ABSTRACT

The influence of extrusion cooking variables on the physico-chemical, functional and sensory properties of African yam bean (AYB) and cassava blends were investigated. The main purpose was to overcome the problems of hard to cook, beany flavour and antinutritional factors associated with AYB and the low protein content of cassava based diets for human nutrition. AYB, Native cassava tubers and Ighu (pregelatinized, shredded and stepped cassava product) were processed into flours. A central composite design (CCD) was used to determine the effect of feed moisture content (MC), feed composition (FC) and the screw speed (SS) of the operations. Twenty extrusion runs were conducted with eight (2) factorial, six star points and six central points. The proximate and amino acids composition, haemaglutinin and hydrocyanic acid contents, expansion ratios, bulk density, solubilities, swelling index, water absorption capacity (WAC), oil absorption capacity (OAC), engineering properties and sensory attributes of the extrudated products were determined using standard procedures. Data obtained were subjected to response surface analysis (RSA) and ANOVA. The RSA plot showed that the maximum protein of 23.72% was obtained at critical values of 152.85rpm screw speed and 0.54% Ighu/99.46 %AYB feed composition. All the amino acids in the raw samples appeared in the extruded products and were proportionally increased as the percentage of AYB increased. The functional properties of the samples improved after extrusion. The maximum expansion ratio occurred at moisture content of 13.26% which was the smallest feed moisture content used in the extrusion, signifring that MC inversely affected the expansion ratio. The feed composition and screw speed had significant quadratic effects (P<0.05) on the solubility. Interaction between SS and MC had significant effect on the solubility of the extruded products. Regressional analysis for extruded blends of Ighu/AYB showed that SME had significant (p <0.05) linear, quadratic and interactive effects on the independent variables.. The response surface regression for the extruded cassavalAYB showed that screw speed had significant (p<0.05) linear and quadratic effects on the haemagglutinin content of the extrudates. There was significantly (P<0.05) better appearance for sample A1(125rpm,16%MC,20%FC) and sample C1 (125rpm,16%MC,20%FC). The results show that the appearance of feed had effect on the appearance of the extruded products. Extrusion cooking of blends of Ighu/AYB and cassavalAYB improved the nutrients contents and functional properties of the products.