Sorghum and tef: Origin distribution and importance

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Statement of problem

Sorghum (Sorghum bieolor (L.) Moench) is an indigenous cereal crop to Afiica, where it is grown in the semi-arid and sub-tropical zone, which includes the large belt in northern Afiica spreading from the Atlantic to Ethiopia and Somalia (Dendy 1995). Due to its drought tolerance and adaptation to semi-arid, sub-tropical and tropical conditions, sorghum can still be produced where agricultural and environmental conditions are unfavorable for the production of other cereal crops. This is ofparticular importance as Global Warming and the growth of the world’s population will require that more marginal lands be used for food production (Taylor and Dewar 2001). World annual sorghum production in 2002 was 54.5 million tons, of which Ethiopia produced about 1.82 million tons (FAa 2003). Nearly all the sorghum grain produced in Ethiopia is used for human consumption. About 80% is used for making leavened bread (injera) and 10% is used to make home brewed beer (telia) (Gebrekidan and GebreHiwot 1q~O). The remainder goe3 into making stiff purridge (genfo), unleavened bread (kitta), boiled whole grain (nifro), popped grain (kollo) and animal feed. The Ethiopian Sorghum Improvement Program (ESIP) conducts research on local landraces and accessions from the world sorghum collection for improvement of sorghum in Ethiopia. The factors requiring consideration in sorghum improvement include: yield increases, resistance to yield limiting biotic and abiotic factors and end use quality traits. Of late, enduse quality as a factor is receiving more attention than ever. The national cultivar release committee of Ethiopia has made it mandatory to include end-use quality data before a cultivar is proposed for release.

Sorghum and tef: Origin distribution and importance

Sorghum (Sorghum Nco lor (L.) Moench) is a tropical grass, which originated in North Africa, cultivated extensively for human consumption in Africa and India, particularly in arid and semi-arid regions of the world. Tef [Eragrostis tef (Zucc.) Trotter] is also a tropical grass, believed to have been domesticated in the northern highlands of Ethiopia (House et al 1995). It is a significant food crop in only one country in the world, Ethiopia (Seyfu 1993, National Research Council 1996). Ethiopia is also considered as the major world center for the genetic diversity of tef (Seyfu 1993). It is estimated that more than 70 percent of the world sorghum crop is consumed as food in the main production areas of Africa and Asia (ICRISATIF AO 1996). In Ethiopia, both sorghum and tef are consumed as staples and are important sources of carbohydrate in the diet, which is cereal-based, specifically in injera. Next to tef, sorghum is the second preferred cereal for making injera (Gebrekidan and GebreHiwot 1982). Of note is the fact that the cultivation productivity of tef is low and therefore it commands a higher market price than other cereals in Ethiopia (Seyfu 1993).

Significance ofendosperm texture

.Endospenn texture has been identified as a factor that most consistently affects the processing and food making properties of sorghum (Rooney et al 1986). Ease of mechanical decortication of the sorghum grain depends on the hardness (related to vitreouness) of the grain (Shepherd 1979, Reichert et al 1982, Lawton and Faubion 1989). With hard grain, the bran is removed with minimum loss of endospenn material. Finer flour is nonnally achieved from more vitreous endospenn, but more energy and time are required (Rooney et al 1986). However, flours from the vitreous endospenn typically contain closely-packed, polygonal starch granules with protein bodies, while starch granules released from the floury endospenn, that are loosely-packed, tend to be released as individuals, as they are not held together by matrix material (Duodu et al 2002). Furthennore, during pasting the association of the protein bodies around the starch granules appears to act as a barrier to starch gelatinization (Chandrashekar and Kirleis 1988). Almeida-Dominguez et al (1997) also demonstrated that floury maize samples developed higher viscosities more rapidly. The authors ascribed this phenomenon to loosely packed starch granules with reduced protein-to-starch bonds in floury maize which hydrated and swelled more rapidly in the presence of heat. This may also apply to sorghum, because a negative correlation was reported between grain hardness and peak paste viscosity (Taylor et al 1997).

Starch

Starch is a major component of many food plants where it occurs as water-insoluble granules (Miles et al 1985). Worldwide, starch provides 70-80% of the calories consumed by people (Whistler and BeMiller 1999). Starch comprises two main polysaccharides amylose, an essentially linear polymer composed of a-l,4 linked D-glucopyranose molecules (Fig 2.4), and amylopectin a very large, branched, D-glucopyranose polymer containing both a-l,4 and a-l,6 linkages (Thomas and Atwell 1999) (Fig 2.5). Recent evidence, suggests that some branches are present on the amylose polymer (Cura et al 1995). Amylose chains are helical with hydrophobic, lipophilic interiors capable of forming complexes with linear hydrophobic portions of molecules that can fit within the lumen of the helix (Whistler and BeMiller 1999). The structure and molecular weight range of amylopectin molecules vary with the source of the starch (Myers et al 2000) and thereby determine its crystallinity and branching patterns (Hizukuri et al. 1997).

READ Technology for temperature monitoring and analysis

Protein

The protein content of sorghum generally ranges from 7 to 14% (Taylor et al 1984b), while tef contains less protein, between 6 to 10% (Lester and Bekele 1981). Protein content is influenced by cultivar and the environment, with considerable environmental variation (House et al 1995). Seed proteins, in general, are composed of three groups, namely storage proteins, structural proteins and biologically active proteins (enzymes) (Fukushima 1991). The storage proteins have been described as a sink for surplus nitrogenous compounds required for physiological processes (Tsai et al 1978). The protein compositions of the vitreous and floury portion of the endosperm are reported to be different. Watterson et al (1993) found that the vitreous endosperm of sorghum contains 1.5-2 times more total protein than the floury endosperm. The sorghum germ contains about 16% of the grain nitrogen, most of which occurs as low molecular weight nitrogen and albumin and globulin proteins (Taylor and Schussler 1986). Albumins and globulins are richer in most of the essential amino acids than other sorghum protein fractions (Youssef 1998).

Lipids

The lipids of sorghum, like those of other cereals, are located mainly in the germ, although there are smaller amounts present in the endosperm (Taylor and Belton 2002). Lipids are mainly stored in spherosomes, lipid-containing organelles, in the germ and aleurone (Morrison 1988). The oil in the germ of cereals is rich in polyunsaturated fatty acids (FAO 1995) with a large number of chemical classes and a much larger number of individual compounds (Hoseney 1994). Fatty acid profiles of sorghum grain lipids revealed that the major acids are palmitic (C16:0) (15.1-24.8%), oleic (C18:1) (29.9 41.8%) and linoleic (C18:2) (35.9-51.3%) (Maestri et al 1996), indicating a high level of unsaturation. The total lipid content of sorghum ranges from 0.5 to 5.2%, but normally in the higher range (Serna-Saldivar and Rooney 1995), while that of tef is in the range of 2.0-3.1 % (National Research Council 1996), which is strange as the germ of tef is also relatively large. For maize, Hoseney (1994) attributed the difference in lipid content between cultivars to differences in germ size and amount of oil in the germ.
This might also apply to sorghum. The proportionally large germ in sorghum results in a high fat content flour when sorghum is milled without decortication. The fat component of whole sorghum flour may also cause rancidity due to oxidation of unsaturated fatty acids. This becomes more important, for example when, as is traditionally the case, households keep enough flour for a few weeks at ambient temperature. In relation to the pasting properties of starches, lipids are reported to cause a higher pasting temperature and a lower starch paste viscosity (Fortuna et al 2000). Free fatty acids, however, increase starch paste viscosity (Nelles et al 2000).

TABLE OF CONTENTS :

Preliminary pages
Title page
Declaration
Dedication
ACKNOWLEDGEMENTS
ABSTRACT
List of tables
List of figures
1. INTRODUCTION
- 1.1. Statement of problem
- 1.2. Hypotheses
- 1.3. Objectives
2. LITERATURE REVIEW
- 2.1. Sorghum and tef: Origin distribution and importance
- 2.2. Anatomy of sorghum and tef grains
- 2.2.1. Starch granules
- 2.2.2. Significance ofendosperm texture
- 2.3. Chemical components of sorghum and tef
- 2.3.1. Starch
- 2.3.2. Protein
- 2.3.3. Lipids
- 2.3.4. Non-starch polysaccharides
- 2.3.5. Tannins
- 2.6. Decortication and milling
- 2.7. Fermentation
- 2.8. Dough making and baking
- 2.9. Staling
- 2.10. Standardized injera making procedures
- 2.11. Sensory evaluation
- 2.12. Sorghumgrain characteristics as related to flat bread quality
- 2.13. Conclusions
3. RESEARCH
- 3.1. Improving the quality of sorghum injera by decortication and compositing with tef
- Abstract
- 3.1.1. Introduction
- 3.1.2. Materials and methods
- 3.1.3. Results and discussion
- 3.1.4. Conclusions
- 3.1.5. References
- 3.2. Effects of sorghum cultivar on injera quality
- Abstract
- 3.2.1. Introduction
- 3.2.2. Materials and methods
- 3.2.3. Results and discussion
- 3.2.4. Conclusions
- 3.2.5. References
- 3.3. Grain and flour quality of Ethiopian sorghums in respect of their injera making potential
- Abstract
- 3.3.1. Introduction
- 3.3.2. Materials and methods
- 3.3.3. Results and discussion
- 3.3.4. Conclusions
- 3.3.5. References
4. GENERAL DISCUSSION
- 4.1. Methodologies: A critical review
- 4.2. Relating physico-chemical parameters to injera sensory Proposed selection criteria
- 4.3.1. Early generation selection criteria
- 4.3.2. Advanced breeding stage selection criteria
- 4.4. Future research needs
- 5. CONCLUSIONS AND RECOMMENDATIONS
- 6. REFERENCES
- Appendix I. Publications and presentations from this work