Extrusion, physico-chemical characterization and nutritional evaluation of sorghum-based high protein, micronutrient fortified blended foods

Abstract

The feasibility of using a wheat flour mill to refine corn, sorghum and cowpea was studied. Milling of white sorghum grain resulted in decrease in fiber content from 1.89% to 0.38% and 0.45% in raw, finely milled and coarsely milled sorghum respectively. Similarly, there was a reduction in fat (3.17% to 1.75% and 0.51%) content from raw to fine and coarse milled fractions. Starch content increased from 61.85% in raw to 69.80% in fine and 72.30% in coarse fractions. Protein content was almost unchanged at about 7.40% in all the fractions. In de-hulling and milling of cowpeas, starch and protein content increased whereas fiber, fat and ash content decreased. There was a significant difference in expansion characteristics between whole and decorticated binary blends on account of different levels of inherent starch content. Sorghum cowpea (SC) blends had the highest specific mechanical energy (SME) range (285.74 – 361.52 kJ/kg), followed by corn soy (CS) (138.73 – 370.99 kJ/kg) and the least SME was found in sorghum soy (SS) blends (66.56 – 332.93 kJ/kg). SME was found to be positively correlated to starch content in the blends. SC blends had the most stable process followed by SSB and CSB in that order. The milling of expanded extrudates was found to be dependent on bulk density and low bulk density extrudates had bigger particle size and vice-versa. The water absorption index (WAI) for SC was 4.17 g/g to 5.97 g/g, SS ranged from 2.85 g/g to 5.91 g/g and CS ranged from 2.63 g/g to 5.40 g/g. Starch gelatinization ranged from 85.42 – 98.83% for SC, 90.70 – 96.27% for SS, and 72.57 – 95.49% for CS. The starch digestibility increased after extrusion and cooking but there was no significant change in protein digestibility. There was a significant reduction in anti-nutritional factors – phytic acid (26.06 – 44.03%), tannins (18.69 – 26.67%) and trypsin inhibitor (16.55 – 50.85%) after extrusion. Thus, the study showed that high protein blends with superior nutrition density needed for preparation of FBFs could be produced by using existing/traditional milling capabilities and extrusion process.