Sorghum is a critically important cereal food crop in Africa because of its drought-tolerance. However, sorghum does not produce good quality flour for dough-based products such as bread. This is on account of the fact that kafirin, its prolamin storage protein does not produce a viscoelastic dough and its starch has a high gelatinisation temperature
Recently, sorghum lines have been developed by genetic modification and by conventional mutation breeding that have altered synthesis of some kafirins, resulting in a high protein digestibility trait, here referred to as GM-HD and HD, respectively. Additionally, lines have been developed with both the conventionally bred HD trait plus the waxy (high amylopectin) trait (WHD).
The aim of this work was to determine the effects of these sorghum types on sorghum flour and dough quality.
The HD lines had altered protein body structure with loosely packed starch granules and a floury endosperm, irrespective of whether they were waxy or non-waxy. WHD lines had higher paste viscosity and formed much softer and less sticky pastes than the non-waxy, normal protein digestibility lines. Flours of the WHD lines also had much higher solubility than the non-waxy-normal protein digestibility lines. At 30oC, the water solubility of WHD lines flour was similar to a commercial wheat bread flour.
At both 30°C and 60°C, GM-HD lines had a significantly higher water soluble fraction compared to their null controls (N). Peak viscosity and holding strength of the GM-HD lines were also significantly higher than the N sorghums. Further, pasting temperature and setback values of GM-HD were significantly lower than N. Rheological analysis revealed that G' (storage modulus) and G" (loss modulus) of GM-HD doughs were higher than N sorghum dough during amplitude sweep and temperature sweep analysis.
SDS-PAGE of the GM-HD sorghums showed that γ-kafirin was missing, unlike their null controls. SDS-PAGE also revealed that the HD had a missing β-kafirin band. 2-D PAGE showed that there were spots of MW about 27 kDa (the apparent molecular weight of γ-kafirin) missing in the GM-HD.
For the first time, viscoelastic doughs were formed from kafirin. This was achieved by first dissolving in glacial acetic acid and then precipitating the dough by rapid cold water addition. These doughs formed fibrils, a characteristic believed to be essential to viscoelasticity. The fibrils in kafirins doughs from GM-HD and HD were more, thinner, and more compact and had a regular (non-ruptured) arrangement compared to their normal sorghums. FTIR did not show differences among the different kafirins in terms of the ratio of α-helical conformation to β-sheet conformation in secondary structure of kafirin (α/β ratio). However, the relative proportion of α-helical conformation increased in kafirin doughs prepared with glacial acetic acid compared to kafirin doughs prepared with dilute acetic acid, i.e. after addition of water.
Suppression of β-kafirin synthesis appeared to be the cause of the floury endosperm trait in the conventionally bred HD lines, which was associated with a less compact protein matrix. Reduction in γ-kafirin in the GM-HD lines seemed to increase sorghum dough elasticity through improving protein-starch interaction. Viscoelastic doughs can be formed from kafirin but only if the kafirin is first dissolved in organic solvent. Although both the combined HDW and GM-HD lines improved the sorghum flour and dough properties, due to the waxy trait the HDW have more potential for improving sorghum end-use quality for making dough-based food products.