Expansion mechanism of extruded foams supplemented with wheat bran Original Research Article

Addition of bran reduces the expansion of extruded starchy foams. This may be attributed to changes in parameters driving expansion. Wheat flour was supplemented with wheat bran to achieve two levels of fibers (12.6% and 22.4%) and extruded under different conditions of barrel temperature (120 or 180 °C), water content in the feed (18% or 22%) or screw speed (400 or 800 rpm). Increasing the bran concentration decreased sectional and volumetric expansions and increased longitudinal expansion. Finer cellular structures consisting of an increased number of small cells with more spherical shapes were generated. This may be linked to an increased nucleation at the die. The reduction in volumetric expansion was associated with an increase in shear viscosity only at the highest bran concentration. At same extrusion conditions the glass transition temperature of starch was reduced by down to 15 °C by supplementation with bran. This may induce a decrease in the viscosity of the starch phase and counteract the matrix viscosity increase linked to the properties of bran. A higher surface porosity was observed in the bran containing recipe due to earlier burst of the bubbles during growth.