Starch-based extruded cereals enriched in fibers: a behavior of composite solid foams

Extruded cereals mainly composed of starch and enriched in fibers were produced with two types of base recipes: (i) one recipe mainly composed of wheat flour and (ii) one recipe mainly composed of corn and soya flours. The addition of fibers was performed through the use of oat bran concentrate or wheat bran, up to 32% of the recipe. The structure of the extrudates, assessed by X-ray tomography, pointed out the decrease of porosity and of mean cells size with the increase of the total dietary fibers content of the recipe. The hardness of the products, i.e. the maximum stress determined by a compression test, was linked to their porosity. The Gibson-Ashby relationship could be applied and the fit was even improved when considering the walls of the solid foam as composite materials. Fibers and proteins can be indeed considered as particles dispersed in the starchy phase. This work thus shows the impact of the structure of the extrudates on their mechanical properties. The structure is taken into account at different length scales; at the level of the porous structure and at the level of the phase of the main biopolymers present in the recipe (starch, proteins and fibers). The mechanical behavior of these products is then discussed according to their characteristics of composite solid foams.