Flow and foam properties of extruded maize flour and its biopolymer blends expanded by microwave

•Elongational viscosity ηE of maize biopolymer melts was determined by Rheoplast®.•Zein content (≤ 15%) had a weak influence on shear viscosity but increased ηE.•Foam cellular structure of microwave expanded blends was thinner than extruded ones.•Normalized mechanical properties of foams followed the model of open cellular solids.•ηE values, varying like storage modulus, explained maize biopolymer melt expansion.

Maize flour and blends from starch and zein biopolymers were processed as dense materials by extrusion (120 °C, 300 J·g− 1) and press-molding (140 °C, 10 min) at a constant moisture content (26% wb), and then foamed by microwave heating. The mechanical properties of foams, determined by a 3-point bending test, were governed by density, in agreement with an open solid foam model. The density and 3D cellular structure of the foams were determined by X-ray tomography. In the same interval of density [0.15, 0.3 g·cm− 3], foams from microwaved materials had a finer cellular structure than directly expanded materials at extruder outlet. The study of melt rheological behavior with Rheoplast® (100-160 °C, SME ≤ 200 J·g− 1) showed that protein content (0-15%) did not affect shear viscosity but increased elongational viscosity. This trend, similar to the one reported for the storage modulus in a rubbery state, could be attributed to dissipative effects in a starch/protein interphase, explaining the difference of expansion between starch, blends and flour.