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.