Optimization of Extrusion Conditions for Production of Instant Millet-Soybean Blended Flour

Abstract

This study developed an optimized protocol for production of instant millet-soybean flour with high solubility. Response surface methodology was used to determine optimal processing conditions for desired water absorption index (WAI) and water solubility index (WSI). Three levels of extrusion temperature (130, 150 and 180 °C) and feed moisture (15, 20 and 25%) were used. Nutritional composition, consumer acceptability and shelf stability of millet-soybean blend produced using optimal extrusion conditions were determined. Analysis of variance showed that the contribution of a quadratic model was significant for WAI and WSI, with high correlation coefficients (R² = 0.80 and 0.75, respectively). Response surface plots showed that increase in temperature significantly increased both WAI and WSI, while increase in feed moisture significantly increased WAI and decreased WSI. The optimal extrusion conditions were found to be: barrel temperature 159 °C and feed moisture of 25% with product predicted WAI of 2.124 g H₂O/g and WSI of 51.30%. Experimental values for responses of products made using optimized conditions were in agreement with the predicted values. Gruels made using the extruded millet-soybean flour were acceptable but with lower consumer acceptability (p < 0.05) scores than the commercially available and extruded millet only flour. Yeasts and molds as well as coliforms were not detected. Total plate count results ranged from 1.35 to 2.9 log cfu/g within one month of storage. This study provides information on the conditions that are suitable for processing an acceptable highly soluble instant millet-soybean flour.