Protein-energy malnutrition (PEM) and micronutrient deficiencies remain burdens among children in Africa due to monotonous cereal-based diets. Compositing with local pulses such as bambara groundnuts can be a solution. Rapidly urbanising communities in Africa, however, demand convenience-type products. To address these needs, the nutritional, physical and functional properties of extrudates of decorticated sorghum and whole grain bambara groundnut flours extruded separately and as a 50:50 composite using a twin screw extrusion cooker was investigated. The effect of two in-barrel moisture conditions, low (24%) and high (40%), were studied. Compositing sorghum with bambara groundnuts increased crude fibre (190%), protein (54%) fat (36%) and mineral (118%) contents. Zinc, calcium and magnesium contents increased by 32, 52, and 11%, respectively with bambara groundnut inclusion. In vitro protein digestibility (IVPD) and nitrogen solubility index (NSI) increased substantially with bambara groundnut inclusion because bambara groundnuts' soluble globulin-type proteins are more digestible than sorghum kafirins. Extrusion cooking caused an increase in IVPD and decrease in NSI. Hot, moist conditions in extrusion cooking probably caused the denaturation of proteins which exposed hydrophobic sites of the protein molecule and exposed more sites for proteolytic attack. Extrusion cooking at high moisture yielded a slightly higher IVPD in composite extrudates and higher NSI in bambara groundnut extrudates, as shear and heat generation would be less severe, compared to low moisture. Phytate content was reduced (12-35%) in the composite and bambara groundnut extrudates after extrusion cooking at both extrusion moistures. Inositol hexaphosphate was possibly hydrolysed to penta- and tetraphosphates. Improved Caco-2 cell zinc uptake (80-84%) was achieved through both extrusion moistures in sorghum extrudates only, probably due to lower phytate x calcium/zinc ratios in sorghum when compared to the composite and bambara groundnut flours. Bambara groundnut inclusion reduced Caco-2 cell zinc uptake due to an increase in phytate content. High extrusion moisture and bambara groundnut inclusion decreased extrudate expansion in sorghum and the composite, probably due to lower viscosity in the extruder barrel and dilution of starch, respectively. High moisture conditions yielded higher overall water absorption index (WAI), lower overall water solubility index (WSI) and higher peak, trough and final paste viscosities, probably due to less starch degradation. Bambara groundnut inclusion caused increased WSI, probably due to its soluble proteins. It also caused lower peak, trough and final paste viscosities, possibly due to starch dilution.
A 50 g (db) serving of sorghum-bambara groundnut composite extrudate can contribute 81% of the daily protein requirement for children aged 2-5 years. Improved IVPD, WAI and paste viscosities for composite extruded at low moisture suggests its application as a nutrient-dense instant porridge. Extrusion cooking of the composite at low moisture also maintains high expansion which can produce a nutritious expanded snack. Sorghum-bambara groundnut composite extruded at low moisture can produce convenience-type products with improved mineral content and excellent protein content and digestibility to address PEM.