Chapter 6 - Nutritional inhibitors and toxic factors

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As with other foodstuffs certain nutritional inhibitors and toxic substances are associated with sorghum and millet grains. Antinutritional factors can be classified broadly as those naturally present in the grains and those due to contamination which may be of fungal origin or may be related to soil and other environmental influences. These factors modify the nutritional value of the individual grains, and some of them have very serious consequences. The following is a brief account of some of the antinutrients and toxic substances associated with sorghum and millets.

Phytate

Phytate represents a complex class of naturally occurring phosphorus compounds that can significantly influence the functional and nutritional properties of foods. Although the presence of these compounds has been known for over a century, their biological role is not completely understood. Phytic acid, myo-inositol 1,2,3,4,5,6-hexakis(dihydrogen phosphate), is the main phosphorus store in mature seeds. Phytic acid has a strong binding capacity, readily forming complexes with multivalent cations and proteins. Most of the phytate-metal complexes are insoluble at physiological pH. Hence phytate binding renders several minerals biologically unavailable to animals and humans.

Doherty, Faubion and Rooney (1982) analysed several varieties of sorghum and found that in the whole grain phytin phosphorus ranged from 170 to 380 mg per 100 g; over 85 percent of the total phosphorus in the whole grain was bound as phytin phosphorus. Wang, Mitchell and Barham ( 1959) studied the distribution of phytin phosphorus in sorghum grain and found that a greater percentage of physic acid was in the germ than in the bran and the least was in the endosperrn. Dehulling can remove 40 to 50 percent of both phytate and total phosphorus. It was observed that phytin phosphorus constituted 82 to 91 percent of total phosphorus in the whole grain, 56 to 84 percent in the dehulled grain and 85 to 95 percent in the bran. In the fractions obtained through traditional milling, phytin phosphorus content was greatest in the bran, less in the whole grain and lowest in the dehulled grain. This suggested that the bran and aleurone layers of the grain are a major reservoir of phytate and total phosphorus in sorghum. As milling of soft-endosperm varieties removes only a small amount of pericarp, milling causes less decrease in phytin phosphorus in these varieties. Bioavailability of iron in sorghum for human subjects was found to be affected more by phytin phosphorus than by tannin content of the grains (Radhakrishnan and Sivaprasad, 1980). On pearling of sorghum grain, a significant increase in ionizable iron and soluble zinc content indicated improved bioavailability of these two micronutrients, which was attributed partially to the removal of phytate, fibre and tannin along with the bran portion during pearling (Sankara Rao and Deosthale, 1980).

In pearl millet, values reported for phytin phosphorus varied from 172 mg per 100 g (Sankara Rao and Deosthale, 1983) to 327 32 mg per 100 g (Chauhan, Suneja and Bhat, 1986). The values reported by Simwemba et al. (1984) were within this range. Ionizable iron was inversely correlated and soluble zinc negatively correlated with phytin phosphorus. Sankara Rao and Deosthale (1983) further observed that malting of the grain significantly reduced the phytin phosphorus content of both pearl and finger millets. This decrease was accompanied by significant increases in ionizable iron and soluble zinc, indicating improved bioavailability of these two elements. Germination of finger millet varieties progressively decreased phytin phosphorus content of the grain (Udayasekhara Rao and Deosthale, 1988). After 48-hour germination with removal of the vegetative portion, the total phosphorus in malted grains decreased by 16, 12 and 9 percent in pearl, finger and foxtail millet, respectively (Malleshi and Desikachar, 1986b). Phytin phosphorus decreased significantly from 38 to 20 percent on germination of pearl millet. However, in finger and foxtail millets the decrease in phytin phosphorus was very small. A weaning food based on germinated wheat, pearl millet, chickpea, mung bean and sesame contained only 4.39 mg phytin phosphorus per 100 g, as against I 0 mg per 100 g in the mix prepared from ungerminated grain (Nattress et al., 1987). In a fermented Indian preparation of pearl millet known as rabadi, the phytin phosphorus after nine-hour fermentation had decreased by 27 to 30 percent ( Dhankher and Chauhan, 1987).

Polyphenols

Widely distributed polyphenols in plants are not directly involved in any metabolic process and are therefore considered secondary metabolites. Some polyphenolic compounds have a role as defence chemicals, protecting the plant from predatory attacks of herbivores, pathogenic fungi and parasitic weeds. Polyphenols in the grains also prevent grain losses from premature germination and damage due to mould (Harris and Burns, 1970, 1973). Dreyer, Reese and Jones ( 1981) observed that polyphenols protect seedlings from insect attack.

Phenolic compounds in sorghum can be classified as phenolic acids, flavonoids and condensed polymeric phenols known as tannins. Phenolic acids, free or bound as esters, are concentrated in the outer layers of the grain. They inhibit growth of microorganisms and probably impart resistance against grain mould.

Flavonoids in sorghum, derivatives of the monomeric polyphenol flavan-4-ol, are called anthocyanidins. The two flavonoids identified to be abundant in sorghum grains are luteoforol (Bate-Smith, 1969) and apiforol (Watterson and Butler, 1983). The latter compound was also found in sorghum leaves. Jambunathan et al. ( 1986) observed that resistance to grain mould rather than to birds (Subramanian et al., 1983) was associated with flavan-4-ol content of the grain. Though low-molecular-weight flavonoids from other plant sources were found to be antinutritional in rats ( Mehansho, Butler and Carlson, 1987 ),so far there has been no evidence that sorghum flavonoids have similar properties.

Tannins are polymers resulting from condensation of flavan-3-ols. Gupta and Haslam (1980) referred to sorghum tannins as procyanidins because they thought that cyanidin was usually the sole anthocyanidin involved. During grain development, flavonoid monomers are synthesized and then condense to form oligomeric proanthocyanidins of four to six units.

Some varieties of sorghum containing high tannin in the grain were found to be bird resistant (Burns,1971; Tipton et al., 1970). Tannins are the most abundant phenolic compound in brown bird-resistant sorghum. During maturation the brown-sorghum grain develops astringence which imparts resistance against bird and grain mould attack. This quality is important in arid and semi-arid regions where other crops fail. In some of these regions, annual losses in grain production as high as 75 percent or sometimes more have been reported (McMillan et al., 1972; Tipton et al., 1970).

Tannins, while conferring the agronomic advantage of bird resistance, adversely affect the grain's nutritional quality (Salunkhe et al., 1982; Salunkhe, Chavan and Kadam, 1990; Butler et al., 1984, 1986). Growth retardation was observed in chicks fed high-tannin sorghums. Tannins in the grain impart an astringent taste which affects palatability, reducing food intake and consequently body growth (Butler et al., 1984). Tannins bind to both exogenous and endogenous proteins including enzymes of the digestive tract, affecting the utilization of proteins (Asquith and Butler 1986; Griffiths, 1985; Eggum and Christensen, 1975). Several studies in rats, chicks and livestock have shown that high tannin in the diet adversely affects digestibility of protein and carbohydrates and reduces growth, feeding efficiency, metabolizable energy and bioavailability of amino acids (Rostango, 1972). Some of the antinutritional effects of high-tannin sorghum may be due to associated lowmolecular-weight flavonoids which are readily absorbed, inhibiting the metabolic utilization of digested and absorbed foodstuffs (Butler, 1988; Mehansho, Butler and Carlson, 1987).

There is no direct evidence regarding antinutritional effects of dietary tannins in human subjects, although high dietary tannin may have some carcinogenic effect (Morton, 1970; Singleton and Kratzer, 1973). Iron absorption in Indian women was lower when they were fed porridge prepared from bird-resistant high-tannin sorghum in place of porridge prepared from tannin-negative sorghum (Gillooly et al., 1984). On the other hand, studies in normal and anaemic subjects (Radhakrishnan and Sivaprasad, 1980) have shown that availability of iron was affected more by physic acid than by the tannin content of the grain. The tannin content of the socalled high-tannin sorghum used by these workers was only 160 mg catechin equivalents per 100 g, and this was much lower than that normally found in birdresistant sorghum varieties.

Considerable efforts have been made to develop methods to improve the nutritional quality of bird-resistant sorghum (Salunkhe, Chavan and Kadam, 1990). Tannins and associated polyphenols are concentrated in the testa or seedcoat and can be removed by milling. However milling by traditional (mortar and pestle) as well as mechanical methods was shown to result in considerable losses in nutrients, and the flour produced was poor in yield as well as in nutritional quality (Chibber, Mertz and Axtell, 1978). Mwasaru, Reichert and Mukuru ( 1988) have suggested that for milling to be commercially economical, varieties of sorghum with round grains and hard endosperm and containing just adequate tannin for bird resistance and other agronomically desirable properties should be developed. They have identified such varieties giving flour extraction levels of 70 percent or higher on abrasive dehulling (Reichert, Mwasaru and Mukuru, 1988).

Price, Hagerman and Butler (1980) observed that the tannin content of sorghum flour decreased when it was mixed into batter, and there was further reduction on cooking. However, the growth of rats fed cooked or uncooked batter was lower than that of animals fed raw flour. Germination was also found to decrease tannin content in sorghum (Osuntogun et al., 1989) and finger millet (Udayasekhara Rao and Deosthale, 1988). However, the tannin content of the germinated sorghum rose again significantly upon drying.

Different methods have been tried to inactivate or detoxify the tannins in birdresistant sorghums to improve their nutritional quality (Salunkhe, Chavan and Kadam, 1990). Moisturizing the grains with alkali several hours prior to utilization, including treatment of whole grain with dilute aqueous ammonia, was found to be quite effective (Price and Butler, 1979). In traditional processing of high-tannin sorghums, prior treatment of the grain with alkalis is an important step. In making sorghum beer, the grains are soaked overnight with moistened wood ash; the alkalis released from the ash were found to inactivate the tannins (Butler, 1988). This observation is very important, since the product before fermentation is used for feeding children in certain parts of eastern Africa. Muindi and Thomke (1981) found that treatment of high-tannin sorghum with Mugadi soda solution was also effective in detoxification of tannins. Other methods suggested to improve the nutritional quality of bird-resistant sorghum include treatment with formaldehyde (Daiber and Taylor, 1982) and polyethylene glycol (Hewitt and Ford, 1982), gelatin (Butler et al., 1986) and high-moisture reconstitution (Teeter e! al., 1986). Supplementation of high-tannin diets with orthophosphoric acid or dicalcium phosphate (Ibrahim et al., 1988) or sodium bicarbonate (BandaNyirenda and Vohra, 1990) also had a positive effect in terms of detoxification of tannins.

Among millets, finger millet was reported to contain high amounts of tannins (Ramachandra, Virupaksha and Shadaksharaswamy, 1977), ranging from 0.04 to 3.47 percent (catachin equivalents). In vitro protein digestibility was negatively associated with the tannin content of finger millet varieties. In studies reported by Udayasekhara Rao and Deosthale (1988), white varieties of finger millet had no detectable tannin, while the tannin content of brown varieties ranged from 351 to 2 392 mg per 100 g. After extraction of tannin the ionizable iron in brown finger millet varieties rose by 85 percent, and addition of the extracted tannin to white varieties reduced the ionizable iron by 52 to 65 percent. These studies indicated that the poor iron availability (represented by low ionizable iron) in brown varieties is due to their high tannin content.

In the fermented pearl millet product rabadi, polyphenols decreased by 10 to 12 percent after nine hours of fermentation (Dhankher and Chauhan, 1987).

Digestive enzyme inhibitors

Inhibitors of amylases and proteases have been identified in sorghum and some millets (Pattabiraman, 1985). Chandrasekher, Raju and Pattabiraman (1981) screened millet varieties for inhibitory activity against human salivary amylase. Japanese barnyard, common, kodo and little millet strains had no detectable activity. One pearl millet and two sorghum strains did not show any inhibitory activity against a-amylase, while other strains of sorghum and pearl, foxtail and finger millets showed appreciable activity, indicating it to be a varietal and species character. Sorghum had the highest inhibitory activity against human, bovine and porcine amylases; foxtail millet did not inhibit human pancreatic amylase, while extracts from pearl and finger millets inhibited all a-amylases tested. The inhibitors were non-dialysable and were inactivated by pepsin treatment. Inhibitors from sorghum and foxtai millets were more thermolabile than those from finger and pearl millets.

Similar screening for protease inhibitors (Chandrasekher, Raju and Pattabiraman, 1982) showed that kodo, common and little millet varieties had no pro/ease-inhibitory properties while pearl, foxtail and barnyard millets displayed only antitrypsin activity. Finger millet extracts were found to have the highest activity against bovine trypsin (33.3 units) and chymotrypsin (12.5 units), as well as against porcine elastase (Pattabiraman, 1985). Extracts from sorghum and pearl, foxtail and barnyard millets inhibited the proteolytic enzymes of both human and bovine pancreatic preparations.

Manjunath, Veerbhadrappa and Virupaksha (1981) purified and characterized trypsin inhibitors from finger millet and found the final preparation homogeneous by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) at pH 4.3 and ultracentrifugation. The purified antitrypsin inhibitor was found to be stable over a wide range of temperature and pH (3 to 12). While it was active against bovine trypsin, it did not inhibit bovine a-chymotrypsin, pepsin, papain or subtilisin. It was found to have inhibitory properties against salivary and pancreatic amylases. Almost simultaneously, Shivara; and Pattabiraman (1981) independently reported that a single bifunctional protein factor in finger millet had inhibitory activity against trypsin and amylase with two separate active sites.

Chandrasekher and Pattabiraman ( 1982) purified and characterized two trypsin inhibitors from pearl millet. Both were active against bovine trypsin but inactive against bovine a-chymotrypsin. Fairly stable to heat, the two inhibitors were also stable under a wide pH range, from 1 to 9.

The nutritional significance of the enzyme inhibitors present in sorghum and millets is not clearly understood: more research on enzyme inhibitors of cereal grains is needed.

Goitrogens

Iodine is an essential micronutrient for all animal species, and iodine deficiency is among the most widely prevalent nutritional problems in many developing countries (DeMaeyer, Lowenstein and Thilly, 1979). Though environmental iodine deficiency is a prerequisite to goitre formation, the incidence of goitre in animals and humans with normal dietary intake of iodine suggests there are other factors in the aetiology of simple goitre. The observation that cabbage feeding produced thyroid hyperplasia in rabbits was the first milestone of progress in this field. A large number of foodstuffs possess antithyroid agents, collectively designated as goitrogens. The isolation and identification of l-5-vinyl-2-thiooxazolidone, a goitrogen of some foods in the Cruciferae family (Astwood, Greer and Ettlinger, 1949), led to the search for similar agents in more commonly eaten foodstuffs. Cyanogenic glycoside, which can be hydrolysed to highly potent antithyroid thiocyanates, was found to be present in cassava tubers, a staple of tropical Africa, and was implicated in the high incidence of goitre in cassava-eating populations.

Another staple food implicated in the aetiology of goitre is pearl millet. In the Sudan, Osman and Fatah ( 1981 ) observed that in rural Darfur Province, where pearl millet was the only staple, the incidence of goitre was higher than in urban regions where other foodgrains such as sorghum were consumed. Consumption of pearl millet is considered one of the factors responsible for the high incidence of goitre in rural populations. A positive correlation observed between the incidence of goitre and per caput production of pearl millet in six African countries (Klopfenstein, Hoseney and Leipold, 1983a) supports this viewpoint. Furthermore, Osman and Fatah (1981) observed that rats fed pearl millet diets developed abnormal thyroidhormone patterns with hyperplasia while animals fed sorghum were unaffected. A thioamide-type goitrogen was suspected to be present in the pearl millet grown and consumed in the region. Sudanese girls with goitre had relatively high serum isothiocyanate which was attributed to their consumption of pearl millet (Osman, Basu and Dickerson, 1983).

Feeding trials in rats showed that the goitrogen inhibited deiodination of thyroxine (T4) to triiodothyronine (T3), the metabolically more active form of the hormone. Iodine supplementation did not alleviate the goitrogenic effect of pearl millet.

Studies reported by Klopfenstein, Hoseney and Leipold (1983b) showed that the goitrogenic principle in pearl millet was present in both the bran and endosperm portions of the grain and was not destroyed by grain fermentation. The observation that autoclaving of the millet reduced its goitrogenic properties suggested a volatile or heat-labile nature of the active principle. Birzer, Klopfenstein and Leipold ( 1987) found that the goitrogenic principle of pearl millet was alcohol extractable and probably present as the c-glucosyl flavones vitexin, glucosyl vitexin and glucosyl orientin. The alcohol extract of wetted and dried pearl millet grain was found to be more goitrogenic; it contained no vitexin nor its glycosides but showed the presence of the phenolic compounds phloroglucinol, resorcinol and p-hydroxybenzoic acid, which are known for their antithyroid properties. Antithyroid activity was reported to be higher in extracts prepared from boiled or stored pearl millet (Gaitan et al., 1989).

Tempering the grain to 26 percent moisture overnight prior to milling resulted in a flour with no goitrogenic activity (Klopfenstein, Leipold and Cecil, 1991). A strong positive correlation was observed between c-glycosyl flavone level and the thyroid histopathology and hormone pattern. Yellowcoloured pearl millet was less goitrogenic than brown or grey millet. More evidence is needed however, to understand the mechanism of the antithyroid action of the flavonoids in pearl millet (Birzer and Klopfenstein 1988).

Amino acid imbalance and pellagra

Dietary deficiency of niacin, a B-complex vitamin, is well accepted as a causative factor of the nutritional disorder known as pellagra in humans. The classical clinical manifestations of pellagra are bilateral and symmetrical photosensitive dermatitis, diarrhoea and dementia or impairment of the mental function. Endemic pellagra in sorghum-eating populations was first described by Gopalan and Srikantia ( 1960), particularly in poor agricultural labourers around Hyderabad in Andhra Pradesh. About I percent of the hospital admissions were pellagrins and about 10 percent of the mental hospital cases showed clinical features of the disease (Gopalan and Vijayaraghavan, 1969).

Traditionally, pellagra has been associated with consumption of maize. It is rarely observed in populations subsisting on other cereals or millets. The pellagragenic properties of maize are largely explained by the poor niacin bigavailability and low tryptophan content of its protein. On the other hand, niacin in sorghum is biologically available (Belavady and Gopalan, 1966) and the tryptophan content of sorghum protein is not low. These observations suggest that the aetiology of pellagra in sorghum eaters might be different. A common feature of sorghum and maize is that the proteins of both these grains contain a relatively high proportion of leucine. It was therefore suggested that an amino acid imbalance from excess leucine might be a factor in the development of pellagra.

Clinical, biochemical and pathological observations in experiments conducted in humans as well as laboratory animals have shown that high leucine in the diet impairs the metabolism of tryptophan and niacin and is responsible for the niacin deficiency in sorghum eaters (Belavady, Srikantia and Gopalan, 1963; Srikantia et al., 1968; Ghafoorunissa and Narasinga Rao, 1973). High leucine is also a factor contributing to the pellagragenic properties of maize, as shown by studies in which dogs fed the low-leucine maize variety Opaque-2 did not suffer from niacin deficiency while those fed high-leucine Deccan hybrid maize showed typical features of the canine form of pellagra (Belavady and Gopalan, 1969). All these observations support the hypothesis that excess leucine in sorghum is aeliologically related to pellagra in sorghumeating populations.

Further studies have shown that the biochemical and clinical manifestations of dietary excess of leucine could be counteracted not only by increasing the intake of niacin or tryptophan but also by supplementation with isoleucine (Belavady and Udayasekhara Rao, 1979; Krishnaswamy and Gopalan, 1971). These studies suggested that the leucine/isoleucine balance is more important than dietary excess of leucine alone in regulating the metabolism of tryptophan and niacin and hence the disease process.

Pellagra is not endemic in all the areas where sorghum is the main staple. This probably suggests that factors other than excess leucine and poor leucine/isoleucine balance in sorghum proteins are responsible for the development of the disease. Recent investigations have shown that vitamin B6 is involved in the metabolism of leucine as well as that of tryptophan and niacin, and it is therefore suggested that regional differences in the prevalence of pellagra might be related to the nutritional status of the population in terms of vitamin B6 (Krishnaswamy et al., 1976).

Hulse, Laing and Pearson (1980), after reviewing the literature available to them, expressed the view that experimental evidence is lacking from other laboratories to support the hypothesis regarding the excess leucine in sorghum as an aetiological factor leading to niacin deficiency. Studies in human subjects (Nakagawa et al., 1975) as well as in rats (Nakagawa and Sasaki, 1977) did not show any effect of excess dietary leucine on tryptophan and niacin metabolism. Similarly, Cook and Carpenter (1987) failed to observe any aberration in niacin metabolism indicative of niacin deficiency resulting from excess leucine in chicks, rats and dogs. In view of these diverse observations, additional research is required to resolve this issue.

Qualitative improvement in the diet as a whole would be the right approach for prevention and control of any nutritional disorder in the population. However, such a blanket solution is not practicable considering economic and socio-cultural constraints. Based on the understanding of the factors that lead to pellagra in sorghum eaters, one of the alternative approaches to combating the disease would be identification of sorghum varieties with low leucine content and hence better leucine/isoleucine balance in the protein. Screening of sorghum varieties from a worldwide sorghum germplasm collection showed that genetic variability in protein content and Iysine and leucine content of the protein is very large (Deosthale, Nagarajan and Visweswar Rao, 1972). Four varieties of sorghum (IS 182, IS 199, IS516 and IS4642) were identified as having a stable low-leucine character (leucine content below 11 g percent in the protein). Experiments in dogs have shown that animals fed sorghum proteins with less than 11 g percent leucine did not suffer from nicotinic acid deficiency (Belavady and Udayasekhara Rao, 1979). The four selected varieties are therefore considered safe and could be beneficially exploited to prevent pellagra in endemic areas (Deosthale, 1980).

Two Ethiopian sorghum varieties were identified for their high protein and high lysine content (Singh and Axtell, 1973a). Analysis of grain samples of those varieties when grown in India not only confirmed their high-protein, highlysine character but also showed that their niacin content was about two to three times higher than that of normal sorghum grains (Pant, 1975). This observation indicates the second alternative approach to increasing the niacin content of the diet. Consumption of such varieties of sorghum may be expected to control and prevent pellagra even if the leucine/isoleucine balance is unfavourable.

Fluorosis, urolithiasis and other trace-element effects

High fluoride content of drinking-water is the most important factor in the aetiology of endemic fluorosis, but it is believed that diet and nutritional status is one of the factors that can influence the course of the disease (Pandit et al., 1940; Siddiqui, 1955). In certain parts of India where fluorosis is endemic, the agroclimatic conditions are conducive for the cultivation of sorghum and millets and these foodgrains are the main staple in the diets of the population. In fluorotic areas of Andhra Pradesh, a clinical manifestation of bone deformation known as genu valgum was seen more frequently in subjects whose staple was sorghum (Krishnamachari and Krishnaswamy, 1974). Furthermore, it was observed that retention of fluoride was significantly higher on a sorghum diet than on rice (Lakshmaiah and Srikantia, 1977).

Several factors including trace-element nutrition have been implicated in the aetio-pathology of fluorosis and genu valgum. In this respect an observation of importance is that grain samples of sorghum and pearl millet grown in a fluorosis area had 60 percent more molybdenum than those from a nonfluorosis area (Deosthale, Krishnamachari and Belavady, 1977). Experiments in human subjects have shown that high intake of molybdenum affects copper metabolism (Deosthale and Gopalan, 1974). Moreover, in areas where the incidence of genu valgum was high, the copper content of water was found to be very low as compared to that in non-genu valgum areas (Krishnamachari, 1976); this perhaps indicates differences in the copper nutritional status of the populations in these areas. Both copper and fluoride have a role in bone formation, and molybdenum promotes fluoride absorption (Underwood, 1971). However, no clear-cut evidence is available to explain the mechanism of this interrelationship with regard to the progression of the disease.

Urolithiasis, which is also endemic in certain parts of India, is a condition in which stones or calculi are formed in the urinary tract. This stone formation is said to be common in millet-eating populations (Patwardhan, 1961a). Several promoters and inhibitors of the lithogenic process have been implicated. There is some evidence to suggest that some trace elements are involved in the genesis of urinary calculi (Eusebio and Elliot, 1967; Satyanarayana et al., 1988). Molybdenum, which is found in greater amounts in sorghum than in other foods, is an integral part of the xanthine oxidase system and is involved as such in the synthesis of uric acid, a component of urinary calculi. In studies conducted in human volunteers, however, dietary intake of molybdenum had no significant effect on uric acid excretion (Deosthale and Gopalan, 1974). Several trace elements have been identified in appreciable amounts in urinary calculi and kidney stones. The significance of their presence in relation to lithogenesis needs investigation.

O'Neill et al. ( 1982) observed that in parts of China, foxtail-millet bran contained very high amounts of silicon, up to 20 percent. High silicon from the soil accumulates in the bristles and is deposited in the grain pericarp. Consumption of this high-silicon foxtail millet has a role in the aetiology of oesophageal cancer in northern China.

Mycotoxins

Like other cereals, sorghum and millets are susceptible to fungal growth and mycotoxin production under certain environmental conditions. Mycotoxins not only threaten consumer health but also affect food quality, causing huge economic losses. To help developing countries improve their mycotoxin prevention and monitoring programmes FAO has published a manual on training in mycotoxin analysis (FAO, 1990a).

Storage fungi, mostly of the genera Aspergillus and Penicillium, are found on foodgrain stored with moisture content greater than 13 percent (Sauer, 1988). Mouldy sorghum earheads were shown to be contaminated with aflatoxins B and G in India (Tripathi, 1973), in Uganda (Alpert et al., 1971) and in the United States (Shotwell et al., 1969). Aflatoxin has been shown to be hepatotoxic, carcinogenic, mutagenic and teratogenic. Proper drying and storage would greatly prevent the contamination of foodgrains. In India, infestation of pearl millet by a parasitic fungus, Claviceps purpurea, has caused an outbreak of ergotism, which is characterized by symptoms of nausea, vomiting, giddiness and somnolence (Patel, Boman and Dalal, 1958; Krishnamachari and Bhat, 1976).

A mouldy-grain toxicosis associated with consumption of sorghum grain was reported from Japan, and the causative fungus was Fusarium sp. (Saito and Ohtsubo, 1974). A strain of Fusarium incarnatum was isolated from naturally infested mouldy sorghum. The toxic metabolite present on mouldy sorghum grain infected with an isolated Fusurium strain was characterized for its chemical and biological properties. It was found to be T2 toxin, 3a-hydroxy4,15-diacetoxy-8a-(3-methylbutyryloxy)- 12,13 epoxy trichothecene.

Fusarium spp., primarily from infected millets, have been implicated in the aetiology of alimentary aleukia in humans in the former Soviet Union (Joffe, 1965).

Infestation

Insect damage during storage not only results in the loss of foodgrain but also affects its nutritional quality. Kapu, Balarabe and Udomah (1989) reported that crude protein values of all the foodstuffs including sorghum decreased significantly with insect damage. Pant and Susheela (1977) observed varietal differences in susceptibility to insect attack in 10 months' storage under ambient temperature and humidity. Moderate insect infestation did not alter the protein quality of the grain, but high infestation (30 percent) decreased it significantly. Insect-infested grain showed significant losses in total fat, mineral matter, thiamine and riboflavin. Sood and Kapoor (1992) have observed reduction in protein and starch digestibility on grain infestation in sorghum, wheat and maize. This effect was found to be dependent on the distribution of protein and starch in the kernel component

as well as on the feeding preferences of the insect. Infestation by the lesser grain borer, Rhizopertha dominica an insect which feeds on endosperm, was found to reduce starch digestibility, while that by the khapra beetle, Trogoderma granarium, which attacks the germ, reduced the protein digestibility.

Conclusion

Several factors as discussed above affect the nutritional quality of sorghum and millets. Fortunately there are methods available to eliminate, inactivate or prevent the antinutritional and/or toxic principles that may be present naturally or because of contamination. Grain processing, discussed in Chapter 3, has a significant role.


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