality of sorghum, pearl millet, pigeon-pea, and chick-pea

R. Jambunathan, U. Singh, and V. Subramanian

International Crops Research Institute for the Semi-arid Tropics, Patancheru, India

Abstract
Nutritional quality
Anti-nutritional factors
Milling characteristics, cooking quality, and consumer acceptability
Conclusions
References

Abstract

The results of analyses of sorghum (Sorghum bicolor (L.) Moench), pearl millet (Pennisetum americanum (L.), pigeon-pea (Cajanus cajan (L.) Millsp.), and chick-pea (Cicer arietinum (L.)), for their chemical composition - including protein content and essential amino acid composition - are discussed. The distribution patterns of various protein fractions in these grains and the levels of some of the anti-nutritional factors present in pigeon-pea and chick-pea are presented. The results of a survey that was carried out in India to study the utilization of these crops, which included the milling characteristics of pigeon-pea and chick-pea, and the relationships between certain physico-chemical characteristics and cooking quality, are discussed.

There are several components to grain quality, such as visual quality; nutritional quality, including digestibility and big-availability of nutrients; anti-nutritional factors; milling characteristics; cooking quality; consumer acceptability; and storage stability. Grain quality deserves as important a place in evaluation of new varieties as high yield and yield stability. Grain produced in a farmer's field passes through several transformations before it is consumed in the form of food. To ensure that these transformations are accomplished efficiently and with minimum nutrient loss, better interactions between different disciplines are needed, and an understanding of various aspects of grain quality becomes vital. This report describes the progress that has been made at ICRISAT in understanding some components of grain quality in sorghum, pearl millet, pigeon-pea, and chick-pea.

Nutritional quality

Protein Quantity and Quality

Sorghum and pearl millet are staple foods that supply a major proportion of calories and protein to large segments of the populations living in the semi-arid tropical regions of Africa and Asia. Hulse et al. (1980) have published an exhaustive review of the chemical composition and nutritive value of sorghum and many millets. Sorghum and millet grain samples analysed in our laboratory exhibited a wide range in chemical composition (see TABLE 1. Seed Weight and Chemical Composition of Sorghum and Pearl Millet Grain). No attempt was made in these analyses to study the genotype-environment interactions.

Protein content, which showed a wide variation, represents the results of analysis of samples obtained from high fertility. low fertility, unirrigated, and irrigated fields. It should be noted that the starch values reported were based on actual analysis and were not calculated values obtained by difference. The wide variability in chemical composition underscores the importance of periodically analysing representative samples of grain from the farmer's field, as normally grown, at low-fertility levels, so that a rough estimate of the availability of protein and other nutrients can be made.

A study conducted at ICRISAT using the two high-protein, high-lysine sorghums, IS-11167 and IS-11758, as parents, suggested that the high-lysine gene may not be stable in normal seed with a plump endosperm background (Riley 1980). However, further intensive study is needed to understand the reasons for this, and possibly to modify the causative factors that hamper the transfer of the high-lysine gene to a normal plump background. Analysis of yield and protein contents in cultivars obtained from four locations indicated that, in pearl millet, it should be possible to select for increased protein content without detriment to grain yield or grain weight (ICRISAT 1980).

In the largely cereal- and legume-based diet of the Indian population, legumes serve as one of the major suppliers of protein. From the large collection of germplasm accessions of pigeon-pea and chick-pea from various parts of the world available at ICRISAT, several thousand samples have been analysed for their protein content and a few for their proximate composition as well (see TABLE 2. Chemical Composition of Pigeon-pea, and Chick-pea Dhala Samples). The mean protein content of pigeon-pea whole seed was 20.4 per cent and that of dhal (decorticated split cotyledons) 23.2 per cent; that of chick-pea whole seed was 20.5 per cent (Jambunathan and Singh 1979, 1980). Cultivars of high-protein content could be potential sources for the development of high-protein lines. So could the wild relatives of pigeon-pea and chick-pea: the protein content in dhal samples of wild relatives of pigeon-pea such as Atylosia, Flemingia, and Rhynchosia spp. ranged between 28.3 and 30.5 per cent (Singh et al. 1981), while some wild relatives of chick-pea showed a protein range from 25.6 to 31.7 per cent.

The quality of a protein may be estimated by comparing its amino acid composition with standard reference patterns (FAO/WHO, 1973), the most limiting amino acid presumably determining the nutritive value. Generally the amino acid score is calculated as fellows: mg of amino acid in 1 g of test protein x 100.

Amino acid score = (mg of amino acid in 1 g of reference protein)/(mg of amino acid in 1 g of reference protein) x 100

The lowest score obtained for any essential amino acid may be taken as a first approximation to the probably efficiency of utilization of the test protein by children (FAO/WHO 1973). Applying calculation to the essential amino acid composition of sorghum, as shown in table 3 (Protein Content and Essential Amino Acid Composition of Sorghum and Pearl Millet Grains^a), it is clear that by far the most limiting amino acid is lysine (amino acid score 38), followed by methionine and cystine (67), and threonine (80). In pearl millet, lysine (52), followed by methionine and cystine (77), are the limiting amino acids. The high level of leucine present in sorghum has been implicated in the human disease, pellagra (Hulse et al. 1980). It has been recognized that to improve the nutritional quality of sorghum and pearl millet, it would be desirable - if resources were available - to analyse the germplasm accessions of these two crops for protein and lysine contents and methods for rapid analysis are available (Jambunathan 1980).

Amino acid composition of pigeon-pea dhal samples (see TABLE 4 Protein Content and Essential Amino Acid Composition of Pigeon-pea and Chick-pea Dhal Samples^a) show that the limiting amino acids are methionine and cystine (chemical score 58), tryptophan (741, followed by valine (92), threonine (95), and isoleucine (97). In chick-pea, methionine, and cystine (67) followed by valine (89) and threonine (90) are the limiting amino acids. Surprisingly, tryptophan was not observed to be the limiting amino acid in chickpea (chemical score 115). However, since the range is large, indicating wide variability, and as no effort was made to sturdy the genotype-environment interaction in these analyses, the data should be treated with caution before drawing any firm conclusions. Our results indicate that screening for either methionine or cystine would be sufficient, as these two amino acids are highly correlated with each other when expressed either as a percentage of protein or as a percentage of sample (Jambunathan and Singh 1982). Using rapid colorimetric procedures, we have estimated methionine content in about 500 chick-pea germplasm accessions (range 0.81-1.59; mean 1.16 9/16 9 N) and about 150 pigeon-pea germplasm accessions (range 0.70-1.48; mean 1.08 9/16 9 N) and tryptophan content in about 100 chickpea accessions (range 0.76-1.27; mean 1.05 9/16 9 N) and about 60 pigeon-pea accessions (range 0.60.95; mean 0.75 9/16 9 N) (Jambunathan et al. unpublished data).

The protein quality of sorghum and millet results from the different proportions of the various protein fractions in the grain. Protein fractionation of these cereals is shown in table 5. Fractions II and III are very low in lysine concentration. In high-lysine sorghum, the proportion of these two fractions is lower and that of the first fraction higher than in normal sorghum, thus improving its overall protein quality (Jambunathan 1980).

TABLE 5. Nitrogen Distribution in Sorghum and Pearl Millet Grain (percentage of total nitrogen)

1. Based on three genotypes, M.35-1, CSH-8, and CSV-3
2. Based on four genotypes, PHB-14, BK-560, WCC75, and Ex-Bornu.

The protein quality of legumes is likewise related to the proportions of various protein classes. The proportions of fractions in the whole seed and cotyledon of chick-pea and pigeon-pea are shown in table 6. The methionine plus cystine contents of various cotyledon protein fractions show wide variability. Globulin is the major protein component in pigeon-pea and chick-pea. Since its content of methionine plus cystine is rather low, there is an overall deficiency of sulphur amino acids in the whole grain.

TABLE 6. Nitrogen Distribution and Levels of Methionine Plus Cystine Content in Pigeon-pea and Chickpea Grain (percentage of total nitrogen)

1. Embryo was removed
2. Figures in parentheses show methionine + cystine content (g/16 g N) determined by ion exchange chromatography.

Digestibility and Bio-availability

Chemical composition and amino acid composition give only an approximation of protein quality. Actual biological utilization of the protein must be determined using animals and, if possible, children. MacLean et al. (1981) conducted a feeding study on children in Peru, using two high-lysine and two conventional varieties of sorghum as the sole source of nitrogen. The mean absorption and retention of nitrogen from the sorghum diet were reported to be 46 ± 17 per cent and 14 ± 10 per cent of intake, respectively, against values of 81 ± 5 per cent and 38 ± 3 per cent, respectively, for casein under similar conditions. This indicates that sorghum is a poor source of nitrogen for children. To our knowledge, no similar studies have been carried out with pearl-millet, pigeon-pea, or chick-pea diets. Axtell et al (1981) have reported that protein from uncooked sorghum has high pepsin digestibility, which was greatly reduced after cooking; however, fermentation of flour improved the pepsin digestibility, indicating that various processing methods should be kept in view while evaluating the grain.

Mineral Elements

Nutritional studies among people of the semi-arid tropics show evidence of deficiencies in calcium, iron, and zinc, all of which elements are rendered insoluble by physic arid (Hulse et al. 1980). Sorghum shows a wide range for these mineral elements (Jambunathan 1980), but the effects of environment and other agronomic factors must be studied in detail before any conclusion is drawn. The analyses revealed interesting variations in the chemical and mineral element composition of desi (small; coloured seed coat) and kabuli (large; white seed coat) types of chick-pea cultivars grown at Hissar (Haryana, India). Desi chick-pea had a higher seed coat percentage and fibre content than kabuli. The calcium and iron contents of the kabuli seed coat, in particular, were much higher than those of the desi seed coat, and a few other minerals also showed significant differences (Jambunathan and Singh 1981).

Anti-nutritional factors

Polyphenolic compounds (commonly referred to as tannins), physic acid, cyanogenic glucosides, etc. act as nutritional inhibitors in coarse grains. They have been reported to reduce the big-availability of protein and other nutrients, but, at the same time, polyphenols have also been shown to contribute some degree of resistance or tolerance to bird depredation, pre-harvest germination, and weathering (Hulse et al. 1980). In legumes, in addition, inhibitors of enzymes such as trypsin, chymotrypsin, and amylase, and of flatulence-causing oligosaccharides such as stachyose, raffinose, and verbascose, play important roles. The levels of these anti-nutritional factors have been determined in pigeon-pea and chick-pea whole seed (Singh and Jambunathan 1981; Singh et al. 1982), as shown in table 7. In general, desi chickpea exhibited higher levels of these factors than kabuli. Genotypes and environments that may affect these levels were not examined in these studies. However, the low levels of these anti-nutritional factors both in pigeon-pea and chick-pea, which would be further reduced or destroyed on cooking, suggest no great need for concern on this score. The levels of these factors and the role of polyphenolic compounds (tannins) in the big-availability of nutrients assume importance in pigeon-pea in areas where it is consumed without cooking when whole and mature, or as a developing green seed (Jambunathan and Singh 1980).

TABLE 7. The Levels of Anti-nutritional Factors and in-vitro Protein Digestibility in Seed Samples of Pigeon-pea and Chick-pea

1. Based on 9 cultivars
2. Based on 15 cultivars
3. Units/mg meal

Milling characteristics, cooking quality, and consumer acceptability

A survey carried out in Haryana, Madbya Pradesh, Maharashtra, Rajasthan, and Uttar Pradesh, which together are the major production and processing areas for pigeon-pea and chick-pea in India (Singh and Jambunathan 1980), yielded the following observations.

Pigeon-pea

Owners of 46 large-scale and 130 small-scale dhal mills were interviewed for their opinions and impressions regarding the milling characteristics of various types of pigeon-pea. Some 130 villagers were interviewed in areas where pigeon-pea seed was reported to give higher dhal yields. Although the dhal yield was reported to vary from one mill to another, depending on the processing method used, there appeared to be a general consensus that round seeds of medium-size and greater hardness yield better dhal recovery. Village-level home processing was reported to give lower dhal recoveries than mechanically operated mills. There were variations in recovery depending on the processing technique used. Consumers seemed to prefer local varieties grown in their own fields. There was a wide variation in colour preference because of long-time associations with a particular grain colour. Interestingly, it was observed that taste seemed to have a priority equal to or higher than cooking time.

Chick-pea

As a common practice, farmers consumed local chick-pea cultivars grown on their own farms. Cultivars with light brown testa colour were generally processed into dhal, desi types of medium seed size being preferred. About 60 to 70 per cent were converted into fine flour, which was used in several recipes. Kabuli cultivars were preferred for consumption whole, as were bold-seeded desi types. Consumption of chick-peas in immature fresh form, or as puffed or parched products, was observed to be a common practice.

A survey of traditional food preparations from sorghum and millet was carried out in 171 villages from seven Indian states (Subramanian and Jambunathan 1980). According to the village housewives, the various foods made from sorghum and millet could be classified into the following food types: roti (coarse unleavened bread), porridge, gruel, or boiled, steamed, or fried foods.

In India, roti is the most common food prepared from sorghum and pearl millet. Studies were carried out at ICPISAT on the relationship between roti quality and physico-chemical characteristics of sorghum (Subramanian and Jambunathan 1981). The grain characteristics of 45 sorghum and 16 millet genotypes are given in table 8 (see TABLE 8. Physico-Chemical Characteristics and Roti Quality of Sorghum^a and Pearl Millet^b Grain). It was observed that sorghum flour with good dough stickiness produced an acceptable roti. The relationships between the physico-chemical characteristics and roti qualities as evaluated by a panel of tasters were tested using the methods of rank correlation and multiple regression. Results revealed that several flour components complement each other to influence roti quality. Water-soluble flour fraction, amylose, water-soluble protein and sugars appear jointly to influence and contribute to the overall roti characteristics of sorghum. In regard to pearl millet, swelling capacity of flour, water-soluble protein content, amylose, and sugars were found to be related to roti quality. Several other products that are prepared in various regions of Africa are also being evaluated at ICRISAT in co-operation with scientists located in different parts of the world.

Farmers and other consumers in Maharashtra stated that they prefer local sorghum to hybrid sorghum (Subramanian et al., unpublished data). Therefore, it seems important to carry out consumer-acceptability studies on advanced breeding materials before they are considered for large-scale testing and subsequent release. In another study on six families in the Rahuri area (Maharashtra), hybrid and local sorghum flours were supplied to each family for a period of four weeks. Of the six families, four could not differentiate between hybrid and local sorghum. Further studies on larger numbers are needed to verify this observation.

Several physico-chemical characteristics of pigeon-pea and chick-pea were determined (see TABLE 9. Relationship between Physico-Chemical Characteristics and Cooking Time of Dhal Samples of Pigeon-pea and Chick-pea) to see whether any of them could be used as a screening index for evaluating the cooking quality and cooking time of these pulses. Important physico-chemical characteristics that showed strong relationships with cooking time included the amount of solids dispersed in cooking water, nitrogen content in the solids dispersed, water absorption of dhal, texture as determined in the Instron Food Testing Instrument (Model 1140 Highwycombe, Berkshire, United Kingdom), physic acid, and minerals such as calcium and magnesium. Thus texture, solids dispersed, water absorption, and cooking time are some of the characteristics that should be studied to evaluate genotypes for cooking quality.

Conclusions

Considering the variability in protein content and the observed deficiency of certain essential amino acids, cereals and legumes should continue to be analysed for these constituents. Proteins of the two cereals, sorghum and pearl millet, and the two legumes, pigeon-pea and chick-pea, were found to be deficient in methionine plus cystine. It may therefore be worthwhile to analyse cereals for methionine plus cystine, as attempts to identify a legume high in sulphur amino acids have not so far been successful. It is important to consider the nutritional quality of the total diet, which should include the changes that occur due to processing, and so on. Such studies may lead to the identification of a traditional product or diet that has more available nutrients for children than other products made from the same ingredients. Cooking quality and consumer acceptability criteria should be evaluated preferably in the early stages of development of cultivars in a breeding programme, and methods should be developed for rapid evaluation of these attributes.

References

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