The aim of this research was to study the effects of calcium hydroxide (0-0.2%) and screw speed (100-180 rpm) on the expansion index (EI), bulk density (BD), penetration force (PF) and specific mechanical energy (SME) values of blue maize meal extrudates. Blue maize meal was extruded using a commercial extruder (Brabender 20DN/8-235-00) with a compression screw ratio of 3:1. A second-order, central composite experimental design was used. It was found that the EI and SME values decreased and the BD and PF values increased when the calcium hydroxide concentration was increased. The screw speed had a significant effect only on the SME and PF values. Quadratic model fitness was shown for all responses, with values of R2 > 0.74, p of F (model) <0.01 and variability coefficient <13.3% (except for PF, 29.18%), and for almost all cases there was no lack of fit (p > 0.055). Calcium hydroxide concentration showed good correlation (p < 0.01) with EI (r = -0.81), PF (r = 0.60), SME (r = -0.76) and BD (r = 0.83). However, screw speed was marginally or not correlated (r < 0.36, p > 0.14) with the responses. The results suggest that it is possible to produce appropriate extruded products from blue maize fortified with calcium in an optimised calcium hydroxide concentration and screw speed range of 0.02-0.078% and 117-180 rpm respectively.© 2001 Society of Chemical Industry
