Increasing the bran content in extruded wheat flour reduces the volumetric and sectional expansion while the longitudinal expansion is increased. Samples enriched with bran show a cellular structure with a higher number of small cells compared to the control flour. At same porosities, the formation of these finer cellular structures with bran leads to samples with a higher breaking strength. The decrease in volumetric expansion with bran can be associated to an increase in the shear viscosity of the melt but only at high bran content. This increase in shear viscosity of the melt may hinder the bubble growth at the die exit. At lower bran content, the decrease in expansion volumes can be explained by the lower extensional capacity of the membrane surrounding the bubble during its growth. The increased surface porosity of the samples with bran addition indicates an early burst of the bubbles at the die exit, limiting their size in the final product. This can be explained by the low adhesion properties between bran particles and starch at their interface.
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