Dietary fibre

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The term dietary fibre is used to describe a variety of indigestible plant polysaccharides including cellulose, hemicelluloses, pectins, oligosaccharides, gums and various lignified compounds. According to the modified definition of Trowell ( 1976), dietary fibre is defined as the sum of the lignin and polysaccharides that are not hydrolysed by the endogenous enzymes of the human digestive tract. Kamath and Belavady ( 1980) found that the major insoluble fibre component of sorghum was cellulose, which varied from 1.19 to 5.23 percent in sorghum varieties. In any seed material there are two sources of dietary fibre, namely the hull or the pericarp and the cell wall structural components. The plant cell walls contain many non-carbohydrate components in addition to lignin, such as protein, lipids and inorganic material, and they modify the properties of the polysaccharides. Several approaches have been suggested for the measurement of total dietary fibre in foods. Each of the methods has certain limitations which may contribute to the observed variations in dietary fibre content reported for various foodstuffs.

Sorghum

Bach Knudsen and Munck (1985) found that a commonly consumed lowtannin Sudanese sorghum variety, Dabar, had total dietary fibre content of 7.6 percent while a high-tannin Sudanese variety, Feterita, contained 9.2 percent. A major proportion of the total dietary fibre in both the varieties was water insoluble (6.5 percent in Dabar and 7.9 percent in Feterita). The acid detergent fibre in the two varieties was also different (2.9 percent in Dabar and 3.6 percent in Feterita). The contribution of polyphenols to the lignin fraction of the dietary fibre was responsible for the higher values of dietary fibre in the high-tannin variety. Cooking of the sorghum as whole-grain porridge decreased the availability of energy, mostly because of the formation of enzyme-resistant starch, therefore apparently increasing the dietary fibre content of both varieties. Fermentation at pH 3.9 helped overcome the formation of resistant starch and also prevented the formation of lignin during cooking. Compared to wheat, rye, barley or maize, the total dietary fibre in the two sorghum varieties was low. The amount of protein bound to total dietary fibre as well as to acid detergent fibre in the sorghum varieties was much higher than in wheat and other foodgrains, and this binding increased on cooking, especially in the high-tannin sorghum. Fermentation or acidification to pH 3.9 inhibited the protein binding. These observations indicate that the traditional Sudanese fermentation method has important nutritional advantages.

Dietary fibre has certain adverse effects on the availability of some nutrients. The concentration of zinc and iron in the tibia of rats on sorghum diets rich in fibre and phytate was significantly lower than in rats on a non-sorghum diet with low fibre content (All and Harland, 1991).

Decortication of the grain is one of the methods to remove fibre. Cornu and Delpeuch (1981) found that the apparent nitrogen digestibility in adult subjects on a diet of 80 percent sorghum decreased from 65.4 to 60.5 percent when the dehusked sorghum in the diet was replaced by whole-grain sorghum. The total faecal matter of subjects on the whole-grain sorghum diet was higher. The nitrogen and formic-acid insoluble material in the faeces also increased.

Karim and Rooney ( 1972) reported that the pentosan content of sorghum varied from 2.51 to 5.57 percent. Pentosans as they occur in the cell walls of cereal grains are a heterogeneous mixture of polysaccharides, many of which contain proteins.

Earp et al. (1983) identified the mixed linked ß-glucans in sorghum pericarp, aleurone and endosperm. These ß-glucans are water soluble and form viscous, sticky solutions. This property is important in the malting of sorghum and brewing of beer. Klopfenstein and Hoseney ( 1987) observed that rats fed bread prepared from white flour fortified with ß-glucan (7 percent by weight) had serum cholesterol significantly lower than those fed bread from unfortified flour. The cholesterol-lowering property was also shown by the glucans isolated from oat, barley, wheat and sorghum.

Millets

Kamath and Belavady (1980), using the method of Southgate, Hudson and Englyst (1978), found that the total dietary fibre in pearl millet (20.4 percent) and finger millet (18.6 percent) was higher than that in sorghum (14.2 percent), wheat (17.2 percent) and rice (8.3 percent). Singh et al. (1987), also using the Southgate method, found that the total dietary fibre content of pearl millet was 17 percent. There are not enough data available on the dietary fibre components of the millets. Bailey, Sumrell and Burton (1979) have isolated pentosan containing a mixture of heterogeneous polysaccharides from the cell wall of pearl millet grains. The pentosan of pearl millet extracted with different solvents including 80 percent ethanol, water and alkali was found to contain seven sugars, the most predominant being arabinose, xylose and galactose, followed by rhamnose and fucose. Emiola and de la Rosa (1981) also studied the water- and alkali-extractable pentosan of pearl millet, but their results were at variance with those of Bailey, Sumrell and Burton (1979), showing an identical pattern for the water- and alkali-soluble pentosan but with ribose rather than fucose as one of the sugars. Emiola and de la Rosa (198 1) found that in pearl millet water-soluble non-starch polysaccharides accounted for 0.66 percent of grain weight and water-insoluble non-starch polysaccharides for 3.88 percent. On further purification these values were reduced to 0.42 percent and 0.97 percent, respectively. Wankhede, Shehnaj and Raghavendra Rao (1979a) reported that in finger and foxtail millet the pentosan content was 6.6 and 5.5 percent, respectively. Muralikrishna, Paramahans and Tharanathan ( I 982) found that the hemicellulose A in little, kodo and barnyard millets was a non-cellulosic ß-glucan and the hemicellulose B was composed of hexose, pentose and uranic acid.

Chapter 5 - Nutritional quality of foods prepared from sorghum and millets

It stands to reason that when a grain is processed, some nutrients must be removed and also that the removal of any but an exactly proportionate part of any constituent of a seed will affect the nutritional quality of what is left. Consequently, the nutritional effect of milling probably depends as much on the amount of material removed as on the method used to remove it. It is therefore difficult to compare different reports involving different preparative techniques. Reichert and Youngs (1977) reported that traditionally decorticated sorghum and millets contained more oil and ash than abrasively decorticated grains but the protein content was similar. Pushpamma ( 1990) reported that decortication reduced total protein and lysine by about 9 and 21 percent respectively, but that it also Improved the utilization of the remaining protein. The loss of minerals was minimal. Decortication improved the biological availability of nutrients and consumer acceptability.

Whether the removal of nutrients (and antinutritional factors) is on balance beneficial is a question that must always be analysed carefully. Organoleptic factors must also be considered. What is actually done is not always nutritionally for the best, and what is best in one type of diet is not always what is best for another.

Germination leads to considerable changes in the nutritive quality of a grain. There will obviously be some changes because of the loss of dry matter, but far more important changes. such as increased enzyme activity and the conversion of starch to sugars, result from the growing process. The toxicity of cyanide in germinated sorghum has already been mentioned. The danger of sickness or death from cyanide ingestion must always be borne in mind.

TABLE 26: Effect of time and temperature on nutritional quality of germinated sorghum seeds^a

a N = 1. Seeds were germinated in quart glass jars, dried at 50°C and ground in a Wiley mill.
b PER = 0.286 + 0.022(RNV). Values rounded to nearest 0.1 PER.
c Non - germinated control.
Source: Wang and Fields, 1978.

Wang and Fields ( 1978) found that germination of sorghum increased the relative nutritive value (RNV) from 54.6 to 63 percent and the protein efficiency ratio (PER) from 1.5 to 1.7. There were substantial increases in Iysine, methionine and tryptophan (Table 26). Malleshi and Desikachar (1986b) reported that germination of finger, pearl and foxtail millets resulted in a slight decrease in total protein and moisture. The main advantage was a reduction in the level of phyate and an increase in the levels of ascorbic acid, lysine and tryptophan. Malleshi, Desikachar and Venkat Rao (1986) also found that germination substantially reduced the amount of phytate, thereby improving the absorption of iron. Sprouting, roasting and sieving reduced the protein content of finger millet from 7.7 to 3.9 percent ( Hemanalini et al., 1980).

TABLE 27: Means of nutrient contents in sorghum meal^a

a n = 5. Means with different letters are significantly different. 'Significant at P < 0.01: "Significant at P < 0.05
Source : Au and Fields, 1981.

Changes that take place during fermentation include increases in amino nitrogen the breakdown of proteins and the destruction of any inhibitors that may be present. Significant increases in various amino acids (particularly methionine) and vitamins have been observed (Kazanas and Fields, 1981; Au and Fields, 1981 ) as a result of fermentation of sorghum (Table 27); an increase in the nutritive value was also reported. Axtell et al. ( 1981 ) found that fermented products of sorghum were more digestible than unfermented products. Fermentation or acidification inhibited the protein-binding effect of polyphenols (Bach Knudsen and Munck, 1985). Obizoba and Atii ( 1991 ) reported that fermentation reduced the level of cyanide in sprouted sorghum. It also reduced enzyme-resistant starch and decreased the concentration of the flatulence-causing sugars raffinose and stachyose (Odunfa and Adeyele, 1987). The starch and protein digestibility of rabadi, a product made from pearl millet, increased with longer fermentation (Dhankher and Chauhan, 1987).

MacLean et al. (1983) showed that decortication and extrusion can markedly improve the apparent digestibility of sorghum protein ted to young children. The addition of calcium hydroxide before extrusion also improved digestibility (Fapojuwo, Maga and Jansen, 1987).

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