The Effect of Wheat and Maize Flours Properties on the Expansion Mechanism During Extrusion Cooking

Abstract

The expansion mechanism in the extrusion cooking process was investigated during maize and wheat flours processing in a twin-screw extruder. An experimental study of extrusion process was carried out at constant moisture and specific mechanical energy. The expansion phenomenon which consists in a growth phase followed by a shrinkage phase was observed through an image processing method and analyzed by use of classical expansion indices: sectional, longitudinal and volumetric expansion indices. It was found that the maximum expansion is obtained with maize flour and that the difference in composition between maize and wheat flours has an impact on the expansion mechanism. Image analysis method enabled to obtain an instantaneous measurement of the growth and shrinkage parameters and appeared as useful tool to characterise expansion in function of extrusion conditions. A model based on the bubble growth in a viscous medium was proposed to analyze this expansion phenomenon. It enabled to study the evolution of expansion rates in function of the characteristic times of the problem: the bubble formation characteristic time and the maximum expansion time in both radial and longitudinal directions.