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. Keywords Extrusion cooking . Maize and wheat flours . Die outlet expansion . Rheological properties . Bubble growth model . Image analysis Nomenclature D :screw diameter (m) D d :die diameter (m) D g :extrudate diameter at the maximum expansion section (m) D s :extrudate diameter at shrinkage equilibrium section (m) L :barrel length (m) (lei) g :longitudinal expansion index of the growth phase (lei) s :longitudinal expansion index of the shrinkage phase n :flow index P a :atmospheric pressure (Pa) P v :vapour pressure at melt temperature in the die (Pa) R :radius of a vapour bubble (m) R i :initial radius of the bubble at the die exit (m) (sei) g :sectional expansion index of the growth phase (sei) s :sectional expansion index of the shrinkage phase t g :growth time duration in the radial direction (s) t gl :growth time duration in the longitudinal direction (s) t s :shrinkage time duration in the radial direction (s) t sl :shrinkage time duration in the longitudinal direction (s) t 0 :bubble formation characteristic time (s) V d :melt velocity in the die (m s -1 ) (vei) g :volumetric expansion index of the growth phase (vei) s :volumetric expansion index of the shrinkage phase V g :average growth velocity in the radial direction (m s -1 ) FoodV gl :average growth velocity in the longitudinal direction (m s -1 ) V s :average shrinkage velocity in the radial direction (m s -1 ) V sl :average shrinkage velocity in the longitudinal direction (m s -1 ) Z g :distance from the die to the maximum expansion section (m) Z s :distance from the maximum expansion section to the shrinkage equilibrium section (m) α :parameter depending on the starch melt type β :parameter depending on the starch melt type ΔP :pressure gradient + w Á v :wall shear rate in the capillary rheometer (s -1 ) + w Á d:wall shear rate in the die (s -1 ) (μ a ) d :me...

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