Sorghum foods contribute significantly to the protein intake of millions of people in developing countries. One limitation to sorghum’s use as a protein source is that its proteins become less digestible on wet cooking, primarily through the formation of disulphide-linked enzymatically resistant protein polymers. Irradiation of foods can modify bonds involved in stabilizing protein structure, resulting in changes in the protein. The effects of irradiating sorghum cultivars BR7 and Madjeri, and maize cultivar PAN 6043 flours under mild (10 kGy dry) and severe (50 kGy dry and 10 and 50 kGy wet) conditions, followed by cooking into porridge on the digestibility, solubility and some molecular properties of their proteins, were investigated. Pepsin and multienzyme methods of determining protein digestibility (PD) were compared. As expected, pepsin PD of sorghum decreased more with cooking alone (12-18%) compared to maize (4%). Sorghum porridges had more disulphide-bonded prolamin dimers than maize as shown by SDS-PAGE under non-reducing conditions. However, the amounts of disulphide bonds in both porridges appeared similar. Prolamin extractability (PE) decreased more with cooking in sorghum compared to maize. There was no significant correlation between the pepsin and multienzyme methods, suggesting the latter may not simulate in vivo PD that has been reported to correlate positively with pepsin PD. Mild and severe irradiation of sorghum flour before cooking alleviated somewhat the reduction in sorghum PD on cooking. Maize porridge digestibility was unaffected by prior irradiation of dry flour but decreased with irradiation of wet flour. Mild irradiation of sorghum alleviated the reduction in PD with cooking most, almost to the level of uncooked flour. The alleviation in PD coincided with alleviation in the reduction in PE. With severe irradiation, the alleviation in PE was not consistent. Pepsin PD was positively correlated with PE for sorghum BR7 (r=0.83, p<0.01) and Madjeri (r=0.75, p<0.05), but not for maize. Copyright / Thesis (PHD)--University of Pretoria, 2006. / Food Science / unrestricted

Lysine deficiency is a major nutritional problem faced by poor people living in the arid and semi-arid tropics who depend on sorghum as their staple food. This is because of poor lysine content and digestibility of sorghum proteins, which aggravates when sorghum is cooked in food. To address this nutritional problem, compositing with locally available lysine-rich legumes has been proposed. Therefore, this study investigated the effects of compositing with the African grain legume, cowpea, on the protein and functional quality of important traditional African sorghum foods. Two sorghum cultivars, a red, tannin (NS 5511) and a white tan plant, non-tannin (Orbit) composited with cowpea at 70:30 ratio, were used to prepare three traditional sorghum foods, ugali (unfermented thick porridge), uji (fermented thin porridge) and injera (fermented flatbread). The protein quality of the traditional sorghum foods was determined by measuring their protein contents, lysine and reactive lysine contents, and in vitro protein digestibility. The functional properties of the foods were studied using instrumental texture analysis. Other sensory properties of ugali were determined using a trained sensory panel. Compositing with cowpea increased the protein contents of the foods by up to 35% and 57% for NS 5511 and Orbit foods, respectively. Lysine contents of the food proteins increased by 67% to 139%. Reactive lysine content increased by 10% to 75%. Protein digestibility of the foods increased by 13% to 62%. There was approximately three- and two-fold increase in protein digestibility corrected amino score (PDCAAS) of NS 5511 and Orbit foods, respectively, due to addition of cowpea. However, Orbit-plus-cowpea foods still had better protein quality than NS 5511-plus-cowpea foods, primarily because of the tannins in the latter which bind the proteins thereby lowering their digestibility. Compositing reduced paste peak viscosity (PV) and cool paste viscosity (CPV) of uji porridge by 6% to 23%, and 6% to 12%, respectively, probably as a result of decreasing porridge starch content. Principal component analysis (PCA) showed that compositing contributed 38% of the variation in 17 sensory attributes of ugali. Compositing imparted cowpea flavour to ugali. Most of the variation in sensory properties (59%) of ugali was due to the quality characteristics of the sorghum cultivars. Compositing increased the stiffness of NS 5511 injera by up to 25%, while it reduced the stiffness of Orbit injera by up to 12%. These differences in stiffness suggested a weakening effect of weaker H-bonding between tannins and other food polymers such as proteins instead of stronger covalent bonds like those involved in proteins-protein interactions. Compositing important traditional sorghum foods with cowpea has potential for helping to solve lysine deficiency faced by sorghum consumers in the semi-arid tropics. However, it introduces cowpea flavour which may need to be eliminated, in foods intended for consumers not accustomed to cowpea flavour. Copyright / Dissertation (MSc)--University of Pretoria, 2009. / Food Science / unrestricted