Starch–zein blends formed by shear flow

Abstract

A newly in-house developed shearing device was used to explore the formation of new types of microstructures in concentrated starch–zein blends. The device allowed processing of the biopolymer blends under homogeneous, simple shear flow conditions. Water and glycerol were added as plasticizers. Different ratios (0–20% zein, dry basis) were used to study the influence of the matrix composition and processing conditions on the properties of the final material. The properties at large deformation were examined by tensile tests in two different directions (i.e. along the flow ( ∥ ) and in the vorticity ( ⊥ ) directions). The morphology of the blends observed by confocal scanning laser microscopy and field emission scanning electron microscopy showed that under shearless conditions, starch–zein formed a co-continuous blend. The application of shear transformed this structure into a dispersion, with zein as the dispersed phase. The size of the zein aggregates increased with zein concentration. A certain degree of anisotropy was found with the 10% zein blend at the highest shear rate applied ( γ ˙ = 120 s - 1 ) . Here, the zein aggregates were slightly deformed along the shear flow ( ∥ ) . Both microscopy and tensile tests indicated that the blends have no or weak adhesion between the zein and starch phases. The effect of zein particles on the mechanical properties followed a modified model used for particle–matrix materials with weak adhesion between the phases.