Lipid composition

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Sorghum

The crude fat content of sorghum is 3 percent, which is higher than that of wheat and rice but lower than that of maize. The germ and aleurone layers are the main contributors to the lipid fraction. The germ itself provides about 80 percent of the total fat (Rooney and Serna-Saldivar,1991). As the kernel tat is mostly located in the germ, in sorghum mutants with a large embryo fraction the fat content is higher (5.8 to 6.6 percent) than normal (Jambunathan, 1980). Variations in reported fat content of the grain can be attributed partly to the different solvent systems used for extraction of kernel fat. Price and Parson (1975) reported that the neutral lipid fraction was 86.2 percent, glycolipid 3.1 percent and phospholipid 10.7 percent in sorghum fat.

No significant difference was reported in let content among several cultivated and wild sorghum races (Stemler et al., 1976). Fatty acid was significantly higher in kafir, caudatum and wild sorghum than in the bicolor, durra and guinea groups. On the other hand, caudatum types had the lowest linoleic acid and bicolor, durra and guinea varieties had more than wild and kafir sorghum. Oleic and linoleic acids were negatively correlated with each other. The fatty acid composition of sorghum fat (linoleic acid 49 percent, oleic 31 percent, palmitic 14 percent, linolenic 2.7 percent, stearic 2.1 percent) was similar to that of corn fat but was more unsaturated (Rooney, 1978).

Millets

Finger, foxtail and kodo millets appeared to contain less fat in the kernel than other millets (Table 17), while the fat content of common millet was similar to that of sorghum. The fat content of pearl millet is the highest among the millets.

Lai and Varriano-Marston (1980) observed significant differences in the fatty acid composition of four different bulk populations of pearl millet. Differences in lipid extraction procedures as well as genetic variability were shown to contribute to differences in the fatty acid content of pearl millet (Jellum and Powell,1971). The principal fatty acids in both free and bound tat were found to be linoleic, oleic and palmitic acids. Distinct differences in fatty acid composition were noted in the neutral lipid, phospholipid and glycolipid fractions (Osagie and Kates, 1984). Neutral lipid was highest in linoleic acid and lowest in palmitic acid; phospholipid was lowest in oleic acid and highest in palmitic acid; and glycolipid was highest in linolenic acid.

The fatty acid composition of common millet and foxtail millet did not differ from that of sorghum (Hulse, Laing and Pearson, 1980). Common millet was found to contain 1.8 to 3.9 percent lipids, and about 24 percent of the grain fat was in the embryo component. The fatty acid profile showed that saturated fatty acids totalled 17.9 to 21.6 percent while unsaturated fatty acids totalled 78 to 82 percent. The unrefined fat extracted from the kernel of common millet contained 8.3 to 10.5 mg vitamin A and 87 to 96 mg vitamin E per 100 g. On refining, all the vitamin A activity was lost and there was significant loss in vitamin E. Vitamin E is also present in the fat extracted from sorghum grain.

Minerals

The mineral composition of sorghum and millet grains (Table 25) is highly variable. More than genetic factors, the environmental conditions prevailing in the growing region affect the mineral content of these foodgrains.

Sorghum

In the sorghum kernel the mineral matter is unevenly distributed and is more concentrated in the germ and the seed-coat (Hubbard, Hall and Earle, 1950). Pedersen and Eggum (1983) have shown that in milled sorghum flours minerals such as phosphorus, iron, zinc and copper decreased with lower extraction rates. Similarly, pearling the grain to remove the fibrous seed-coat resulted in considerable reduction in the mineral contents of sorghum (Sankara Rao and Deosthale, 1980). However, these studies also showed that the in vitro availability of iron as judged by the ionizable iron as percentage of total iron was higher in pearled grain. Dehulling improves iron availability because the hull is rich in phytate, a compound that binds iron and certain other minerals and makes them biologically unavailable (see Chapter 6). Mbofung and Ndjouenkeu ( 1990) observed that the percentage of soluble and ionizable iron was higher in gruels prepared from mechanically dehulled sorghum than in those prepared from grain milled traditionally using mortar and pestle. The increase in iron availability was attributed partly to the efficient removal of the phytate-rich hull in mechanical milling and partly to the greater destruction of phytate during soaking of the grain prior to dehulling.

In studies in Indian women, absorption of iron was higher from tannin-free than from high-tannin sorghum cultivars (Gillooly et al., 1984). Pearling of the grain improved the absorption of iron from both high- and low-tannin cultivars. Radhakrishnan and Sivaprasad (1980) assessed the bioavailability of iron in normal and anaemic subjects fed diets based on two varieties of sorghum containing 20 and 136 mg of tannin respectively and 160 and 273 mg of phytin phosphorus respectively per 100 g. In normal subjects iron absorption from low- and high-tannin sorghum was essentially similar. However, in anaemic subjects it was significantly lower with high-tannin sorghum. On equalization of the phytate content of the two sorghum meals the difference in iron absorption disappeared. It was concluded that at the levels of tannins present in the two varieties of sorghum, tannins had a minor role in determining the iron bioavailability.

TABLE 25: Mineral composition of sorghum and millets (mg %) a

Grain Number of cultivars P Mg Ca Fe Zn Cu Mn Mo Cr
Sorghum 6 352 171 15 4.2 2.5 0.44 1.15 0.06 0.017
Pearl millet 9 379 137 46 8.0 3.1 1.06 1.15 0.07 0.023
Finger millet 6 320 137 398 3.9 2.3 0.47 5.49 0.10 0.028
Foxtail millet 5                  
Whole   422 81 38 5.3 2.9 1.60 0.85 - 0.070
Dehulled   360 68 21 2.8 2.4 1.40 0.60 - 0.030
Common millet 5                  
Whole   281 117 23 4.0 2.4 5.80 1.20 - 0.040
Dehulled   156 78 8 0.8 1.4 1.60 0.60 - 0.020
Little millet 5                  
Whole   251 133 12 13.9 3.5 1.60 1.03 - 0.240
Dehulled   220 139 13 9.3 3.7 1.00 0.68 - 0.180
Barnyard millet 5                  
Whole   340 82 21 9.2 2.6 1.30 1.33 - 0.140
Dehulled   267 39 28 5.0 3.0 0.60 0.96 - 0.090
Kodo millet 5                  
Whole   215 166 31 3.6 1.5 5.80 2.90 - 0.080
Dehulled   161 82 20 0.5 0.7 1.60 1.10 - 0.020

a Expressed on a dry-weight basis.
Sources: Sankara Rao and Deosthale.1980 (sorghum) 1983 (pearl and finger millets), unpublished ( other millets).

Gillooly et al. (1984) found no difference in the iron absorption from porridges prepared from malted and unmelted sorghum. They observed that addition of ascorbic acid facilitated the iron absorption from both porridges, while consumption of tea adversely affected the iron absorption. Iron absorption varied in a narrow range of 72 to 83 percent in rats fed acidic, basic or neutral sorghum t˘, maize gruel or the fermented sorghum porridge aceda (Stuart et al., 1987). However, absorption of zinc was found to be significantly higher, 97 percent, in rats fed fermented sorghum aceda than in those fed maize gruel or any of the three types of sorghum t˘ (67 to 78 percent).

Beers brewed with sorghum adjuncts and maize grits are very common in African countries. Derman et al. ( 1980) observed that the iron absorption from beer brewed from sorghum or maize was more than 12 times higher than that from gruel prepared from these two grains. Beer brewed with sorghum adjunct was found to be a concentrated source not only of vitamins such as thiamin and nicotinic acid but also of several minerals including copper, manganese, iron, magnesium, potassium and phosphorus (van Heerden, 1989). With appreciable amounts of protein and starch and no detectable phytate, sorghum beer could make an important contribution to the daily intake of vitamins and minerals in African populations.

Pearl millet

Wide variations have been reported in the mineral and trace-element composition of pearl millet, and as with sorghum the composition and nature of the soil was considered the main environmental factor determining the mineral content of the grain (Hoseney, Andrews and Clark, 1987; Jambunathan and Subramanian, 1988). Milling of pearl millet to a flour with an extraction rate of 75 percent reduced the calcium and iron content by about 66 percent (de Wit and Schweigart, 1970). Dassenko ( 1980) observed significant losses of calcium, magnesium and sodium but not of iron and potassium on milling pearl millet to a flour with 67 percent extraction rate.

In rat feeding studies, absorption of iron by anaemic animals fed pearl millet as a source of iron (2 mg per kilogram body weight) was 35.7 percent as against 29.7 percent with sorghum,37.5 percent with maize, 40 percent with soybean and 33.3 percent with bambara nuts (Ifon,1981). In bioavailability studies with chicks, the magnesium availability was higher from pearl millet than from sorghum (Nwokolo, 1987). However, the millet was found to be poor in available zinc, iron and manganese compared with sorghum.

Malting enhanced severalfold the ionizable iron content of pearl millet and finger millet grains and also significantly increased their soluble zinc content, indicating an improvement in in vitro availability of these two elements (Sankara Rao and Deosthale, 1983).

Klopfenstein, Hoseney and Leipold (1985) observed that rats fed pearl millet supplemented with calcium carbonate in the diet continued to grow well after seven weeks of feeding, while those fed unsupplemented millet in the diet ceased to grow after four weeks. It was concluded that calcium was more limiting than lysine or other nutrients in pearl millet when fed to growing rats.

Finger millet

Except for very high calcium and manganese content, the mineral and trace element composition of finger millet is comparable to that of sorghum. Some high-protein (8 to 12.1 percent) and high-yielding varieties of finger millet were also rich in calcium (294 to 390 mg per 100 g) (Babu, Ramana and Radhakrishnan, 1987). Studies conducted in nine- to ten-year-old girls showed that replacement of rice in a rice-based diet with finger millet not only maintained positive nitrogen balance but also improved calcium retention (Joseph et al., 1959). Thus finger millet could be used to overcome the calcium deficiency of a rice diet. In vitro studies showed that bioavailability of iron was poor in commonly cultivated and highly pigmented varieties of finger millet grain because of their tannin content. Removal or reduction of tannin either by extraction with solvent or by grain germination enhanced the ionizable iron content. These studies also showed that iron availability in terms of ionizable iron content was higher in white grain, no-tannin finger millet varieties (Udayasekhara Rao and Deosthale, 1988).

Other millets

The total mineral matter as ash content was higher in common, little, foxtail, kodo and barnyard millets than in most commonly consumed cereal grains including sorghum. These minor millets have a highly fibrous hull which is usually removed before consumption. Dehulling was found to result in considerable nutrient losses in all five millets. The extent of these losses was variable and depended upon the mineral content of the species (Sankara Rao and Deosthale, unpublished) (Table 25).

Lorenz (1983) observed that the phytate content of common millet varieties ranged from 170 to 470 mg per 100 g whole grain, and dehulling resulted in a 27 to 53 percent reduction in phytate content. On dehulling, phytin phosphorus decreased 12 percent in common millet, 39 percent in little millet, 25 percent in kodo millet and 23 percent in barnyard millet (Sankara Rao and Deosthale, unpublished).

Vitamins

Sorghum

Sorghum and millets in general are rich sources of B-complex vitamins. Some yellow-endosperm varieties of sorghum contain ▀-carotene which can be converted to vitamin A by the human body. Blessin, VanEtten and Wiebe (1958) isolated carotenoids of sorghum and identified lutein, zeaxanthin and ▀carotene. Suryanarayana Rao, Rukmini and Mohan (1968) analysed several varieties of sorghum for their ▀-carotene content. The variations were very large, with values ranging from 0 to 0.097 mg per 100 g of grain sample. In view of the photosensitive nature of carotenes and variability due to environmental factors, yellow-endosperm varieties of sorghum are likely to be of little importance as a dietary source of vitamin A precursor.

Detectable amounts of other fat-soluble vitamins, namely D, E and K, have also been found in sorghum grain. Sorghum as it is generally consumed is not a source of vitamin C. On germination, some amount of vitamin C is synthesized in the grain and on fermentation there is a further rise in the vitamin content (Taur, Pawar and Ingle, 1984). ln feeding trials in guinea pigs on diets based on wheat, rice, maize or pearl millet, the vitamin C requirement of the animals for optimal growth was five times higher than that of animals fed casein in their diets (Klopfenstein, Varriano-Marston and Hoseney, 1981 a,b: Klopfenstein, Hoseney and Varriano-Marston, 1981). Guinea pigs on isonitrogenous, isocaloric, nutritionally adequate diets based on sorghum required 40 mg vitamin C per day as against 2 mg on the casein-based diet. Higher levels of dietary ascorbic acid apparently had a niacin-sparing effect on the sorghum-based diet. Interestingly, the animals fed 40 mg ascorbic acid had low levels of cholesterol in their blood and liver. The significance of these observations in relation to the nutrition of predominantly sorghum-eating populations needs further investigation.

Among B-group vitamins, concentrations of thiamin, riboflavin and niacin in sorghum were comparable to those in maize (Table 17). Wide variations have been observed in the values reported, particularly for niacin (Hulse, Laing and Pearson, 1980). The highest niacin content, 9.16 mg per 100 g sorghum, was reported by Tanner, Pfeiffer and Curtis (1947). Ethiopian high-lysine sorghum varieties were also very high in niacin; values per 100 g were 10.5 mg in IS 11167 and 11.5 mg in IS 11758, as against 2.9 to 4.9 mg in normal sorghum (Pant, 1975).

Niacin in cereal grains exists in a bound form which is alkali soluble but considered biologically unavailable to humans (Goldsmith et al., 1956). Ghosh, Sarkar and Guha (1963) observed that 80 to 90 percent of the niacin in sorghum grains was in bound form and was available for the growth of the microorganism used for niacin assay only after alkali treatment. Adrian, Murias de Queroz and Frangne (1970) followed different extraction procedures and found that in sorghum 20 to 28 percent of the niacin was cold-water extractable and thus biologically available, compared to about 45 percent in maize. Belavady and Gopalan (1966) in their studies in dogs observed that niacin in sorghum grain was completely cold-water soluble and thus available, an observation that was quite different from those of Ghosh, Sarkar and Guha (1963) and Adrian, Murias de Queroz and Frangne (1970). Other studies (Carter and Carpenter,1981,1982) showed that niacin in sorghum grain was present as a high-molecular-weight complex and was biologically available to rats after alkali treatment of the grain but not after boiling in water. In boiled grains total niacin per 100 g was 7.07 mg in rice, 5.73 mg in wheat, 4.53 mg in sorghum and 1.88 mg in maize. The proportion of total niacin available to rats was 41 percent in rice, 31 percent in wheat, 33 percent in sorghum and 37 percent in maize. Thus niacin bioavailability in cereal grains was found to be limited (Wall and Carpenter' 1988).

Other B-complex vitamins present in sorghum in significant amounts are vitamin B6 (0.5 mg per 100 g), folacin (0.02 mg), pantothenic acid ( 1.25 ma) and biotin (0.042 ma) (United States National Research Council/National Academy of Sciences, 1982).

Millets

Available data are very meagre regarding the vitamin content of pearl millet, finger millet and minor millets. In thiamin and riboflavin content these millets differed little from sorghum (Table 17). Niacin content, however, was lower in some of them. Ghosh, Sarkar and Guha ( 1963) found that, as in sorghum, 80 to 90 percent of the niacin in pearl millet grains was biologically unavailable. Adrian, Murias de Queroz and Frangne (1970), however, found that 31 to 40 percent of the niacin in pearl millet was cold-water extractable and thus available. In little millet total niacin was very high ( 10.88 mg percent), about two to three times higher than in other cereals, but only 13 percent of it was cold-water extractable.

Khalil and Sawaya (1984) found that bread prepared from pearl millet flour by a traditional method was significantly lower in thiamin, pantothenic acid and folic acid than the flour itself. The millet flour was relatively high in pantothenic acid. In nine pearl millet varieties thiamin content varied from 0.29 to 0.4 mg per 100 g, with a mean of 0.34 mg (Chauhan, Suneja and Bhat, 1986). Germination of pearl, finger and foxtail millet grains for 48 hours increased ascorbic acid to 8, 5 and 6 mg per 100 g, respectively. There was also a small but significant increase in thiamin content (Malleshi and Desikachar, 1986a). Opoku, Ohenhen and Ejiofor ( 1981 ) observed increases in thiamin, riboflavin, ascorbic acid, vitamin A and tocopherol in pearl millet germinated for 48 hours and kilned at 45░C. Niacin, however, decreased by about 30 percent. Aliya and Geervani ( 1981 ) observed increases in thiamin (to 90 percent) and riboflavin (to 85 percent) on fermentation of pearl millet batter. However, steaming the fermented batter decreased the thiamin (to 64 percent) and riboflavin (to 28 percent) below the initial values of unfermented batter. Similar vitamin losses on fermentation of pearl millet flour were observed by Dassenko (1980). On cooking there was no change in the vitamin content of the fermented product.


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