Modeling of Residual Polyphenols, Phytic Acid and Protein Digestibility of Extruded Sorghum-Cowpea Formulated Foods

Blends of cowpea (10%, 20% and 30%) and sorghum flour were extruded at 20%, 22.5% and 25% moisture levels and 120oC, 140oC and 160oC barrel temperatures using a single-screw extruder. Response surface methodology with central composite face-centered (CCF) design was used to model the residual phytic acid and polyphenols, protein solubility and protein digestibility of the extrudates. Increasing the barrel temperature caused a reduction in protein solubility, residual polyphenols and phytic acid but increased the in vitro protein digestibility of the extrudates. The coefficients of determination (R2) were 0.97, 0.99, 0.98 and 0.89 for polyphenols, phytic acid, protein solubility and protein digestibility respectively with non-significant lack of fit in all cases. The correlation coefficients (r2) of observed and predicted values ranged from 0.99 to 0.98 suggesting a good fit for the model. Barrel temperature had the most effect on the responses. The second order polynomial was found appropriate for the prediction of polyphenols, phytic acid and protein digestibility of the sorghum- cowpea extrudates. Keywords: response surface, digestibility, polyphenols, phytic acid, extrusion